Adapter.notifyDataSetChanged()源码分析以及与ListView.setAdapter的区别

一直很好奇，notifyDataSetChanged究竟是重绘了整个ListView还是只重绘了被修改的那些Item，它与重新设置适配器即调用setAdapter的区别在哪里？所以特地追踪了一下源码，过程如下：

一、notifyDataSetChanged实现机制

自定义Activity中有如下调用语句：

checkoutAdapter.notifyDataSetChanged();

点击notifyDataSetChanged()进行代码跟踪。首先，进入到BaseAdapter的notifyDataSetChanged方法：

public void notifyDataSetChanged() {
        mDataSetObservable.notifyChanged();
    }

我们发现其实就是DataSetObservable这个对象在发生作用，点击notifyChanged进行追踪。

public class DataSetObservable extends Observable<DataSetObserver> {
    /**
     * Invokes onChanged on each observer. Called when the data set being observed has
     * changed, and which when read contains the new state of the data.
     */
    public void notifyChanged() {
        synchronized(mObservers) {
            // since onChanged() is implemented by the app, it could do anything, including
            // removing itself from {@link mObservers} - and that could cause problems if
            // an iterator is used on the ArrayList {@link mObservers}.
            // to avoid such problems, just march thru the list in the reverse order.
            for (int i = mObservers.size() - 1; i >= 0; i--) {
                mObservers.get(i).onChanged();
            }
        }
    }

继续跟踪onChanged()，我们发现DataSetObserver 是个抽象类，其派生类实例对象是在哪里指定的呢？根据经验，我们需要回溯至adapter的构造过程。

public abstract class DataSetObserver {
    /**
     * This method is called when the entire data set has changed,
     * most likely through a call to {@link Cursor#requery()} on a {@link Cursor}.
     */
    public void onChanged() {
        // Do nothing
    }

    /**
     * This method is called when the entire data becomes invalid,
     * most likely through a call to {@link Cursor#deactivate()} or {@link Cursor#close()} on a
     * {@link Cursor}.
     */
    public void onInvalidated() {
        // Do nothing
    }
}

先看adapter的构造函数

        CheckOut_DishListViewAdapter checkoutAdapter;
// 绑定适配器
        checkoutAdapter = new CheckOut_DishListViewAdapter(
                CheckOutActivity.this, list_dish);
        list_view_dish.setAdapter(checkoutAdapter);

public class CheckOut_DishListViewAdapter extends BaseAdapter {
    private DecimalFormat df = new DecimalFormat("######0.00");// 用于double保留两位小数
    private LayoutInflater mInflater;
    private List<HashMap<String, Object>> list;

    public CheckOut_DishListViewAdapter(Context con,
            List<HashMap<String, Object>> list) {
        mInflater = LayoutInflater.from(con);
        this.list = list;
    }

显然没有DataSetObserver的有关信息。

再看ListView中的setAdapter方法，我们省略其他代码，只看与DataSetObserver相关的部分，从mDataSetObserver = new AdapterDataSetObserver();可知，AdapterDataSetObserver是DataSetObserver的实例化类。

@Override
    public void setAdapter(ListAdapter adapter) {
        ...
        if (mAdapter != null) {
            ...27 
            mDataSetObserver = new AdapterDataSetObserver();
            mAdapter.registerDataSetObserver(mDataSetObserver);

            ...46         } else {
            ...51         }

        requestLayout();
    }

查看AdapterDataSetObserver的onChanged方法：

class AdapterDataSetObserver extends DataSetObserver
  {
    private Parcelable mInstanceState = null;

    AdapterDataSetObserver() {
    }
    public void onChanged() { 
      mDataChanged = true;
      mOldItemCount = mItemCount;
      mItemCount = getAdapter().getCount();

      if ((getAdapter().hasStableIds()) && (mInstanceState != null) && (mOldItemCount == 0) && (mItemCount > 0))
      {
        onRestoreInstanceState(mInstanceState);
        mInstanceState = null;
      } else {
        rememberSyncState();
      }
      checkFocus();
      requestLayout();
    }
    //...省略不必要代码
}

在第20行，我们看见了requestLayout()，它就是用来重绘界面的，点击追踪requestLayout时，无法继续追踪，这时通过查找系统源码，我们发现AdapterDataSetObserver原来是抽象类AdapterView的内部类

public abstract class AdapterView<T extends Adapter> extends ViewGroup {
    ...
}

    class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;

        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = getAdapter().getCount();

            // Detect the case where a cursor that was previously invalidated has
            // been repopulated with new data.
            if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
                    && mOldItemCount == 0 && mItemCount > 0) {
                AdapterView.this.onRestoreInstanceState(mInstanceState);
                mInstanceState = null;
            } else {
                rememberSyncState();
            }
            checkFocus();
            requestLayout();
        }

        @Override
        public void onInvalidated() {
            mDataChanged = true;

            if (AdapterView.this.getAdapter().hasStableIds()) {
                // Remember the current state for the case where our hosting activity is being
                // stopped and later restarted
                mInstanceState = AdapterView.this.onSaveInstanceState();
            }

            // Data is invalid so we should reset our state
            mOldItemCount = mItemCount;
            mItemCount = 0;
            mSelectedPosition = INVALID_POSITION;
            mSelectedRowId = INVALID_ROW_ID;
            mNextSelectedPosition = INVALID_POSITION;
            mNextSelectedRowId = INVALID_ROW_ID;
            mNeedSync = false;

            checkFocus();
            requestLayout();
        }

        public void clearSavedState() {
            mInstanceState = null;
        }
    }

在21行，我们又看见了requestLayout()，Ctrl+单击该方法，进入到View类的同名方法

    /**
     * Call this when something has changed which has invalidated the
     * layout of this view. This will schedule a layout pass of the view
     * tree.
     */
    public void requestLayout() {
        if (ViewDebug.TRACE_HIERARCHY) {
            ViewDebug.trace(this, ViewDebug.HierarchyTraceType.REQUEST_LAYOUT);
        }

        mPrivateFlags |= FORCE_LAYOUT;
        mPrivateFlags |= INVALIDATED;

        if (mParent != null) {
            if (mLayoutParams != null) {
                mLayoutParams.resolveWithDirection(getResolvedLayoutDirection());
            }
            if (!mParent.isLayoutRequested()) {
                mParent.requestLayout();
            }
        }
    }

在第19行，我们发现该方法将requestLayout()任务上抛至其mParent，因此我们需要追踪mParent，先来看看谁为它赋值：

    /*
     * Caller is responsible for calling requestLayout if necessary.
     * (This allows addViewInLayout to not request a new layout.)
     */
    void assignParent(ViewParent parent) {
        if (mParent == null) {
            mParent = parent;
        } else if (parent == null) {
            mParent = null;
        } else {
            throw new RuntimeException("view " + this + " being added, but"
                    + " it already has a parent");
        }
    }

原来是assignParent，因此在构造子view的过程中，子view一定有assignParent的操作。根据View Tree的层级关系，我们可以猜测，这样一层层的上抛请求，最后应该上抛至Activity的根View，这个根View是谁？根据我们对Activity加载布局流程的理解，这个根View其实就是DecorView，那么我们先来看看DecorView中是否有requestLayout方法的具体实现。

我们知道DecorView是PhoneWindow的内部类，进入DecorView类，

private final class DecorView extends FrameLayout implements RootViewSurfaceTaker {

发现DecorView继承自FrameLayout ，也即间接继承自View，但DecorView中并未重写requestLayout方法，说明DecorView并不是requestLayout的最终执行者，DecorView存在mParent，要想弄清楚DecorView的mParent是谁，我们有必要回顾一下DecorView是如何装载到Activity的。

我们按照流程图一级一级的找，在WindowManagerImpl中找到addView方法，发现新建了一个ViewRootImpl对象，并在最后调用ViewRootImpl的setView方法，接下来我们继续跟进setView方法。

private void addView(View view, ViewGroup.LayoutParams params,
            CompatibilityInfoHolder cih, boolean nest) {
            ...
        
            ViewRootImpl root;
            ...
            
            root = new ViewRootImpl(view.getContext());
            ...
            root.setView(view, wparams, panelParentView);
    } 

在ViewRootImpl的setView方法中找到如下代码：view.assignParent(this);也即将DecorView的mParent指定为ViewRootImpl实例，并且在第6行发现调用了requestLayout方法。

     public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
         synchronized (this) {
             if (mView == null) {
                 mView = view;
                 ...
　　　　　　　　　　requestLayout();
　　　　　　　　　　...

                 view.assignParent(this);
                 ...
     } 

进入到ViewRootImpl的requestLayout方法：

    public void requestLayout() {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }

之后的流程参考从ViewRootImpl类分析View绘制的流程一文。

从以上分析可知，每一次notifyDataSetChange()都会引起界面的重绘，重绘的最终实现是在ViewRootImpl.java中。

二、notifyDataSetChanged与setAdapter区别

仔细阅读ListView的setAdapter方法，当ListView之前绑定过adapter信息时，在这里会清除原有Adapter和数据集观察者等信息，重置了ListView当前选中项等信息，并在方法的最后一句调用requestLayout进行界面的重绘。

public void setAdapter(ListAdapter adapter) {
        // 与原有观察者解绑定
        if (mAdapter != null && mDataSetObserver != null) {
            mAdapter.unregisterDataSetObserver(mDataSetObserver);
        }

        resetList();
        mRecycler.clear();

        if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
            mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter);
        } else {
            mAdapter = adapter;
        }
        
        mOldSelectedPosition = INVALID_POSITION;
        mOldSelectedRowId = INVALID_ROW_ID;

        // AbsListView#setAdapter will update choice mode states.
        super.setAdapter(adapter);

        if (mAdapter != null) {
            mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
            mOldItemCount = mItemCount;
            mItemCount = mAdapter.getCount();
            checkFocus();
            // 重新绑定新的数据集观察者
            mDataSetObserver = new AdapterDataSetObserver();
            mAdapter.registerDataSetObserver(mDataSetObserver);

            mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());

            int position;
            if (mStackFromBottom) {
                position = lookForSelectablePosition(mItemCount - 1, false);
            } else {
                position = lookForSelectablePosition(0, true);
            }
            setSelectedPositionInt(position);
            setNextSelectedPositionInt(position);

            if (mItemCount == 0) {
                // Nothing selected
                checkSelectionChanged();
            }
        } else {
            mAreAllItemsSelectable = true;
            checkFocus();
            // Nothing selected
            checkSelectionChanged();
        }
        // 重绘
        requestLayout();
    }

由此可知，调用adapter.notifyDataSetChanged与listView.setAdapter函数都会引起界面重绘，区别是前者会保留原有位置、数据信息，后者是回到初始状态。

 

 注：以上过程纯属个人探索，如有错误敬请批评指正。

参考文献：

1.从ViewRootImpl类分析View绘制的流程（http://blog.csdn.net/feiduclear_up/article/details/46772477）

2.从源代码的角度分析--在BaseAdapter调用notifyDataSetChanged()之后发生了什么（http://www.cnblogs.com/kissazi2/p/3721941.html ）