2015-07-28 17:29:19
这一篇主要看看布局过程
一、布局过程肯定要不可避免的涉及到layout()和onLayout()方法，这两个方法都是定义在View.java中，源码如下：
1 /** 2 * Assign a size and position to a view and all of its 3 * descendants 4 * 5 * <p>This is the second phase of the layout mechanism. 6 * (The first is measuring). In this phase, each parent calls 7 * layout on all of its children to position them. 8 * This is typically done using the child measurements 9 * that were stored in the measure pass().</p> 10 * 11 * <p>Derived classes should not override this method. 12 * Derived classes with children should override 13 * onLayout. In that method, they should 14 * call layout on each of their children.</p> 15 * 16 * @param l Left position, relative to parent 17 * @param t Top position, relative to parent 18 * @param r Right position, relative to parent 19 * @param b Bottom position, relative to parent 20 */ 21 @SuppressWarnings({"unchecked"}) 22 public void layout(int l, int t, int r, int b) { 23 if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) { 24 onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec); 25 mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; 26 } 27 28 int oldL = mLeft; 29 int oldT = mTop; 30 int oldB = mBottom; 31 int oldR = mRight; 32 33 boolean changed = isLayoutModeOptical(mParent) ? 34 setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); 35 if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) { 36 /// M: Monitor onLayout time if longer than 3s print log. 37 long logTime = System.currentTimeMillis(); 38 onLayout(changed, l, t, r, b); 39 long nowTime = System.currentTimeMillis(); 40 if (nowTime - logTime > DBG_TIMEOUT_VALUE) { 41 Xlog.d(VIEW_LOG_TAG, "[ANR Warning]onLayout time too long, this =" + this + "time =" + (nowTime - logTime)); 42 } 43 mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED; 44 45 ListenerInfo li = mListenerInfo; 46 if (li != null && li.mOnLayoutChangeListeners != null) { 47 ArrayList<OnLayoutChangeListener> listenersCopy = 48 (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone(); 49 int numListeners = listenersCopy.size(); 50 for (int i = 0; i < numListeners; ++i) { 51 listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB); 52 } 53 } 54 } 55 56 mPrivateFlags &= ~PFLAG_FORCE_LAYOUT; 57 mPrivateFlags3 |= PFLAG3_IS_LAID_OUT; 58 } 59 60 /** 61 * Called from layout when this view should 62 * assign a size and position to each of its children. 63 * 64 * Derived classes with children should override 65 * this method and call layout on each of 66 * their children. 67 * @param changed This is a new size or position for this view 68 * @param left Left position, relative to parent 69 * @param top Top position, relative to parent 70 * @param right Right position, relative to parent 71 * @param bottom Bottom position, relative to parent 72 */ 73 protected void onLayout(boolean changed, int left, int top, int right, int bottom) { 74 }
简单的翻译一下layout()方法的那段注释哈～E文不好～
“指定一个view以及它的所有子孙节点的大小和位置，这是布局机制的第二阶段(第一阶段是测量)，在这一阶段，父view调用所有子view的layout()方法以确定他们所在的位置，通常是使用子View存储的自身的尺寸。派生类不应该重写此方法，应该重写onLayout()方法，在派生类重写的onLayout()方法中，应该调用每一个子View的layout方法。”啰嗦一句，int l, int t, int r, int b都是相对于父节点的坐标值。
注意layout方法中的红色代码，调用了onLayout。而onLayout在view中实现为空。现在来看看ViewGroup中的这两个方法。
1 /** 2 * {@inheritDoc} 3 */ 4 @Override 5 public final void layout(int l, int t, int r, int b) { 6 if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) { 7 if (mTransition != null) { 8 mTransition.layoutChange(this); 9 } 10 super.layout(l, t, r, b); 11 } else { 12 // record the fact that we noop'd it; request layout when transition finishes 13 mLayoutCalledWhileSuppressed = true; 14 } 15 } 16 17 /** 18 * {@inheritDoc} 19 */ 20 @Override 21 protected abstract void onLayout(boolean changed, 22 int l, int t, int r, int b);
在ViewGroup的layout方法中，mSuppressLayout用来控制是否禁止调用layout()，该值由如下方法来控制：
1 /** 2 * Tells this ViewGroup to suppress all layout() calls until layout 3 * suppression is disabled with a later call to suppressLayout(false). 4 * When layout suppression is disabled, a requestLayout() call is sent 5 * if layout() was attempted while layout was being suppressed. 6 * 7 * @hide 8 */ 9 public void suppressLayout(boolean suppress) { 10 mSuppressLayout = suppress; 11 if (!suppress) { 12 if (mLayoutCalledWhileSuppressed) { 13 requestLayout(); 14 mLayoutCalledWhileSuppressed = false; 15 } 16 } 17 }
这个方法不是对外公开的，所以不了解它也行。可以简单地理解ViewGroup的layout方法，它直接调用了父类View的layout()方法即可。至于onLayout方法，竟然被搞成了abstract的，这是逼着ViewGroup的子类必须得去实现啊～当然了，你必须得实现啊，你作为一个容器类，如何摆放你的子孙控件，是你义不容辞的责任啊。
至此我们已经明白了几点：
1. 派生类不需要重写layout()，而应该重写onLayout()方法，因为在layout()方法中就调用了onLayout()。
2. 在重写onLayout()方法时，我们需要显式的调用每一个childView的layout方法，把它摆放在合适的位置上。前提是在调用之前，得先计算好该childView的坐标。
3. 如果直接继承自View,那么可以不用重写onLayout()方法，比如ImageView、ImageButton等都没有重写该方法，所以不重写这个方法对于自定义View影响不大，至于TextView比较特殊，它重写了该方法，如下：
1 @Override 2 protected void onLayout(boolean changed, int left, int top, int right, int bottom) { 3 super.onLayout(changed, left, top, right, bottom); 4 if (mDeferScroll >= 0) { 5 int curs = mDeferScroll; 6 mDeferScroll = -1; 7 bringPointIntoView(Math.min(curs, mText.length())); 8 } 9 } 10 11 /** 12 * Move the point, specified by the offset, into the view if it is needed. 13 * This has to be called after layout. Returns true if anything changed. 14 */ 15 public boolean bringPointIntoView(int offset) { 16 if (isLayoutRequested()) { 17 mDeferScroll = offset; 18 return false; 19 } 20 boolean changed = false; 21 22 Layout layout = isShowingHint() ? mHintLayout: mLayout; 23 24 if (layout == null) return changed; 25 26 int line = layout.getLineForOffset(offset); 27 28 int grav; 29 30 switch (layout.getParagraphAlignment(line)) { 31 case ALIGN_LEFT: 32 grav = 1; 33 break; 34 case ALIGN_RIGHT: 35 grav = -1; 36 break; 37 case ALIGN_NORMAL: 38 grav = layout.getParagraphDirection(line); 39 break; 40 case ALIGN_OPPOSITE: 41 grav = -layout.getParagraphDirection(line); 42 break; 43 case ALIGN_CENTER: 44 default: 45 grav = 0; 46 break; 47 } 48 49 // We only want to clamp the cursor to fit within the layout width 50 // in left-to-right modes, because in a right to left alignment, 51 // we want to scroll to keep the line-right on the screen, as other 52 // lines are likely to have text flush with the right margin, which 53 // we want to keep visible. 54 // A better long-term solution would probably be to measure both 55 // the full line and a blank-trimmed version, and, for example, use 56 // the latter measurement for centering and right alignment, but for 57 // the time being we only implement the cursor clamping in left to 58 // right where it is most likely to be annoying. 59 final boolean clamped = grav > 0; 60 // FIXME: Is it okay to truncate this, or should we round? 61 final int x = (int)layout.getPrimaryHorizontal(offset, clamped); 62 final int top = layout.getLineTop(line); 63 final int bottom = layout.getLineTop(line + 1); 64 65 int left = (int) FloatMath.floor(layout.getLineLeft(line)); 66 int right = (int) FloatMath.ceil(layout.getLineRight(line)); 67 int ht = layout.getHeight(); 68 69 int hspace = mRight - mLeft - getCompoundPaddingLeft() - getCompoundPaddingRight(); 70 int vspace = mBottom - mTop - getExtendedPaddingTop() - getExtendedPaddingBottom(); 71 if (!mHorizontallyScrolling && right - left > hspace && right > x) { 72 // If cursor has been clamped, make sure we don't scroll. 73 right = Math.max(x, left + hspace); 74 } 75 76 int hslack = (bottom - top) / 2; 77 int vslack = hslack; 78 79 if (vslack > vspace / 4) 80 vslack = vspace / 4; 81 if (hslack > hspace / 4) 82 hslack = hspace / 4; 83 84 int hs = mScrollX; 85 int vs = mScrollY; 86 87 if (top - vs < vslack) 88 vs = top - vslack; 89 if (bottom - vs > vspace - vslack) 90 vs = bottom - (vspace - vslack); 91 if (ht - vs < vspace) 92 vs = ht - vspace; 93 if (0 - vs > 0) 94 vs = 0; 95 96 if (grav != 0) { 97 if (x - hs < hslack) { 98 hs = x - hslack; 99 } 100 if (x - hs > hspace - hslack) { 101 hs = x - (hspace - hslack); 102 } 103 } 104 105 if (grav < 0) { 106 if (left - hs > 0) 107 hs = left; 108 if (right - hs < hspace) 109 hs = right - hspace; 110 } else if (grav > 0) { 111 if (right - hs < hspace) 112 hs = right - hspace; 113 if (left - hs > 0) 114 hs = left; 115 } else /* grav == 0 */ { 116 if (right - left <= hspace) { 117 /* 118 * If the entire text fits, center it exactly. 119 */ 120 hs = left - (hspace - (right - left)) / 2; 121 } else if (x > right - hslack) { 122 /* 123 * If we are near the right edge, keep the right edge 124 * at the edge of the view. 125 */ 126 hs = right - hspace; 127 } else if (x < left + hslack) { 128 /* 129 * If we are near the left edge, keep the left edge 130 * at the edge of the view. 131 */ 132 hs = left; 133 } else if (left > hs) { 134 /* 135 * Is there whitespace visible at the left? Fix it if so. 136 */ 137 hs = left; 138 } else if (right < hs + hspace) { 139 /* 140 * Is there whitespace visible at the right? Fix it if so. 141 */ 142 hs = right - hspace; 143 } else { 144 /* 145 * Otherwise, float as needed. 146 */ 147 if (x - hs < hslack) { 148 hs = x - hslack; 149 } 150 if (x - hs > hspace - hslack) { 151 hs = x - (hspace - hslack); 152 } 153 } 154 } 155 156 if (hs != mScrollX || vs != mScrollY) { 157 if (mScroller == null) { 158 scrollTo(hs, vs); 159 } else { 160 long duration = AnimationUtils.currentAnimationTimeMillis() - mLastScroll; 161 int dx = hs - mScrollX; 162 int dy = vs - mScrollY; 163 164 if (duration > ANIMATED_SCROLL_GAP) { 165 mScroller.startScroll(mScrollX, mScrollY, dx, dy); 166 awakenScrollBars(mScroller.getDuration()); 167 invalidate(); 168 } else { 169 if (!mScroller.isFinished()) { 170 mScroller.abortAnimation(); 171 } 172 173 scrollBy(dx, dy); 174 } 175 176 mLastScroll = AnimationUtils.currentAnimationTimeMillis(); 177 } 178 179 changed = true; 180 } 181 182 if (isFocused()) { 183 // This offsets because getInterestingRect() is in terms of viewport coordinates, but 184 // requestRectangleOnScreen() is in terms of content coordinates. 185 186 // The offsets here are to ensure the rectangle we are using is 187 // within our view bounds, in case the cursor is on the far left 188 // or right. If it isn't withing the bounds, then this request 189 // will be ignored. 190 if (mTempRect == null) mTempRect = new Rect(); 191 mTempRect.set(x - 2, top, x + 2, bottom); 192 getInterestingRect(mTempRect, line); 193 ///M: ALPS00605613 requestRectangleOnScreen() will return error result if setting the mTempRect to mScrollX, mScrollY 194 //mTempRect.offset(mScrollX, mScrollY); 195 196 if (requestRectangleOnScreen(mTempRect)) { 197 changed = true; 198 } 199 } 200 201 return changed; 202 }
有点没看明白bringPointIntoView方法的作用是什么，还得再研究。不过这不影响我们分析布局过程。
二、结合自己写的一个小Demo继续分析，代码如下：
MyLinear2.java
1 public class MyLinear2 extends ViewGroup { 2 private static final String TAG = "David_MyLinear2"; 3 4 public MyLinear2(Context context) { 5 super(context); 6 } 7 8 public MyLinear2(Context context, AttributeSet attrs) { 9 super(context, attrs); 10 } 11 12 @Override 13 protected void onLayout(boolean changed, int l, int t, int r, int b) { 14 int count = getChildCount(); 15 int cWidth = 0; 16 int cHeight = 0; 17 int top = 0; 18 MarginLayoutParams params = null; 19 Log.e(TAG + " onLayout", "l = " + l); 20 Log.e(TAG + " onLayout", "t = " + t); 21 Log.e(TAG + " onLayout", "r = " + r); 22 Log.e(TAG + " onLayout", "b = " + b); 23 24 for (int i = 0; i < count; i++) { 25 Log.e(TAG + " onLayout", "====================i = " + i); 26 View childView = getChildAt(i); 27 cWidth = childView.getMeasuredWidth(); 28 cHeight = childView.getMeasuredHeight(); 29 params = (MarginLayoutParams) childView.getLayoutParams(); 30 31 Log.e(TAG + " onLayout", "params.height = " + params.height); 32 Log.e(TAG + " onLayout", "params.width = " + params.width); 33 int leftMargin = params.leftMargin; 34 Log.e(TAG + " onLayout", "leftMargin = " + leftMargin); 35 Log.e(TAG + " onLayout", "params.rightMargin = " + params.rightMargin); 36 /*Log.e(TAG + " onLayout", "cWidth = " + cWidth); 37 Log.e(TAG + " onLayout", "cHeight = " + cHeight); 38 Log.e(TAG + " onLayout", "getWidth() = " + childView.getWidth()); 39 Log.e(TAG + " onLayout", "getHeight() = " + childView.getHeight());*/ 40 int cl = 0, ct = 0, cr = 0, cb = 0; 41 cl = leftMargin; 42 cr = cl + cWidth; 43 ct = top; 44 cb = cHeight + ct; 45 /*Log.e(TAG + " onLayout", "cr = " + cr); 46 Log.e(TAG + " onLayout", "ct = " + ct); 47 Log.e(TAG + " onLayout", "cb = " + cb); 48 Log.e(TAG + " onLayout", "top = " + top);*/ 49 childView.layout(cl, ct, cr, cb); 50 top += cHeight; 51 } 52 } 53 54 @Override 55 public ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) { 56 Log.e(TAG, "generateLayoutParams attrs"); 57 return new MarginLayoutParams(getContext(), attrs); 58 } 59 60 @Override 61 protected ViewGroup.LayoutParams generateDefaultLayoutParams() { 62 Log.e(TAG, "generateDefaultLayoutParams"); 63 return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT); 64 } 65 66 @Override 67 protected boolean checkLayoutParams(LayoutParams p) { 68 return super.checkLayoutParams(p); 69 } 70 71 @Override 72 protected ViewGroup.LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) { 73 Log.e(TAG, "generateLayoutParams p"); 74 return new MarginLayoutParams(p); 75 } 76 77 @Override 78 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { 79 int measuredHeight = measureHeight(heightMeasureSpec); 80 int measuredWidth = measureWidth(widthMeasureSpec); 81 Log.e(TAG, "onMeasure measuredHeight = " + measuredHeight); 82 Log.e(TAG, "onMeasure measuredWidth = " + measuredWidth); 83 setMeasuredDimension(measuredWidth, measuredHeight); 84 measureChildren(widthMeasureSpec, heightMeasureSpec); 85 } 86 87 private int measureHeight(int measureSpec) { 88 int specMode = MeasureSpec.getMode(measureSpec); 89 int specSize = MeasureSpec.getSize(measureSpec); 90 91 int result = 500; 92 if (specMode == MeasureSpec.AT_MOST){ 93 result = specSize; 94 } else if (specMode == MeasureSpec.EXACTLY){ 95 result = specSize; 96 } 97 return result; 98 } 99 100 private int measureWidth(int measureSpec) { 101 int specMode = MeasureSpec.getMode(measureSpec); 102 int specSize = MeasureSpec.getSize(measureSpec); 103 104 int result = 100; 105 if (specMode == MeasureSpec.AT_MOST){ 106 result = specSize; 107 } else if (specMode == MeasureSpec.EXACTLY){ 108 result = specSize; 109 } 110 return result; 111 } 112 }
MyLinear2时模仿纵向布局的LinearLayout。
MyTextView.java
1 public class MyTextView extends View { 2 private static final String TAG = "David___MyTextView"; 3 4 public MyTextView(Context context, AttributeSet attrs) { 5 super(context, attrs); 6 } 7 8 public MyTextView(Context context) { 9 super(context); 10 } 11 12 @Override 13 protected void onDraw(Canvas canvas) { 14 super.onDraw(canvas); 15 Paint paint = new Paint(); 16 paint.setTextSize(22); 17 paint.setTextAlign(Align.CENTER); 18 Log.e(TAG, "onDraw getTop() = " + getTop()); 19 Log.e(TAG, "onDraw getLeft() = " + getLeft()); 20 canvas.drawText("nihao ----", getTop(), getLeft(), paint); 21 } 22 23 @Override 24 protected void onLayout(boolean changed, int left, int top, int right, int bottom) { 25 Log.e(TAG, "onLayout left = " + left); 26 Log.e(TAG, "onLayout top = " + top); 27 Log.e(TAG, "onLayout right = " + right); 28 Log.e(TAG, "onLayout bottom = " + bottom); 29 //super.onLayout(changed, left, top, right, bottom); 30 } 31 32 @Override 33 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { 34 int measuredHeight = measureHeight(heightMeasureSpec); 35 int measuredWidth = measureWidth(widthMeasureSpec); 36 Log.e(TAG, "onMeasure measuredHeight = " + measuredHeight); 37 Log.e(TAG, "onMeasure measuredWidth = " + measuredWidth); 38 setMeasuredDimension(measuredWidth, measuredHeight); 39 } 40 41 private int measureHeight(int measureSpec) { 42 int specMode = MeasureSpec.getMode(measureSpec); 43 int specSize = MeasureSpec.getSize(measureSpec); 44 45 int result = 500; 46 if (specMode == MeasureSpec.AT_MOST){ 47 result = specSize; 48 } else if (specMode == MeasureSpec.EXACTLY){ 49 result = specSize; 50 } 51 return result; 52 } 53 54 private int measureWidth(int measureSpec) { 55 int specMode = MeasureSpec.getMode(measureSpec); 56 int specSize = MeasureSpec.getSize(measureSpec); 57 58 int result = 500; 59 if (specMode == MeasureSpec.AT_MOST){ 60 result = specSize; 61 } else if (specMode == MeasureSpec.EXACTLY){ 62 result = specSize; 63 } 64 return result; 65 } 66 }
这是一个简易的View,先不要关心onDraw()方法。布局文件如下：
1 <com.test.touch.MyLinear2 xmlns:android="http://schemas.android.com/apk/res/android" 2 android:layout_width="300dp" 3 android:layout_height="200dp" 4 android:paddingTop="22dp" 5 android:layout_marginLeft="10dp" > 6 7 <com.test.touch.MyTextView 8 android:layout_width="match_parent" 9 android:layout_height="120dp" 10 android:background="#0f0" 11 android:layout_marginLeft="30dp" 12 android:paddingTop="22dp" 13 android:text="Hello World" /> 14 15 <TextView 16 android:id="@+id/tv1" 17 android:layout_width="match_parent" 18 android:layout_height="110dp" 19 android:background="#f00" 20 android:gravity="center_vertical" 21 android:layout_marginLeft="10dp" 22 android:text="Hello World Text" /> 23 24 25 <com.test.touch.MyTextView 26 android:layout_width="match_parent" 27 android:layout_height="60dp" 28 android:background="#00f" 29 android:text="Hello World88" /> 30 31 </com.test.touch.MyLinear2>
OK,运行打印的日志如下：
这段日志其实是会被打印好几遍的，我只截取了完整的一次日志。从日志可以看出，首先执行的是测量过程，这个之前分析过了，请参考http://www.cnblogs.com/wlrhnh/p/4680636.html。然后才是布局过程，由于我们在MyLinear2的onLayout()中显式调用childView的layout()方法，代码如下：
1 @Override 2 protected void onLayout(boolean changed, int l, int t, int r, int b) { 3 int count = getChildCount(); 4 int cWidth = 0; 5 int cHeight = 0; 6 int top = 0; 7 MarginLayoutParams params = null; 8 Log.e(TAG + " onLayout", "l = " + l); 9 Log.e(TAG + " onLayout", "t = " + t); 10 Log.e(TAG + " onLayout", "r = " + r); 11 Log.e(TAG + " onLayout", "b = " + b); 12 13 for (int i = 0; i < count; i++) { 14 Log.e(TAG + " onLayout", "====================i = " + i); 15 View childView = getChildAt(i); 16 cWidth = childView.getMeasuredWidth(); 17 cHeight = childView.getMeasuredHeight(); 18 params = (MarginLayoutParams) childView.getLayoutParams(); 19 20 Log.e(TAG + " onLayout", "params.height = " + params.height); 21 Log.e(TAG + " onLayout", "params.width = " + params.width); 22 int leftMargin = params.leftMargin; 23 Log.e(TAG + " onLayout", "leftMargin = " + leftMargin); 24 Log.e(TAG + " onLayout", "params.rightMargin = " + params.rightMargin); 25 int cl = 0, ct = 0, cr = 0, cb = 0; 26 cl = leftMargin; 27 cr = cl + cWidth; 28 ct = top; 29 cb = cHeight + ct; 30 childView.layout(cl, ct, cr, cb); 31 top += cHeight; 32 } 33 }
所以日志中显示:先调用了ViewGroup的onLayout,然后遍历每一个childView,取出它们已经计算好的坐标值，按照ViewGroup的既定布局策略，给childView布局，调用他们的layout()方法，进而调用了onLayout()。而很多继承自View的组件，比如ImageView等，由于没有实现onLayout()方法，那么其实只调用了View的layout()和View的空实现的onLayout()方法。当然了，我在MyTextView中重写onLayout()只是为了打印日志而已，并没有做什么实际操作。在日志的最后，打印了onDraw()方法，可见，先测量、后布局、最后才统一draw，并不是布局完一个就draw一个。而且我发现一个有意思的现象，那就是如果你的父控件拥有的space不足以显示所有的子View，那么不能显示出来的子View的onDraw方法是不会被调用的，这点好理解～
三、关于LayoutParams
注意上面的一行红色代码，每个childView都有LayoutParams,而且可以强转为MarginLayoutParams呢？注意看这几行代码：
1 @Override 2 public ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) { 3 Log.e(TAG, "generateLayoutParams attrs"); 4 return new MarginLayoutParams(getContext(), attrs); 5 } 6 7 @Override 8 protected ViewGroup.LayoutParams generateDefaultLayoutParams() { 9 Log.e(TAG, "generateDefaultLayoutParams"); 10 return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT); 11 } 12 13 @Override 14 protected boolean checkLayoutParams(LayoutParams p) { 15 return super.checkLayoutParams(p); 16 } 17 18 @Override 19 protected ViewGroup.LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) { 20 Log.e(TAG, "generateLayoutParams p"); 21 return new MarginLayoutParams(p); 22 }
看一下日志：
看黄色部分日志，主要执行了generateLayoutParams(AttributeSet attrs)方法，而且是先于测量过程执行的，这个方法我们有自己的实现，具体的请参考http://www.cnblogs.com/wlrhnh/p/4683542.html
简单的总结一下布局过程：
Android View绘制系统首先取到Activity布局的根View，当然这一般是一个ViewGroup了。先测量，后布局。布局的时候调用ViewGroup子类比如MyLinear2的layout()方法，由于ViewGroup强制子类实现onLayout()方法，所以会调到MyLinear2的onLayout()方法，在这个方法中，需要遍历子View，按照布局策略，计算每一个子View的坐标，然后将它放在合适的位置上。至于继承自View的子类，则不需要实现onLayout，毕竟onLayout的作是布局，这是容器类该干的事情。而子View要做的就是根据父View在指定给自己的空间中draw。
通过以上三篇文章，想必应该对Android系统测量、布局View的流程有了一个大概的了解，这是一条主线。当然了，这里面涉及到的细节其实有很多，最好的办法是自己去看源码喽～
项目代码和View.java ViewGroup.java LinearLayout.java Button.java等源码请下载
后记：分析完三步骤之后觉得不过瘾，想起网上有人写过这样的布局，就顺手写了个，抄袭人家的创意了，但是代码是自己的写的～
1 public class MyLinear1 extends ViewGroup { 2 private static final String TAG = "David_MyLinear1"; 3 4 public MyLinear1(Context context) { 5 super(context); 6 } 7 8 public MyLinear1(Context context, AttributeSet attrs) { 9 super(context, attrs); 10 } 11 12 @Override 13 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { 14 int measuredHeightSize = MeasureSpec.getSize(heightMeasureSpec); 15 int measuredWidthSize = MeasureSpec.getSize(widthMeasureSpec); 16 17 int measuredHeightMode = MeasureSpec.getMode(heightMeasureSpec); 18 int measuredWidthMode = MeasureSpec.getMode(widthMeasureSpec); 19 20 measureChildren(widthMeasureSpec, heightMeasureSpec); 21 22 int lHeight = 0, rHeight = 0; 23 int tWidth = 0, bWidth = 0; 24 25 View childView = null; 26 MarginLayoutParams params = null; 27 for (int i = 0; i < getChildCount(); i++) { 28 childView = getChildAt(i); 29 params = (MarginLayoutParams) childView.getLayoutParams(); 30 31 switch (i) { 32 case 0: 33 lHeight += childView.getMeasuredHeight(); 34 tWidth += childView.getMeasuredWidth(); 35 break; 36 case 1: 37 rHeight += childView.getMeasuredHeight(); 38 tWidth += childView.getMeasuredWidth(); 39 break; 40 case 2: 41 lHeight += childView.getMeasuredHeight(); 42 bWidth += childView.getMeasuredWidth(); 43 break; 44 case 3: 45 rHeight += childView.getMeasuredHeight(); 46 bWidth += childView.getMeasuredWidth(); 47 break; 48 49 default: 50 break; 51 } 52 } 53 54 int realHeightSize = Math.max(lHeight, rHeight); 55 int realWidthSize = Math.max(bWidth, tWidth); 56 int h = measuredHeightMode == MeasureSpec.EXACTLY ? measuredHeightSize : realHeightSize; 57 int w = measuredWidthMode == MeasureSpec.EXACTLY ? measuredWidthSize : realWidthSize; 58 setMeasuredDimension(w, h); 59 } 60 61 @Override 62 protected void onLayout(boolean changed, int l, int t, int r, int b) { 63 int realLeft = 0; 64 int realTop = 0; 65 int realRight = 0; 66 int realBottom = 0; 67 int count = getChildCount(); 68 69 View childView = null; 70 MarginLayoutParams params = null; 71 for (int i = 0; i < count; i++) { 72 Log.e(TAG, "------------------------- i = " + i); 73 childView = getChildAt(i); 74 params = (MarginLayoutParams) childView.getLayoutParams(); 75 Log.d(TAG, "childView.getMeasuredWidth() = " + childView.getMeasuredWidth()); 76 Log.d(TAG, "childView.getMeasuredHeight() = " + childView.getMeasuredHeight()); 77 switch (i) { 78 case 0: 79 realLeft = l; 80 realTop = t; 81 realRight = realLeft + childView.getMeasuredWidth(); 82 realBottom = realTop + childView.getMeasuredHeight(); 83 break; 84 case 1: 85 realLeft = r - childView.getMeasuredWidth(); 86 realTop = t; 87 realRight = realLeft + childView.getMeasuredWidth(); 88 realBottom = realTop + childView.getMeasuredHeight(); 89 break; 90 case 2: 91 realLeft = l; 92 realTop = b - childView.getMeasuredHeight(); 93 realRight = realLeft + childView.getMeasuredWidth(); 94 realBottom = realTop + childView.getMeasuredHeight(); 95 break; 96 case 3: 97 realLeft = r - childView.getMeasuredWidth(); 98 realTop = b - childView.getMeasuredHeight(); 99 realRight = realLeft + childView.getMeasuredWidth(); 100 realBottom = realTop + childView.getMeasuredHeight(); 101 break; 102 103 default: 104 break; 105 } 106 childView.layout(realLeft, realTop, realRight, realBottom); 107 } 108 } 109 110 @Override 111 public ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) { 112 return new MarginLayoutParams(getContext(), attrs); 113 } 114 }
代码随手写的，有些问题考虑的还不够周全，各位自己完善吧～而且没有做注释，但是相信看完这三篇文章，不需要看注释了吧～
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