当我们调用Adapter.notifyDataSetChanged()时候, 会调用被观察者的()方法，

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public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {
	//数据集被观察者
    private final DataSetObservable mDataSetObservable = new DataSetObservable();
    public boolean hasStableIds() {
        return false;
    }
    
    public void registerDataSetObserver(DataSetObserver observer) {
        mDataSetObservable.registerObserver(observer);
    }
    public void unregisterDataSetObserver(DataSetObserver observer) {
        mDataSetObservable.unregisterObserver(observer);
    }
    
        /**
     * Notifies the attached observers that the underlying data has been changed
     * and any View reflecting the data set should refresh itself.
     */
    public void notifyDataSetChanged() {
    	//通知观察者更新
        mDataSetObservable.notifyChanged();
    }
    
}

//

在DataSetObservable.notifyChanged()方法中，遍历每个观察者，并调用观察者的onChanged()方法。

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public class DataSetObservable extends Observable<DataSetObserver> {
    /**
     * Invokes {@link DataSetObserver#onChanged} on each observer.
     * Called when the contents of the data set have changed.  The recipient
     * will obtain the new contents the next time it queries the data set.
     */
    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();
            }
        }
    }
}

//
这些观察者从哪里来的，又是什么时候绑定到DataSetObservable中的?
查看ListView的setAdapter()方法

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  /**
   * Sets the data behind this ListView.
   *
   * The adapter passed to this method may be wrapped by a {@link WrapperListAdapter},
   * depending on the ListView features currently in use. For instance, adding
   * headers and/or footers will cause the adapter to be wrapped.
   *
   * @param adapter The ListAdapter which is responsible for maintaining the
   *        data backing this list and for producing a view to represent an
   *        item in that data set.
   *
   * @see #getAdapter() 
   */
  @Override
  public void setAdapter(ListAdapter adapter) {
   //如果已存在adapter先注销原来Adapter对应的观察者
      if (mAdapter != null && mDataSetObserver != null) {
          mAdapter.unregisterDataSetObserver(mDataSetObserver);
      }
      resetList();
      mRecycler.clear();
      if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
          mAdapter = wrapHeaderListAdapterInternal(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();
//创建一个数据集观察者
          mDataSetObserver = new AdapterDataSetObserver();
          
          	//注册观察者
          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();
      }
      requestLayout();
  }

///
AdapterDataSetObserver是AbsListView中定义的内部类

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class AdapterDataSetObserver extends AdapterView<ListAdapter>.AdapterDataSetObserver {
    @Override
    public void onChanged() {
    	//调用父类的onChange()方法。
        super.onChanged();
        if (mFastScroll != null) {
            mFastScroll.onSectionsChanged();
        }
    }
    @Override
    public void onInvalidated() {
        super.onInvalidated();
        if (mFastScroll != null) {
            mFastScroll.onSectionsChanged();
        }
    }
}

而它又继承AdapterView的AdapterDataSetObserver

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  class AdapterDataSetObserver extends DataSetObserver {
      private Parcelable mInstanceState = null;
//最终调用
      @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;
      }
  }

我们只关注onChanged()方法

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@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();
}

它会调用requestLayout()方法进行重新布局，从而刷新listvew(GridView ， AdapterView)界面