BaseAdapter.notifyDataSetChanged()之观察者设计模式及源码分析

BaseAdapter.notifyDataSetChanged()的实现涉及到设计模式-观察者模式，详情请参考我之前的博文设计模式之观察者模式

Ok,回到notifyDataSetChanged进行源码分析，

Adapter本身不是被观察者Observable，而是adapter里面有一个被观察者DataSetObservable mDataSetObservable

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

notifyChanged源码分析，notifyChanged实际上是去调用了观察者Observer的onChanged方法.

 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();
             }
         }
     }

被观察者subject ListAdapter添加注册观察者Observer AdapterDataSetObserver

1    //ListView.java - setAdapter()
2    //被观察者subject ListAdapter添加注册观察者Observer AdapterDataSetObserver
3    mDataSetObserver = new AdapterDataSetObserver();
4    mAdapter.registerDataSetObserver(mDataSetObserver);

AdapterDataSetObserver的onChanged方法里最后调用了requestLayout去更新ListView

 //AdapterView.java
     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;
         }
     }