android listview DataSetObserver

AbsListView 内部定义了：

 class AdapterDataSetObserver extends AdapterView<ListAdapter>.AdapterDataSetObserver {
        @Override
        public void onChanged() {
            super.onChanged();
            if (mFastScroll != null) {
                mFastScroll.onSectionsChanged();
            }
        }

        @Override
        public void onInvalidated() {
            super.onInvalidated();
            if (mFastScroll != null) {
                mFastScroll.onSectionsChanged();
            }
        }
    }

AttachedToWindow的时候，创建Observer，并添注册该观察者，
该用户用于刷新，mFastScroll.onSectionsChanged();

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        ...........................
        if (mAdapter != null && mDataSetObserver == null) {
            mDataSetObserver = new AdapterDataSetObserver();
            mAdapter.registerDataSetObserver(mDataSetObserver);
            // Data may have changed while we were detached. Refresh.
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = mAdapter.getCount();
        }
    }

什么时候调用了？
当我们调用BaseAdapter.notifyDataSetChanged的时候

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

    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();
            }
        }
    }

就会回调到上面在AbsListView 内部类中的AdapterDataSetObserver 的
onChanged方法中。