ViewDragHelper详解

http://blog.csdn.net/daditao/article/details/29826035

2013年谷歌i/o大会上介绍了两个新的layout： SlidingPaneLayout和DrawerLayout，现在这俩个类被广泛的运用，其实研究他们的源码你会发现这两个类都运用了ViewDragHelper来处理拖动。ViewDragHelper是framework中不为人知却非常有用的一个工具。

ViewDragHelper解决了android中手势处理过于复杂的问题，在DrawerLayout出现之前，侧滑菜单都是由第三方开源代码实现的，其中著名的当属MenuDrawer ，MenuDrawer重写onTouchEvent方法来实现侧滑效果，代码量很大，实现逻辑也需要很大的耐心才能看懂。如果每个开发人员都从这么原始的步奏开始做起，那对于安卓生态是相当不利的。所以说ViewDragHelper等的出现反映了安卓开发框架已经开始向成熟的方向迈进。

本文先介绍ViewDragHelper的基本用法，然后介绍一个能真正体现ViewDragHelper实用性的例子。

其实ViewDragHelper并不是第一个用于分析手势处理的类，gesturedetector也是，但是在和拖动相关的手势分析方面gesturedetector只能说是勉为其难。

关于ViewDragHelper有如下几点：

ViewDragHelper.Callback是连接ViewDragHelper与view之间的桥梁（这个view一般是指拥子view的容器即parentView）；

   ViewDragHelper的实例是通过静态工厂方法创建的；

   你能够指定拖动的方向；

   ViewDragHelper可以检测到是否触及到边缘；

   ViewDragHelper并不是直接作用于要被拖动的View，而是使其控制的视图容器中的子View可以被拖动，如果要指定某个子view的行为，需要在Callback中想办法；

   ViewDragHelper的本质其实是分析onInterceptTouchEvent和onTouchEvent的MotionEvent参数，然后根据分析的结果去改变一个容器中被拖动子View的位置（ 通过offsetTopAndBottom(int offset)和offsetLeftAndRight(int offset)方法 ），他能在触摸的时候判断当前拖动的是哪个子View；

   虽然ViewDragHelper的实例方法 ViewDragHelper create(ViewGroup forParent, Callback cb) 可以指定一个被ViewDragHelper处理拖动事件的对象 ，但ViewDragHelper类的设计决定了其适用于被包含在一个自定义ViewGroup之中，而不是对任意一个布局上的视图容器使用ViewDragHelper。

 

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本文最先发表在我的个人网站 http://jcodecraeer.com/a/anzhuokaifa/androidkaifa/2014/0911/1680.html


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用法：

1.ViewDragHelper的初始化

ViewDragHelper一般用在一个自定义ViewGroup的内部，比如下面自定义了一个继承于LinearLayout的DragLayout，DragLayout内部有一个子viewmDragView作为成员变量：

public class DragLayout extends LinearLayout {
private final ViewDragHelper mDragHelper;
private View mDragView;
public DragLayout(Context context) {
  this(context, null);
}
public DragLayout(Context context, AttributeSet attrs) {
  this(context, attrs, 0);
}
public DragLayout(Context context, AttributeSet attrs, int defStyle) {
  super(context, attrs, defStyle);
}

创建一个带有回调接口的ViewDragHelper

 

 

public DragLayout(Context context, AttributeSet attrs, int defStyle) {
  super(context, attrs, defStyle);
  mDragHelper = ViewDragHelper.create(this, 1.0f, new DragHelperCallback());
}

 

 

其中1.0f是敏感度参数参数越大越敏感。第一个参数为this，表示该类生成的对象，他是ViewDragHelper的拖动处理对象，必须为ViewGroup。

要让ViewDragHelper能够处理拖动需要将触摸事件传递给ViewDragHelper，这点和gesturedetector是一样的：

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
  final int action = MotionEventCompat.getActionMasked(ev);
  if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
      mDragHelper.cancel();
      return false;
  }
  return mDragHelper.shouldInterceptTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
  mDragHelper.processTouchEvent(ev);
  return true;
}

 

接下来，你就可以在回调中处理各种拖动行为了。

 

2.拖动行为的处理

处理横向的拖动：

在DragHelperCallback中实现clampViewPositionHorizontal方法， 并且返回一个适当的数值就能实现横向拖动效果，clampViewPositionHorizontal的第二个参数是指当前拖动子view应该到达的x坐标。所以按照常理这个方法原封返回第二个参数就可以了，但为了让被拖动的view遇到边界之后就不在拖动，对返回的值做了更多的考虑。

@Override
public int clampViewPositionHorizontal(View child, int left, int dx) {
  Log.d("DragLayout", "clampViewPositionHorizontal " + left + "," + dx);
  final int leftBound = getPaddingLeft();
  final int rightBound = getWidth() - mDragView.getWidth();
  final int newLeft = Math.min(Math.max(left, leftBound), rightBound);
  return newLeft;
}

 

 

 

同上，处理纵向的拖动：

在DragHelperCallback中实现clampViewPositionVertical方法，实现过程同clampViewPositionHorizontal

@Override
public int clampViewPositionVertical(View child, int top, int dy) {
  final int topBound = getPaddingTop();
  final int bottomBound = getHeight() - mDragView.getHeight();
  final int newTop = Math.min(Math.max(top, topBound), bottomBound);
  return newTop;
}

 

clampViewPositionHorizontal 和 clampViewPositionVertical必须要重写，因为默认它返回的是0。事实上我们在这两个方法中所能做的事情很有限。 个人觉得这两个方法的作用就是给了我们重新定义目的坐标的机会。

通过DragHelperCallback的tryCaptureView方法的返回值可以决定一个parentview中哪个子view可以拖动，现在假设有两个子views (mDragView1和mDragView2)  ，如下实现tryCaptureView之后，则只有mDragView1是可以拖动的。

1 2 3 4

@Override public boolean tryCaptureView(View child, int pointerId) {   returnchild == mDragView1; }

 

滑动边缘:

分为滑动左边缘还是右边缘：EDGE_LEFT和EDGE_RIGHT，下面的代码设置了可以处理滑动左边缘：

mDragHelper.setEdgeTrackingEnabled(ViewDragHelper.EDGE_LEFT);

假如如上设置，onEdgeTouched方法会在左边缘滑动的时候被调用，这种情况下一般都是没有和子view接触的情况。

 

@Override
public void onEdgeTouched(int edgeFlags, int pointerId) {
    super.onEdgeTouched(edgeFlags, pointerId);
    Toast.makeText(getContext(), "edgeTouched", Toast.LENGTH_SHORT).show();
}

如果你想在边缘滑动的时候根据滑动距离移动一个子view，可以通过实现onEdgeDragStarted方法，并在onEdgeDragStarted方法中手动指定要移动的子View

 

 

@Override
public void onEdgeDragStarted(int edgeFlags, int pointerId) {
    mDragHelper.captureChildView(mDragView2, pointerId);
}

 

 

ViewDragHelper让我们很容易实现一个类似于YouTube视频浏览效果的控件，效果如下：

 

代码中的关键点：

1.tryCaptureView返回了唯一可以被拖动的header view；

2.拖动范围drag range的计算是在onLayout中完成的；

3.注意在onInterceptTouchEvent和onTouchEvent中使用的ViewDragHelper的若干方法；

4.在computeScroll中使用continueSettling方法（因为ViewDragHelper使用了scroller）

5.smoothSlideViewTo方法来完成拖动结束后的惯性操作。

需要注意的是代码仍然有很大改进空间。

activity_main.xml

<FrameLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    <ListView
            android:id="@+id/listView"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:tag="list"
            />
    <com.example.vdh.YoutubeLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:id="@+id/youtubeLayout"
            android:orientation="vertical"
            android:visibility="visible">
        <TextView
                android:id="@+id/viewHeader"
                android:layout_width="match_parent"
                android:layout_height="128dp"
                android:fontFamily="sans-serif-thin"
                android:textSize="25sp"
                android:tag="text"
                android:gravity="center"
                android:textColor="@android:color/white"
                android:background="#AD78CC"/>
        <TextView
                android:id="@+id/viewDesc"
                android:tag="desc"
                android:textSize="35sp"
                android:gravity="center"
                android:text="Loreum Loreum"
                android:textColor="@android:color/white"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:background="#FF00FF"/>
    </com.example.vdh.YoutubeLayout>
</FrameLayout>

YoutubeLayout.java

public class YoutubeLayout extends ViewGroup {
private final ViewDragHelper mDragHelper;
private View mHeaderView;
private View mDescView;
private float mInitialMotionX;
private float mInitialMotionY;
private int mDragRange;
private int mTop;
private float mDragOffset;
public YoutubeLayout(Context context) {
  this(context, null);
}
public YoutubeLayout(Context context, AttributeSet attrs) {
  this(context, attrs, 0);
}
@Override
protected void onFinishInflate() {
    mHeaderView = findViewById(R.id.viewHeader);
    mDescView = findViewById(R.id.viewDesc);
}
public YoutubeLayout(Context context, AttributeSet attrs, int defStyle) {
  super(context, attrs, defStyle);
  mDragHelper = ViewDragHelper.create(this, 1f, new DragHelperCallback());
}
public void maximize() {
    smoothSlideTo(0f);
}
boolean smoothSlideTo(float slideOffset) {
    final int topBound = getPaddingTop();
    int y = (int) (topBound + slideOffset * mDragRange);
    if (mDragHelper.smoothSlideViewTo(mHeaderView, mHeaderView.getLeft(), y)) {
        ViewCompat.postInvalidateOnAnimation(this);
        return true;
    }
    return false;
}
private class DragHelperCallback extends ViewDragHelper.Callback {
  @Override
  public boolean tryCaptureView(View child, int pointerId) {
        return child == mHeaderView;
  }
    @Override
  public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
      mTop = top;
      mDragOffset = (float) top / mDragRange;
        mHeaderView.setPivotX(mHeaderView.getWidth());
        mHeaderView.setPivotY(mHeaderView.getHeight());
        mHeaderView.setScaleX(1 - mDragOffset / 2);
        mHeaderView.setScaleY(1 - mDragOffset / 2);
        mDescView.setAlpha(1 - mDragOffset);
        requestLayout();
  }
  @Override
  public void onViewReleased(View releasedChild, float xvel, float yvel) {
      int top = getPaddingTop();
      if (yvel > 0 || (yvel == 0 && mDragOffset > 0.5f)) {
          top += mDragRange;
      }
      mDragHelper.settleCapturedViewAt(releasedChild.getLeft(), top);
  }
  @Override
  public int getViewVerticalDragRange(View child) {
      return mDragRange;
  }
  @Override
  public int clampViewPositionVertical(View child, int top, int dy) {
      final int topBound = getPaddingTop();
      final int bottomBound = getHeight() - mHeaderView.getHeight() - mHeaderView.getPaddingBottom();
      final int newTop = Math.min(Math.max(top, topBound), bottomBound);
      return newTop;
  }
}
@Override
public void computeScroll() {
  if (mDragHelper.continueSettling(true)) {
      ViewCompat.postInvalidateOnAnimation(this);
  }
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
  final int action = MotionEventCompat.getActionMasked(ev);
  if (( action != MotionEvent.ACTION_DOWN)) {
      mDragHelper.cancel();
      return super.onInterceptTouchEvent(ev);
  }
  if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
      mDragHelper.cancel();
      return false;
  }
  final float x = ev.getX();
  final float y = ev.getY();
  boolean interceptTap = false;
  switch (action) {
      case MotionEvent.ACTION_DOWN: {
          mInitialMotionX = x;
          mInitialMotionY = y;
            interceptTap = mDragHelper.isViewUnder(mHeaderView, (int) x, (int) y);
          break;
      }
      case MotionEvent.ACTION_MOVE: {
          final float adx = Math.abs(x - mInitialMotionX);
          final float ady = Math.abs(y - mInitialMotionY);
          final int slop = mDragHelper.getTouchSlop();
          if (ady > slop && adx > ady) {
              mDragHelper.cancel();
              return false;
          }
      }
  }
  return mDragHelper.shouldInterceptTouchEvent(ev) || interceptTap;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
  mDragHelper.processTouchEvent(ev);
  final int action = ev.getAction();
    final float x = ev.getX();
    final float y = ev.getY();
    boolean isHeaderViewUnder = mDragHelper.isViewUnder(mHeaderView, (int) x, (int) y);
    switch (action & MotionEventCompat.ACTION_MASK) {
      case MotionEvent.ACTION_DOWN: {
          mInitialMotionX = x;
          mInitialMotionY = y;
          break;
      }
      case MotionEvent.ACTION_UP: {
          final float dx = x - mInitialMotionX;
          final float dy = y - mInitialMotionY;
          final int slop = mDragHelper.getTouchSlop();
          if (dx * dx + dy * dy < slop * slop && isHeaderViewUnder) {
              if (mDragOffset == 0) {
                  smoothSlideTo(1f);
              } else {
                  smoothSlideTo(0f);
              }
          }
          break;
      }
  }
  return isHeaderViewUnder && isViewHit(mHeaderView, (int) x, (int) y) || isViewHit(mDescView, (int) x, (int) y);
}
private boolean isViewHit(View view, int x, int y) {
    int[] viewLocation = new int[2];
    view.getLocationOnScreen(viewLocation);
    int[] parentLocation = new int[2];
    this.getLocationOnScreen(parentLocation);
    int screenX = parentLocation[0] + x;
    int screenY = parentLocation[1] + y;
    return screenX >= viewLocation[0] && screenX < viewLocation[0] + view.getWidth() &&
            screenY >= viewLocation[1] && screenY < viewLocation[1] + view.getHeight();
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    measureChildren(widthMeasureSpec, heightMeasureSpec);
    int maxWidth = MeasureSpec.getSize(widthMeasureSpec);
    int maxHeight = MeasureSpec.getSize(heightMeasureSpec);
    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, 0),
            resolveSizeAndState(maxHeight, heightMeasureSpec, 0));
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
  mDragRange = getHeight() - mHeaderView.getHeight();
    mHeaderView.layout(
            0,
            mTop,
            r,
            mTop + mHeaderView.getMeasuredHeight());
    mDescView.layout(
            0,
            mTop + mHeaderView.getMeasuredHeight(),
            r,
            mTop  + b);
}

代码下载地址：https://github.com/flavienlaurent/flavienlaurent.com

 

不管是menudrawer 还是本文实现的DragLayout都体现了一种设计哲学，即可拖动的控件都是封装在一个自定义的Layout中的，为什么这样做？为什么不直接将ViewDragHelper.create(this, 1f, new DragHelperCallback())中的this替换成任何已经布局好的容器，这样这个容器中的子View就能被拖动了，而往往是单独定义一个Layout来处理?个人认为如果在一般的布局中去拖动子view并不会出现什么问题，只是原本规则的世界被打乱了，而单独一个Layout来完成拖动，无非是说，他本来就没有什么规则可言，拖动一下也无妨。

源码解析：

/*
 * Copyright (C) 2013 The <a href="http://www.it165.net/pro/ydad/" target="_blank" class="keylink">Android</a> Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


package android.support.v4.widget;

import android.content.Context;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.VelocityTrackerCompat;
import android.support.v4.view.ViewCompat;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewGroup;
import android.view.animation.Interpolator;

import java.util.Arrays;

/**
 * @tranlator AlexTam
 * ViewDragHelper是自定义ViewGroup时的实用类.它提供大量有用的操作和状态,来追踪用户在父View内的
 * 拖曳子view和重新定位子view.(看到这里,估计就会想,要同步监听拖曳事件,肯定少不了在onTouch事件中随处用到的
 * MotionEvent这个对象.是的,下面的确有它.)
 * 
 * ViewDragHelper is a utility class for writing custom ViewGroups. It offers a number
 * of useful operations and state tracking for allowing a user to drag and reposition
 * views within their parent ViewGroup.
 */
public class ViewDragHelper {
    private static final String TAG = "ViewDragHelper";

    /**
     * 空/无效的pointer ID
     * A null/invalid pointer ID.
     */
    public static final int INVALID_POINTER = -1;

    /**
     * 状态量:(IDLE是闲置的 意思)表示 view当前没有被拖曳或者运行的动画结束
     * A view is not currently being dragged or animating as a result of a fling/snap.
     */
    public static final int STATE_IDLE = 0;

    /**
     * 状态量: view当前正被拖曳.根据用户的输入或者模拟用户的输入,view当前位置发生改变.(表示,
     * 用户怎么拖曳移动,view就根据拖曳的动作而发生位置改变.)
     * A view is currently being dragged. The position is currently changing as a result
     * of user input or simulated user input.
     */
    public static final int STATE_DRAGGING = 1;

    /**
     * 状态量: 由于fling动作,或者预定的无交互的运动,view被"安置"到一个结束的地方.(可以想象,一个view被用户快速拖曳
     * 并甩动,从而view被甩到某个结束的位置,的过程.)
     * A view is currently settling into place as a result of a fling or
     * predefined non-interactive motion.
     */
    public static final int STATE_SETTLING = 2;

    /**
     * 标记可从左边缘拖曳.
     * Edge flag indicating that the left edge should be affected.
     */
    public static final int EDGE_LEFT = 1 << 0;

    /**
     * 标记可从右边缘拖曳.
     * Edge flag indicating that the right edge should be affected.
     */
    public static final int EDGE_RIGHT = 1 << 1;

    /**
     * 标记可从顶部拖曳.
     * Edge flag indicating that the top edge should be affected.
     */
    public static final int EDGE_TOP = 1 << 2;

    /**
     * 标记可从底部拖曳.
     * Edge flag indicating that the bottom edge should be affected.
     */
    public static final int EDGE_BOTTOM = 1 << 3;

    /**
     * 标记所有地方(边缘的上下左右)都能被拖曳.
     * Edge flag set indicating all edges should be affected.
     */
    public static final int EDGE_ALL = EDGE_LEFT | EDGE_TOP | EDGE_RIGHT | EDGE_BOTTOM;

    /**
     * 指引值:
     * 
     * 表示a check(指引) 应该沿着水平轴发生.
     * Indicates that a check should occur along the horizontal axis
     */
    public static final int DIRECTION_HORIZONTAL = 1 << 0;

    /**
     * 表示a check 应该沿着垂直轴发生.
     * Indicates that a check should occur along the vertical axis
     */
    public static final int DIRECTION_VERTICAL = 1 << 1;

    /**
     * 表示a check可水平可垂直的发生.
     * Indicates that a check should occur along all axes
     */
    public static final int DIRECTION_ALL = DIRECTION_HORIZONTAL | DIRECTION_VERTICAL;

    //将边缘大小定位20dp
    private static final int EDGE_SIZE = 20; // dp

    //时间值
    private static final int BASE_SETTLE_DURATION = 256; // ms
    private static final int MAX_SETTLE_DURATION = 600; // ms

    // 当前的拖曳状态,值为idle, dragging or settling.
    // Current drag state; idle, dragging or settling
    private int mDragState;

    // 在拖曳开始前的滑动位移.(可以这么理解,触发拖曳的最大临界值.)
    // Distance to travel before a drag may begin
    private int mTouchSlop;
    
    // 上一次的位置或点
    // Last known position/pointer tracking
    private int mActivePointerId = INVALID_POINTER;
    //初始化的X坐标
    private float[] mInitialMotionX;
    //初始化的Y坐标
    private float[] mInitialMotionY;
    //下面这些变量不写了噻,看名字也能知道.
    private float[] mLastMotionX;
    private float[] mLastMotionY;
    private int[] mInitialEdgesTouched;
    private int[] mEdgeDragsInProgress;
    private int[] mEdgeDragsLocked;
    private int mPointersDown;

    private VelocityTracker mVelocityTracker;
    private float mMaxVelocity;
    private float mMinVelocity;
    //边缘的大小,单位px
    private int mEdgeSize;
    private int mTrackingEdges;

    //兼容新API所提供的Scroller
    private ScrollerCompat mScroller;
    
    //内部抽象类,提供一些规范的接口方法
    private final Callback mCallback;

    private View mCapturedView;
    private boolean mReleaseInProgress;

    private final ViewGroup mParentView;

    /**
     * 这个Callback是作为通信接口,当ViewDragHelper返回父view时使用."on"为首的方法是重要事件的回调方法,几个
     * 接口方法用于提供更多关于请求父view的状态的信息给ViewDragHelper.这个抽象类同时提供子view拖曳的一些细节信息.
     * 
     * A Callback is used as a communication channel with the ViewDragHelper back to the
     * parent view using it. <code>on*</code>methods are invoked on siginficant events and several
     * accessor methods are expected to provide the ViewDragHelper with more information
     * about the state of the parent view upon request. The callback also makes decisions
     * governing the range and draggability of child views.
     */
    public static abstract class Callback {
        /**
         * 当拖曳状态变更时回调该方法.可看"STATE_"为首的常量了解更多信息.
         * Called when the drag state changes. See the <code>STATE_*</code> constants
         * for more information.
         *
         * @param state The new drag state
         *
         * @see #STATE_IDLE
         * @see #STATE_DRAGGING
         * @see #STATE_SETTLING
         */
        public void onViewDragStateChanged(int state) {}

        /**
         * 当捕获view由于拖曳或者设定而发生位置变更时回调..
         * Called when the captured view's position changes as the result of a drag or settle.
         *
         * @param changedView View whose position changed - 发生位置变更的view
         * @param left New X coordinate of the left edge of the view - 新的左边缘X坐标
         * @param top New Y coordinate of the top edge of the view    - 新的顶部边缘Y坐标
         * @param dx Change in X position from the last call    - 从旧到新位置发生的X偏移值
         * @param dy Change in Y position from the last call    - 从旧到新位置发生的Y偏移值
         */
        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {}

        /**
         * 当子view被由于拖曳或设置(settle有点难翻译)而被捕获时回调的方法.提供拖曳的pointer的ID.
         * 如果activePointerId被标记为{@link #INVALID_POINTER},它会代替没有初始化的pointer.
         * 
         * Called when a child view is captured for dragging or settling. The ID of the pointer
         * currently dragging the captured view is supplied. If activePointerId is
         * identified as {@link #INVALID_POINTER} the capture is programmatic instead of
         * pointer-initiated.
         *
         * @param capturedChild Child view that was captured
         * @param activePointerId Pointer id tracking the child capture
         */
        public void onViewCaptured(View capturedChild, int activePointerId) {}

        /**
         * 当子view不再被拖曳时调用.如果有需要,fling的速度也会被提供.速度值会介于系统最小化和最大值之间.
         * 
         * Called when the child view is no longer being actively dragged.
         * The fling velocity is also supplied, if relevant. The velocity values may
         * be clamped to system minimums or maximums.
         *
         * <p>Calling code may decide to fling or otherwise release the view to let it
         * settle into place. It should do so using {@link #settleCapturedViewAt(int, int)}
         * or {@link #flingCapturedView(int, int, int, int)}. If the Callback invokes
         * one of these methods, the ViewDragHelper will enter {@link #STATE_SETTLING}
         * and the view capture will not fully end until it comes to a complete stop.
         * If neither of these methods is invoked before <code>onViewReleased</code> returns,
         * the view will stop in place and the ViewDragHelper will return to
         * {@link #STATE_IDLE}.</p>
         *
         * @param releasedChild The captured child view now being released
         *         - 被捕获到的要释放的子view
         * @param xvel X velocity of the pointer as it left the screen in pixels per second.
         *         - pointer离开屏幕X轴方向每秒运动的速率,单位是px.
         * @param yvel Y velocity of the pointer as it left the screen in pixels per second.
         *         - pointer离开屏幕Y轴方向每秒运动的速率,单位是px.
         */
        public void onViewReleased(View releasedChild, float xvel, float yvel) {}

        /**
         * 当父view其中一个被标记可拖曳的边缘被用户触摸, 同时父view里没有子view被捕获响应时回调该方法.
         * Called when one of the subscribed edges in the parent view has been touched
         * by the user while no child view is currently captured.
         *
         * @param edgeFlags A combination of edge flags describing the edge(s) currently touched
         *             - 描述所当前所触摸的位置的边缘标记, 如EDGE_LEFT,EDGE_RIGHT等等.
         * @param pointerId ID of the pointer touching the described edge(s)
         *             - 触摸的点的ID.
         * 
         * @see #EDGE_LEFT
         * @see #EDGE_TOP
         * @see #EDGE_RIGHT
         * @see #EDGE_BOTTOM
         */
        public void onEdgeTouched(int edgeFlags, int pointerId) {}

        /**
         * 该方法当原来可以拖曳的边缘被锁定不可拖曳时回调.如果边缘在初始化开始拖曳前被拒绝拖曳,就会发生前面说的这种情况.
         * 但这个方法会在{@link #onEdgeTouched(int, int)}之后才会被回调.这个方法会返回true来锁定该边缘.或者
         * 返回false来释放解锁该屏幕.默认的行为是后者(返回false来释放解锁该屏幕).
         * 
         * Called when the given edge may become locked. This can happen if an edge drag
         * was preliminarily rejected before beginning, but after {@link #onEdgeTouched(int, int)}
         * was called. This method should return true to lock this edge or false to leave it
         * unlocked. The default behavior is to leave edges unlocked.
         *
         * @param edgeFlags A combination of edge flags describing the edge(s) locked
         *         - 描述被锁定的边缘的边缘标记,如EDGE_LEFT等.
         * @return true to lock the edge, false to leave it unlocked
         *         - 返回true来锁定该边缘.或者 返回false来释放解锁该屏幕.
         */
        public boolean onEdgeLock(int edgeFlags) {
            return false;
        }

        /**
         * 当用户开始从父view中"订阅的"(之前约定允许拖曳的)屏幕边缘拖曳,并且父view中没有子view响应时调用.
         * 
         * Called when the user has started a deliberate drag away from one
         * of the subscribed edges in the parent view while no child view is currently captured.
         *
         * @param edgeFlags A combination of edge flags describing the edge(s) dragged
         *         - 描述该边缘的边缘标记,如EDGE_LEFT等.
         * @param pointerId ID of the pointer touching the described edge(s)
         *         - pointer的ID.
         * @see #EDGE_LEFT
         * @see #EDGE_TOP
         * @see #EDGE_RIGHT
         * @see #EDGE_BOTTOM
         */
        public void onEdgeDragStarted(int edgeFlags, int pointerId) {}

        /**
         * 调用设置子view z轴次序的参数.
         * Called to determine the Z-order of child views.
         *
         * @param index the ordered position to query for
         * @return index of the view that should be ordered at position <code>index</code>
         */
        public int getOrderedChildIndex(int index) {
            return index;
        }

        /**
         * 返回拖曳的子view水平移动范围的值,单位为px.这个方法如果返回0,那么该view则不能水平移动.
         * Return the magnitude of a draggable child view's horizontal range of motion in pixels.
         * This method should return 0 for views that cannot move horizontally.
         *
         * @param child Child view to check - 目标子view
         * @return range of horizontal motion in pixels    - 水平拖曳的值,单位为px.
         */
        public int getViewHorizontalDragRange(View child) {
            return 0;
        }

        /**
         * 返回拖曳的子view垂直移动范围的值,单位为px.这个方法如果返回0,那么该view则不能垂直移动.
         * Return the magnitude of a draggable child view's vertical range of motion in pixels.
         * This method should return 0 for views that cannot move vertically.
         *
         * @param child Child view to check
         * @return range of vertical motion in pixels
         */
        public int getViewVerticalDragRange(View child) {
            return 0;
        }

        /**
         * 当用户通过pointerId 输入特定值令目标子view移动时回调该方法.callback接口如果返回true,则表示用户
         * 允许通过用于引导的pointer来拖曳该子view.
         * Called when the user's input indicates that they want to capture the given child view
         * with the pointer indicated by pointerId. The callback should return true if the user
         * is permitted to drag the given view with the indicated pointer.
         *
         * 如果该子view已经被捕获, ViewDragHelper可能多次重复的调用该方法.多次的调用会导致新的pointer尝试去控制这个view.
         * <p>ViewDragHelper may call this method multiple times for the same view even if
         * the view is already captured; this indicates that a new pointer is trying to take
         * control of the view.</p>
         *
         * 如果该方法返回true,并且当成功捕获到该子view时,方法{@link #onViewCaptured(android.view.View, int)}会随即被调用.
         * <p>If this method returns true, a call to {@link #onViewCaptured(android.view.View, int)}
         * will follow if the capture is successful.</p>
         *
         * @param child Child the user is attempting to capture    - 用户视图捕获的子view
         * @param pointerId ID of the pointer attempting the capture    - 捕获该子view的pointerID.
         * @return true if capture should be allowed, false otherwise    - 如果允许并且捕获成功应该返回true.否则返回false.
         */
        public abstract boolean tryCaptureView(View child, int pointerId);

        /**
         * 该方法用于限制子view沿水平拖曳的手势.默认的实现是,不允许水平手势.如果有类继承了该类,
         * 必须覆盖重写该方法,并且提供值去限制该拖曳手势.
         * Restrict the motion of the dragged child view along the horizontal axis.
         * The default implementation does not allow horizontal motion; the extending
         * class must override this method and provide the desired clamping.
         *
         *
         * @param child Child view being dragged    -  被拖曳的子view.
         * @param left Attempted motion along the X axis    - 沿X轴(水平)的手势
         * @param dx Proposed change in position for left    - view的left变更值
         * @return The new clamped position for left - 对left返回新的位置值
         */
        public int clampViewPositionHorizontal(View child, int left, int dx) {
            return 0;
        }

        /**
         * 该方法用于限制子view沿垂直拖曳的手势.默认的实现是,不允许垂直手势...(同上面的方法类似,就不过多解释了.)
         * Restrict the motion of the dragged child view along the vertical axis.
         * The default implementation does not allow vertical motion; the extending
         * class must override this method and provide the desired clamping.
         *
         *
         * @param child Child view being dragged
         * @param top Attempted motion along the Y axis
         * @param dy Proposed change in position for top
         * @return The new clamped position for top
         */
        public int clampViewPositionVertical(View child, int top, int dy) {
            return 0;
        }
    }

    /**
     * 定义曲线动画的插值器
     * Interpolator defining the animation curve for mScroller
     */
    private static final Interpolator sInterpolator = new Interpolator() {
        public float getInterpolation(float t) {
            t -= 1.0f;
            return t * t * t * t * t + 1.0f;
        }
    };

    // 实现Runnable接口
    private final Runnable mSetIdleRunnable = new Runnable() {
        public void run() {
            setDragState(STATE_IDLE);
        }
    };

    /**
     * 创建ViewDragHelper的工厂方法
     * Factory method to create a new ViewDragHelper.
     *
     * @param forParent Parent view to monitor  - 所要监听的父view
     * @param cb Callback to provide information and receive events - 提供信息的Callback对象
     * @return a new ViewDragHelper instance
     */
    public static ViewDragHelper create(ViewGroup forParent, Callback cb) {
        return new ViewDragHelper(forParent.getContext(), forParent, cb);
    }

    /**
     * Factory method to create a new ViewDragHelper.
     *
     * @param forParent Parent view to monitor
     * @param sensitivity Multiplier for how sensitive the helper should be about detecting
     *                    the start of a drag. Larger values are more sensitive. 1.0f is normal.
     * @param cb Callback to provide information and receive events
     * @return a new ViewDragHelper instance
     */
    public static ViewDragHelper create(ViewGroup forParent, float sensitivity, Callback cb) {
        final ViewDragHelper helper = create(forParent, cb);
        helper.mTouchSlop = (int) (helper.mTouchSlop * (1 / sensitivity));
        return helper;
    }

    /**
     * 应用应该使用ViewDragHelper.create()去获取新的实例.这将允许ViewDragHelper使用内部实现去兼容不同的平台版本.
     * Apps should use ViewDragHelper.create() to get a new instance.
     * This will allow VDH to use internal compatibility implementations for different
     * platform versions.
     *
     * @param context Context to initialize config-dependent params from
     * @param forParent Parent view to monitor
     */
    private ViewDragHelper(Context context, ViewGroup forParent, Callback cb) {
        if (forParent == null) {
            throw new IllegalArgumentException("Parent view may not be null");
        }
        if (cb == null) {
            throw new IllegalArgumentException("Callback may not be null");
        }

        mParentView = forParent;
        mCallback = cb;

        // ViewConfiguration是一个包含配置信息,如时间,位移等的配置类.
        final ViewConfiguration vc = ViewConfiguration.get(context);
        final float density = context.getResources().getDisplayMetrics().density;
        mEdgeSize = (int) (EDGE_SIZE * density + 0.5f);

        mTouchSlop = vc.getScaledTouchSlop();
        mMaxVelocity = vc.getScaledMaximumFlingVelocity();
        mMinVelocity = vc.getScaledMinimumFlingVelocity();
        mScroller = ScrollerCompat.create(context, sInterpolator);
    }

    /**
     * 设置最小速率.大于0px/s的速率能更好的被检测到.这样Callback就能恰当的运用该值去约束移动的速率.
     * Set the minimum velocity that will be detected as having a magnitude greater than zero
     * in pixels per second. Callback methods accepting a velocity will be clamped appropriately.
     *
     * @param minVel Minimum velocity to detect
     */
    public void setMinVelocity(float minVel) {
        mMinVelocity = minVel;
    }

    /**
     * 获取最小速率. 值得注意的是,如果最小速率小于0, 那么直接返回0,不会返回比0小的值.
     * Return the currently configured minimum velocity. Any flings with a magnitude less
     * than this value in pixels per second. Callback methods accepting a velocity will receive
     * zero as a velocity value if the real detected velocity was below this threshold.
     *
     * @return the minimum velocity that will be detected
     */
    public float getMinVelocity() {
        return mMinVelocity;
    }

    /**
     * 获取当前helper的拖曳状态,返回结果为{@link #STATE_IDLE}, {@link #STATE_DRAGGING} 
     * or {@link #STATE_SETTLING}.中的其一.
     * 
     * Retrieve the current drag state of this helper. This will return one of
     * {@link #STATE_IDLE}, {@link #STATE_DRAGGING} or {@link #STATE_SETTLING}.
     * @return The current drag state
     */
    public int getViewDragState() {
        return mDragState;
    }

    /**
     * 设置允许父view的某个边缘可追踪.CallBack对象的{@link Callback#onEdgeTouched(int, int)} and
     * {@link Callback#onEdgeDragStarted(int, int)}方法只有在边缘允许被追踪时才会调用. 
     * (就是说,如果不设置上下左右的某个边缘可追踪,那么这2个方法是不可用的.)
     * 
     * Enable edge tracking for the selected edges of the parent view.
     * The callback's {@link Callback#onEdgeTouched(int, int)} and
     * {@link Callback#onEdgeDragStarted(int, int)} methods will only be invoked
     * for edges for which edge tracking has been enabled.
     *
     * @param edgeFlags Combination of edge flags describing the edges to watch
     * @see #EDGE_LEFT
     * @see #EDGE_TOP
     * @see #EDGE_RIGHT
     * @see #EDGE_BOTTOM
     */
    public void setEdgeTrackingEnabled(int edgeFlags) {
        mTrackingEdges = edgeFlags;
    }

    /**
     * 返回边缘大小的值.单位为px.这个值是该view边缘可以被监测或追踪的值的范围.
     * Return the size of an edge. This is the range in pixels along the edges of this view
     * that will actively detect edge touches or drags if edge tracking is enabled.
     *
     * @return The size of an edge in pixels
     * @see #setEdgeTrackingEnabled(int)
     */
    public int getEdgeSize() {
        return mEdgeSize;
    }

    /**
     * 在父view内捕获指定的子view用于拖曳.同时callback对象会被通知.但{@link Callback#tryCaptureView(android.view.View, int)}
     * 不会被要求获取权限来捕获该view.
     * 
     * Capture a specific child view for dragging within the parent. The callback will be notified
     * but {@link Callback#tryCaptureView(android.view.View, int)} will not be asked permission to
     * capture this view.
     *
     * @param childView Child view to capture
     * @param activePointerId ID of the pointer that is dragging the captured child view
     */
    public void captureChildView(View childView, int activePointerId) {
        if (childView.getParent() != mParentView) {
            throw new IllegalArgumentException("captureChildView: parameter must be a descendant " +
                    "of the ViewDragHelper's tracked parent view (" + mParentView + ")");
        }

        mCapturedView = childView;
        mActivePointerId = activePointerId;
        mCallback.onViewCaptured(childView, activePointerId);
        setDragState(STATE_DRAGGING);
    }

    /**
     * 返回当前捕获的view.如果没有捕获到的view,则返回null.
     * @return The currently captured view, or null if no view has been captured.
     */
    public View getCapturedView() {
        return mCapturedView;
    }

    /**
     * 当前拖曳捕获的view的点(pointer)的ID.
     * @return The ID of the pointer currently dragging the captured view,
     *         or {@link #INVALID_POINTER}.
     */
    public int getActivePointerId() {
        return mActivePointerId;
    }

    /**
     * 获取最小触发和初始化拖曳动作的值,单位px.
     * @return The minimum distance in pixels that the user must travel to initiate a drag
     */
    public int getTouchSlop() {
        return mTouchSlop;
    }

    /**
     * 这方法等价于onTouch中MotionEvent的ACTION_CANCEL事件.
     * The result of a call to this method is equivalent to
     * {@link #processTouchEvent(android.view.MotionEvent)} receiving an ACTION_CANCEL event.
     */
    public void cancel() {
        mActivePointerId = INVALID_POINTER;
        clearMotionHistory();

        if (mVelocityTracker != null) {
            mVelocityTracker.recycle();
            mVelocityTracker = null;
        }
    }

    /**
     * 中止取所有手势.并且直接结束动画.
     * {@link #cancel()}, but also abort all motion in progress and snap to the end of any
     * animation.
     */
    public void abort() {
        cancel();
        if (mDragState == STATE_SETTLING) {
            final int oldX = mScroller.getCurrX();
            final int oldY = mScroller.getCurrY();
            mScroller.abortAnimation();
            final int newX = mScroller.getCurrX();
            final int newY = mScroller.getCurrY();
            mCallback.onViewPositionChanged(mCapturedView, newX, newY, newX - oldX, newY - oldY);
        }
        //中止了,当然要设置拖曳状态为闲置(或者说初始态)
        setDragState(STATE_IDLE);
    }

    /**
     * (使用这个方法,可以有动画效果的移动子view到特定位置,该位置需要给出的finalLeft和 finalTop值.)
     * 随着动画,子view移动到既定(给定left和top值)的位置.如果这个方法返回true,会在后面随着手势移动的
     * 每一帧中回调{@link #continueSettling(boolean)}方法,直至返回false.如果这个方法返回false,
     * 就不会再移动去完成手势动作的事件. 
     * 
     * Animate the view <code>child</code> to the given (left, top) position.
     * If this method returns true, the caller should invoke {@link #continueSettling(boolean)}
     * on each subsequent frame to continue the motion until it returns false. If this method
     * returns false there is no further work to do to complete the movement.
     *
     * 要注意的是,即使方法{@link #getCapturedView()}在这个滑动过程中仍会一直有效,可以获取catureView的值,
     * 但这个操作过程不看做是一个捕获事件(我们应当知道,捕获子view不是我们决定的,是Helper自动在父view和
     * 子view之间去自动完成的过程,无论这个过程成功还是失败).
     * 
     * <p>This operation does not count as a capture event, though {@link #getCapturedView()}
     * will still report the sliding view while the slide is in progress.</p>
     *
     * @param child Child view to capture and animate - 要捕获和添加动画移动的view对象
     * @param finalLeft Final left position of child - 最终位置的left值
     * @param finalTop Final top position of child - 最终位置的top值
     * @return true if animation should continue through {@link #continueSettling(boolean)} calls
     */
    public boolean smoothSlideViewTo(View child, int finalLeft, int finalTop) {
        mCapturedView = child;
        mActivePointerId = INVALID_POINTER;

        boolean continueSliding = forceSettleCapturedViewAt(finalLeft, finalTop, 0, 0);
        if (!continueSliding && mDragState == STATE_IDLE && mCapturedView != null) {
            // If we're in an IDLE state to begin with and aren't moving anywhere, we
            // end up having a non-null capturedView with an IDLE dragState
            mCapturedView = null;
        }

        return continueSliding;
    }

    /**
     * (通过这个方法,我们应当知道settle和slide的区别.前者是直接跳到结束位置,而后者是有过渡效果的.)
     * 将捕获的view设置(settle)在给定的left,top值的位置.(表示,直接忽略过程,直接将view显示在特定位置)
     * 这个过程中,该view(如果在此时已经有)适当的速度,则该速度会影响settle的过程.
     * 如果这个方法返回true,方法{@link #continueSettling(boolean)}在整个settle过程中会被回调,直至返回false.
     * 如果这个方法返回false,(表示此时该view已经在给定的位置)这个settle的过程就会结束,不会再工作完成事件.
     * 
     * Settle the captured view at the given (left, top) position.
     * The appropriate velocity from prior motion will be taken into account.
     * If this method returns true, the caller should invoke {@link #continueSettling(boolean)}
     * on each subsequent frame to continue the motion until it returns false. If this method
     * returns false there is no further work to do to complete the movement.
     *
     * @param finalLeft Settled left edge position for the captured view
     * @param finalTop Settled top edge position for the captured view
     * @return true if animation should continue through {@link #continueSettling(boolean)} calls
     */
    public boolean settleCapturedViewAt(int finalLeft, int finalTop) {
        if (!mReleaseInProgress) {
            throw new IllegalStateException("Cannot settleCapturedViewAt outside of a call to " +
                    "Callback#onViewReleased");
        }

        return forceSettleCapturedViewAt(finalLeft, finalTop,
                (int) VelocityTrackerCompat.getXVelocity(mVelocityTracker, mActivePointerId),
                (int) VelocityTrackerCompat.getYVelocity(mVelocityTracker, mActivePointerId));
    }

    /**
     * 同样是将view直接设到特定位置(给定left, top值).
     * (看该方法的实现,整个过程 也是靠scroller的scroll去实现的).
     * Settle the captured view at the given (left, top) position.
     *
     * @param finalLeft Target left position for the captured view
     * @param finalTop Target top position for the captured view
     * @param xvel Horizontal velocity    - 水平速度
     * @param yvel Vertical velocity    - 垂直速度
     * @return true if animation should continue through {@link #continueSettling(boolean)} calls
     *         - settleing的过程中会一直返回true,否则返回false表示结束.
     */
    private boolean forceSettleCapturedViewAt(int finalLeft, int finalTop, int xvel, int yvel) {
        final int startLeft = mCapturedView.getLeft();
        final int startTop = mCapturedView.getTop();
        final int dx = finalLeft - startLeft;
        final int dy = finalTop - startTop;

        if (dx == 0 && dy == 0) {
            // Nothing to do. Send callbacks, be done.
            mScroller.abortAnimation();
            setDragState(STATE_IDLE);
            return false;
        }
        // 仔细看computeSettleDuration()这个计算时间的方法,其实挺复杂的.使用了相当多的运算处理.因此可以不看该方法的实现;
        // 除非要继承ViewDrarHelper实现子类,实现更多效果...
        final int duration = computeSettleDuration(mCapturedView, dx, dy, xvel, yvel);
        mScroller.startScroll(startLeft, startTop, dx, dy, duration);

        setDragState(STATE_SETTLING);
        return true;
    }

    //该方法计算settle的时间
    private int computeSettleDuration(View child, int dx, int dy, int xvel, int yvel) {
        //clampMag(...)方法保证水平和垂直速度值不大于最大值, 也不小于最小值.
        xvel = clampMag(xvel, (int) mMinVelocity, (int) mMaxVelocity);
        yvel = clampMag(yvel, (int) mMinVelocity, (int) mMaxVelocity);
        final int absDx = Math.abs(dx);
        final int absDy = Math.abs(dy);
        final int absXVel = Math.abs(xvel);
        final int absYVel = Math.abs(yvel);
        final int addedVel = absXVel + absYVel;
        final int addedDistance = absDx + absDy;

        final float xweight = xvel != 0 ? (float) absXVel / addedVel :
                (float) absDx / addedDistance;
        final float yweight = yvel != 0 ? (float) absYVel / addedVel :
                (float) absDy / addedDistance;
        //要注意的是getViewHorizontalDragRange(...)方法默认返回0,但一般都会在创建helper时传进的mCallback中重写该方法
        int xduration = computeAxisDuration(dx, xvel, mCallback.getViewHorizontalDragRange(child));
        int yduration = computeAxisDuration(dy, yvel, mCallback.getViewVerticalDragRange(child));

        return (int) (xduration * xweight + yduration * yweight);
    }
    
    //该方法计算settle的时间,三个输入的参数依次分别是:水平或垂直方向的移动距离,水平或垂直方向的速度大小,拖曳范围值
    private int computeAxisDuration(int delta, int velocity, int motionRange) {
        if (delta == 0) {
            return 0;
        }

        final int width = mParentView.getWidth();
        final int halfWidth = width / 2;
        final float distanceRatio = Math.min(1f, (float) Math.abs(delta) / width);
        final float distance = halfWidth + halfWidth *
                distanceInfluenceForSnapDuration(distanceRatio);

        int duration;
        velocity = Math.abs(velocity);
        if (velocity > 0) {
            duration = 4 * Math.round(1000 * Math.abs(distance / velocity));
        } else {
            final float range = (float) Math.abs(delta) / motionRange;
            duration = (int) ((range + 1) * BASE_SETTLE_DURATION);
        }
        return Math.min(duration, MAX_SETTLE_DURATION);
    }

    /**
     * 该方法通过最大和最小值,算出区间值.低于最小值返回0,大于最大值则返回最大值.
     * Clamp the magnitude of value for absMin and absMax.
     * If the value is below the minimum, it will be clamped to zero.
     * If the value is above the maximum, it will be clamped to the maximum.
     *
     * @param value Value to clamp
     * @param absMin Absolute value of the minimum significant value to return
     * @param absMax Absolute value of the maximum value to return
     * @return The clamped value with the same sign as <code>value</code>
     */
    private int clampMag(int value, int absMin, int absMax) {
        final int absValue = Math.abs(value);
        if (absValue < absMin) return 0;
        if (absValue > absMax) return value > 0 ? absMax : -absMax;
        return value;
    }

    /**
     * 这个方法和上面的clampMag(int value, int absMin, int absMax)几乎一样,只是换了浮点型.
     * Clamp the magnitude of value for absMin and absMax.
     * If the value is below the minimum, it will be clamped to zero.
     * If the value is above the maximum, it will be clamped to the maximum.
     *
     * @param value Value to clamp
     * @param absMin Absolute value of the minimum significant value to return
     * @param absMax Absolute value of the maximum value to return
     * @return The clamped value with the same sign as <code>value</code>
     */
    private float clampMag(float value, float absMin, float absMax) {
        final float absValue = Math.abs(value);
        if (absValue < absMin) return 0;
        if (absValue > absMax) return value > 0 ? absMax : -absMax;
        return value;
    }

    private float distanceInfluenceForSnapDuration(float f) {
        f -= 0.5f; // center the values about 0.
        f *= 0.3f * Math.PI / 2.0f;
        return (float) Math.sin(f);
    }

    /**
     * 该方法类似上面的forceSettleCapturedViewAt(...),可参考之.
     * 
     * Settle the captured view based on standard free-moving fling behavior.
     * The caller should invoke {@link #continueSettling(boolean)} on each subsequent frame
     * to continue the motion until it returns false.
     *
     * @param minLeft Minimum X position for the view's left edge
     * @param minTop Minimum Y position for the view's top edge
     * @param maxLeft Maximum X position for the view's left edge
     * @param maxTop Maximum Y position for the view's top edge
     */
    public void flingCapturedView(int minLeft, int minTop, int maxLeft, int maxTop) {
        if (!mReleaseInProgress) {
            throw new IllegalStateException("Cannot flingCapturedView outside of a call to " +
                    "Callback#onViewReleased");
        }

        mScroller.fling(mCapturedView.getLeft(), mCapturedView.getTop(),
                (int) VelocityTrackerCompat.getXVelocity(mVelocityTracker, mActivePointerId),
                (int) VelocityTrackerCompat.getYVelocity(mVelocityTracker, mActivePointerId),
                minLeft, maxLeft, minTop, maxTop);

        setDragState(STATE_SETTLING);
    }

    /**
     * 这个方法在上面好几地方都被提及了.
     * 在整个settle的过程中,这个方法会返回true.直至返回false,表示settle的过程结束.
     * (该方法是内部调用的,外部建议不适用.)
     * 
     * Move the captured settling view by the appropriate amount for the current time.
     * If <code>continueSettling</code> returns true, the caller should call it again
     * on the next frame to continue.
     *
     * 参数deferCallbacks - 如果要推迟滑动,比如在{@link android.view.View#computeScroll()}里面回调,或者view还在layout或者draw
     * 的过程中,该参数应当传true;
     * @param deferCallbacks true if state callbacks should be deferred via posted message.
     *                       Set this to true if you are calling this method from
     *                       {@link android.view.View#computeScroll()} or similar methods
     *                       invoked as part of layout or drawing.
     * @return true if settle is still in progress
     */
    public boolean continueSettling(boolean deferCallbacks) {
        if (mDragState == STATE_SETTLING) {
            // 由于整个settle的过程都借助Scroller去实现,
            // 因此keepGoing这个值也来自mScroller.computeScrollOffset();
            // mScroller.computeScrollOffset()这方法,表示只要view处于scroll状态,都会返回true.停止scroll则返回false.
            boolean keepGoing = mScroller.computeScrollOffset();
            final int x = mScroller.getCurrX();
            final int y = mScroller.getCurrY();
            final int dx = x - mCapturedView.getLeft();
            final int dy = y - mCapturedView.getTop();

            if (dx != 0) {
                mCapturedView.offsetLeftAndRight(dx);
            }
            if (dy != 0) {
                mCapturedView.offsetTopAndBottom(dy);
            }

            if (dx != 0 || dy != 0) {
                // 可见该方法在整个settle的过程中,由于位置的不断变化
                // 会一直回调mCallback.onViewPositionChanged(...)的方法
                mCallback.onViewPositionChanged(mCapturedView, x, y, dx, dy);
            }
            
            //这里很明显,当view已经去到最终位置,XY的坐标均相等时,即使keepGoing依然为true,系统以为
            //该view依旧处于滑动中,但很显然,应该结束了.于是方法里面强制调用Scroller.abortAnimation()去中止动画,并
            //向mScroller标记完成状态.keepGoing自然就为false了.
            if (keepGoing && x == mScroller.getFinalX() && y == mScroller.getFinalY()) {
                // Close enough. The interpolator/scroller might think we're still moving
                // but the user sure doesn't.
                mScroller.abortAnimation();
                keepGoing = false;
            }
            
            //此处推迟滑动,借助Runable接口去实现
            if (!keepGoing) {
                if (deferCallbacks) {
                    mParentView.post(mSetIdleRunnable);
                } else {
                    //来到这里,keepGoing和deferCallbacks为false,表示整个settle过程都结束了.
                    //更改拖曳状态,continueSettling(...)不会再被回调.
                    setDragState(STATE_IDLE);
                }
            }
        }

        return mDragState == STATE_SETTLING;
    }

    /**
     * (该方法是当完成settle过程后释放捕获到的view对象, 内部方法,不必了解详细过程.)
     * 正如所有接口事件的方法,这个方法也必须在UI主线程中使用.在释放的过程中,只会调用一次
     * {@link #settleCapturedViewAt(int, int)}或者{@link #flingCapturedView(int, int, int, int)}方法.
     * 
     * Like all callback events this must happen on the UI thread, but release
     * involves some extra semantics. During a release (mReleaseInProgress)
     * is the only time it is valid to call {@link #settleCapturedViewAt(int, int)}
     * or {@link #flingCapturedView(int, int, int, int)}.
     */
    private void dispatchViewReleased(float xvel, float yvel) {
        mReleaseInProgress = true;
        mCallback.onViewReleased(mCapturedView, xvel, yvel);
        mReleaseInProgress = false;

        if (mDragState == STATE_DRAGGING) {
            // onViewReleased didn't call a method that would have changed this. Go idle.
            setDragState(STATE_IDLE);
        }
    }

    //下面几个"clear"为首的方法都是清空历史记录了
    private void clearMotionHistory() {
        if (mInitialMotionX == null) {
            return;
        }
        Arrays.fill(mInitialMotionX, 0);
        Arrays.fill(mInitialMotionY, 0);
        Arrays.fill(mLastMotionX, 0);
        Arrays.fill(mLastMotionY, 0);
        Arrays.fill(mInitialEdgesTouched, 0);
        Arrays.fill(mEdgeDragsInProgress, 0);
        Arrays.fill(mEdgeDragsLocked, 0);
        mPointersDown = 0;
    }

    private void clearMotionHistory(int pointerId) {
        if (mInitialMotionX == null) {
            return;
        }
        mInitialMotionX[pointerId] = 0;
        mInitialMotionY[pointerId] = 0;
        mLastMotionX[pointerId] = 0;
        mLastMotionY[pointerId] = 0;
        mInitialEdgesTouched[pointerId] = 0;
        mEdgeDragsInProgress[pointerId] = 0;
        mEdgeDragsLocked[pointerId] = 0;
        mPointersDown &= ~(1 << pointerId);
    }
    
    //这个方法很明显是内部调用的,在saveInitialMotion(...)中调用.
    //因为mInitialMotionX数组里面保存有触摸X坐标的缓存信息,该方法确保mInitialMotionX一直保存最新的pointerId值
    private void ensureMotionHistorySizeForId(int pointerId) {
        if (mInitialMotionX == null || mInitialMotionX.length <= pointerId) {
            float[] imx = new float[pointerId + 1];
            float[] imy = new float[pointerId + 1];
            float[] lmx = new float[pointerId + 1];
            float[] lmy = new float[pointerId + 1];
            int[] iit = new int[pointerId + 1];
            int[] edip = new int[pointerId + 1];
            int[] edl = new int[pointerId + 1];
            
            //这个过程,将触摸的X,Y坐标,上次触摸的X,Y坐标等信息复制过去
            if (mInitialMotionX != null) {
                //这里调用本地C方法去将mInitialMotionX的内存复制给imx数组,没源码...
                System.arraycopy(mInitialMotionX, 0, imx, 0, mInitialMotionX.length);
                System.arraycopy(mInitialMotionY, 0, imy, 0, mInitialMotionY.length);
                System.arraycopy(mLastMotionX, 0, lmx, 0, mLastMotionX.length);
                System.arraycopy(mLastMotionY, 0, lmy, 0, mLastMotionY.length);
                System.arraycopy(mInitialEdgesTouched, 0, iit, 0, mInitialEdgesTouched.length);
                System.arraycopy(mEdgeDragsInProgress, 0, edip, 0, mEdgeDragsInProgress.length);
                System.arraycopy(mEdgeDragsLocked, 0, edl, 0, mEdgeDragsLocked.length);
            }

            mInitialMotionX = imx;
            mInitialMotionY = imy;
            mLastMotionX = lmx;
            mLastMotionY = lmy;
            mInitialEdgesTouched = iit;
            mEdgeDragsInProgress = edip;
            mEdgeDragsLocked = edl;
        }
    }

    // 在这里,连同pointerId,保存X,Y轴坐标信息
    // 也许看到这里,你已经猜到,pointerId这个值是递增的,由系统自动分配.
    private void saveInitialMotion(float x, float y, int pointerId) {
        ensureMotionHistorySizeForId(pointerId);
        mInitialMotionX[pointerId] = mLastMotionX[pointerId] = x;
        mInitialMotionY[pointerId] = mLastMotionY[pointerId] = y;
        mInitialEdgesTouched[pointerId] = getEdgesTouched((int) x, (int) y);
        // 或运算后再进行左移运算. 
        mPointersDown |= 1 << pointerId;
    }

    private void saveLastMotion(MotionEvent ev) {
        final int pointerCount = MotionEventCompat.getPointerCount(ev);
        for (int i = 0; i < pointerCount; i++) {
            final int pointerId = MotionEventCompat.getPointerId(ev, i);
            final float x = MotionEventCompat.getX(ev, i);
            final float y = MotionEventCompat.getY(ev, i);
            mLastMotionX[pointerId] = x;
            mLastMotionY[pointerId] = y;
        }
    }

    /**
     * 检查给定id的pointer是否当前按下的pointer.
     * Check if the given pointer ID represents a pointer that is currently down (to the best
     * of the ViewDragHelper's knowledge).
     *
     * 被用于报告这个pointer信息的有以下几个方法:shouldInterceptTouchEvent()和processTouchEvent().
     * 如果这其中一个方法都没有被相关的触摸事件回调,那么该方法中所汇报的信息是不准确或者过时的.
     * (很明显,最新的触摸信息,必须是当前InterceptTouchEvent事件中能回调的.)
     * 
     * <p>The state used to report this information is populated by the methods
     * {@link #shouldInterceptTouchEvent(android.view.MotionEvent)} or
     * {@link #processTouchEvent(android.view.MotionEvent)}. If one of these methods has not
     * been called for all relevant MotionEvents to track, the information reported
     * by this method may be stale or incorrect.</p>
     *
     * @param pointerId pointer ID to check; corresponds to IDs provided by MotionEvent
     * @return true if the pointer with the given ID is still down
     */
    public boolean isPointerDown(int pointerId) {
        return (mPointersDown & 1 << pointerId) != 0;
    }

    void setDragState(int state) {
        mParentView.removeCallbacks(mSetIdleRunnable);
        if (mDragState != state) {
            mDragState = state;
            mCallback.onViewDragStateChanged(state);
            if (mDragState == STATE_IDLE) {
                mCapturedView = null;
            }
        }
    }

    /**
     * 通过传进的pointerId,试图捕获view.如果之前已成功捕获过,则不再调用mCallback.tryCaptureView()方法,而直接返回true.
     * Attempt to capture the view with the given pointer ID. The callback will be involved.
     * This will put us into the "dragging" state. If we've already captured this view with
     * this pointer this method will immediately return true without consulting the callback.
     *
     * @param toCapture View to capture
     * @param pointerId Pointer to capture with
     * @return true if capture was successful
     */
    boolean tryCaptureViewForDrag(View toCapture, int pointerId) {
        if (toCapture == mCapturedView && mActivePointerId == pointerId) {
            // Already done!
            return true;
        }
        if (toCapture != null && mCallback.tryCaptureView(toCapture, pointerId)) {
            mActivePointerId = pointerId;
            captureChildView(toCapture, pointerId);
            return true;
        }
        return false;
    }

    /**
     * 测试是否view v是否能滑动
     * Tests scrollability within child views of v given a delta of dx.
     *
     * @param v View to test for horizontal scrollability
     * @param checkV Whether the view v passed should itself be checked for scrollability (true),
     *               or just its children (false).
     * @param dx Delta scrolled in pixels along the X axis
     * @param dy Delta scrolled in pixels along the Y axis
     * @param x X coordinate of the active touch point
     * @param y Y coordinate of the active touch point
     * @return true if child views of v can be scrolled by delta of dx.
     */
    protected boolean canScroll(View v, boolean checkV, int dx, int dy, int x, int y) {
        if (v instanceof ViewGroup) {
            final ViewGroup group = (ViewGroup) v;
            final int scrollX = v.getScrollX();
            final int scrollY = v.getScrollY();
            final int count = group.getChildCount();
            // Count backwards - let topmost views consume scroll distance first.
            for (int i = count - 1; i >= 0; i--) {
                // TODO: Add versioned support here for transformed views.
                // This will not work for transformed views in Honeycomb+
                final View child = group.getChildAt(i);
                if (x + scrollX >= child.getLeft() && x + scrollX < child.getRight() &&
                        y + scrollY >= child.getTop() && y + scrollY < child.getBottom() &&
                        canScroll(child, true, dx, dy, x + scrollX - child.getLeft(),
                                y + scrollY - child.getTop())) {
                    return true;
                }
            }
        }

        return checkV && (ViewCompat.canScrollHorizontally(v, -dx) ||
                ViewCompat.canScrollVertically(v, -dy));
    }

    /**
     * 检测这个作为被提供给父view的onInterceptTouchEvent的事件是否令父view拦截到当前的触摸事件流.
     * Check if this event as provided to the parent view's onInterceptTouchEvent should
     * cause the parent to intercept the touch event stream.
     *
     * @param ev MotionEvent provided to onInterceptTouchEvent - 提供给onInterceptTouchEvent()方法的触摸事件对象
     * @return true if the parent view should return true from onInterceptTouchEvent
     */
    public boolean shouldInterceptTouchEvent(MotionEvent ev) {
        final int action = MotionEventCompat.getActionMasked(ev);
        final int actionIndex = MotionEventCompat.getActionIndex(ev);

        if (action == MotionEvent.ACTION_DOWN) {
            // Reset things for a new event stream, just in case we didn't get
            // the whole previous stream.
            cancel();
        }

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        switch (action) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();
                final int pointerId = MotionEventCompat.getPointerId(ev, 0);
                saveInitialMotion(x, y, pointerId);

                final View toCapture = findTopChildUnder((int) x, (int) y);

                // Catch a settling view if possible.
                if (toCapture == mCapturedView && mDragState == STATE_SETTLING) {
                    tryCaptureViewForDrag(toCapture, pointerId);
                }

                final int edgesTouched = mInitialEdgesTouched[pointerId];
                if ((edgesTouched & mTrackingEdges) != 0) {
                    mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);
                }
                break;
            }

            case MotionEventCompat.ACTION_POINTER_DOWN: {
                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);
                final float x = MotionEventCompat.getX(ev, actionIndex);
                final float y = MotionEventCompat.getY(ev, actionIndex);

                saveInitialMotion(x, y, pointerId);

                // A ViewDragHelper can only manipulate one view at a time.
                if (mDragState == STATE_IDLE) {
                    final int edgesTouched = mInitialEdgesTouched[pointerId];
                    if ((edgesTouched & mTrackingEdges) != 0) {
                        mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);
                    }
                } else if (mDragState == STATE_SETTLING) {
                    // Catch a settling view if possible.
                    final View toCapture = findTopChildUnder((int) x, (int) y);
                    if (toCapture == mCapturedView) {
                        tryCaptureViewForDrag(toCapture, pointerId);
                    }
                }
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                // First to cross a touch slop over a draggable view wins. Also report edge drags.
                final int pointerCount = MotionEventCompat.getPointerCount(ev);
                for (int i = 0; i < pointerCount; i++) {
                    final int pointerId = MotionEventCompat.getPointerId(ev, i);
                    final float x = MotionEventCompat.getX(ev, i);
                    final float y = MotionEventCompat.getY(ev, i);
                    final float dx = x - mInitialMotionX[pointerId];
                    final float dy = y - mInitialMotionY[pointerId];

                    final View toCapture = findTopChildUnder((int) x, (int) y);
                    final boolean pastSlop = toCapture != null && checkTouchSlop(toCapture, dx, dy);
                    if (pastSlop) {
                        // check the callback's
                        // getView[Horizontal|Vertical]DragRange methods to know
                        // if you can move at all along an axis, then see if it
                        // would clamp to the same value. If you can't move at
                        // all in every dimension with a nonzero range, bail.
                        final int oldLeft = toCapture.getLeft();
                        final int targetLeft = oldLeft + (int) dx;
                        final int newLeft = mCallback.clampViewPositionHorizontal(toCapture,
                                targetLeft, (int) dx);
                        final int oldTop = toCapture.getTop();
                        final int targetTop = oldTop + (int) dy;
                        final int newTop = mCallback.clampViewPositionVertical(toCapture, targetTop,
                                (int) dy);
                        final int horizontalDragRange = mCallback.getViewHorizontalDragRange(
                                toCapture);
                        final int verticalDragRange = mCallback.getViewVerticalDragRange(toCapture);
                        if ((horizontalDragRange == 0 || horizontalDragRange > 0
                                && newLeft == oldLeft) && (verticalDragRange == 0
                                || verticalDragRange > 0 && newTop == oldTop)) {
                            break;
                        }
                    }
                    reportNewEdgeDrags(dx, dy, pointerId);
                    if (mDragState == STATE_DRAGGING) {
                        // Callback might have started an edge drag
                        break;
                    }

                    if (pastSlop && tryCaptureViewForDrag(toCapture, pointerId)) {
                        break;
                    }
                }
                saveLastMotion(ev);
                break;
            }

            case MotionEventCompat.ACTION_POINTER_UP: {
                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);
                clearMotionHistory(pointerId);
                break;
            }

            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL: {
                cancel();
                break;
            }
        }

        return mDragState == STATE_DRAGGING;
    }

    /**
     * 加工从父view中获取的触摸事件.这个方法将分发callback回调事件.父view的触摸事件实现中应该调用该方法.
     * Process a touch event received by the parent view. This method will dispatch callback events
     * as needed before returning. The parent view's onTouchEvent implementation should call this.
     *
     * @param ev The touch event received by the parent view
     */
    public void processTouchEvent(MotionEvent ev) {
        final int action = MotionEventCompat.getActionMasked(ev);
        final int actionIndex = MotionEventCompat.getActionIndex(ev);

        if (action == MotionEvent.ACTION_DOWN) {
            // Reset things for a new event stream, just in case we didn't get
            // the whole previous stream.
            cancel();
        }

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        switch (action) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();
                final int pointerId = MotionEventCompat.getPointerId(ev, 0);
                final View toCapture = findTopChildUnder((int) x, (int) y);

                saveInitialMotion(x, y, pointerId);

                // Since the parent is already directly processing this touch event,
                // there is no reason to delay for a slop before dragging.
                // Start immediately if possible.
                tryCaptureViewForDrag(toCapture, pointerId);

                final int edgesTouched = mInitialEdgesTouched[pointerId];
                if ((edgesTouched & mTrackingEdges) != 0) {
                    mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);
                }
                break;
            }

            case MotionEventCompat.ACTION_POINTER_DOWN: {
                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);
                final float x = MotionEventCompat.getX(ev, actionIndex);
                final float y = MotionEventCompat.getY(ev, actionIndex);

                saveInitialMotion(x, y, pointerId);

                // A ViewDragHelper can only manipulate one view at a time.
                if (mDragState == STATE_IDLE) {
                    // If we're idle we can do anything! Treat it like a normal down event.

                    final View toCapture = findTopChildUnder((int) x, (int) y);
                    tryCaptureViewForDrag(toCapture, pointerId);

                    final int edgesTouched = mInitialEdgesTouched[pointerId];
                    if ((edgesTouched & mTrackingEdges) != 0) {
                        mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);
                    }
                } else if (isCapturedViewUnder((int) x, (int) y)) {
                    // We're still tracking a captured view. If the same view is under this
                    // point, we'll swap to controlling it with this pointer instead.
                    // (This will still work if we're "catching" a settling view.)

                    tryCaptureViewForDrag(mCapturedView, pointerId);
                }
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                if (mDragState == STATE_DRAGGING) {
                    final int index = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
                    final float x = MotionEventCompat.getX(ev, index);
                    final float y = MotionEventCompat.getY(ev, index);
                    final int idx = (int) (x - mLastMotionX[mActivePointerId]);
                    final int idy = (int) (y - mLastMotionY[mActivePointerId]);

                    dragTo(mCapturedView.getLeft() + idx, mCapturedView.getTop() + idy, idx, idy);

                    saveLastMotion(ev);
                } else {
                    // Check to see if any pointer is now over a draggable view.
                    final int pointerCount = MotionEventCompat.getPointerCount(ev);
                    for (int i = 0; i < pointerCount; i++) {
                        final int pointerId = MotionEventCompat.getPointerId(ev, i);
                        final float x = MotionEventCompat.getX(ev, i);
                        final float y = MotionEventCompat.getY(ev, i);
                        final float dx = x - mInitialMotionX[pointerId];
                        final float dy = y - mInitialMotionY[pointerId];

                        reportNewEdgeDrags(dx, dy, pointerId);
                        if (mDragState == STATE_DRAGGING) {
                            // Callback might have started an edge drag.
                            break;
                        }

                        final View toCapture = findTopChildUnder((int) x, (int) y);
                        if (checkTouchSlop(toCapture, dx, dy) &&
                                tryCaptureViewForDrag(toCapture, pointerId)) {
                            break;
                        }
                    }
                    saveLastMotion(ev);
                }
                break;
            }

            case MotionEventCompat.ACTION_POINTER_UP: {
                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);
                if (mDragState == STATE_DRAGGING && pointerId == mActivePointerId) {
                    // Try to find another pointer that's still holding on to the captured view.
                    int newActivePointer = INVALID_POINTER;
                    final int pointerCount = MotionEventCompat.getPointerCount(ev);
                    for (int i = 0; i < pointerCount; i++) {
                        final int id = MotionEventCompat.getPointerId(ev, i);