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) {
             ...
             mDataSetObserver = new AdapterDataSetObserver();
             mAdapter.registerDataSetObserver(mDataSetObserver);

             ...         } else {
             ...         }

         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 ）