Override ListView getAdapter造成的后果

近期工作中，发现了一个bug，是和ListView Adapter有关的。产生了FC，描写叙述信息大约是

"The content of the adapter has changed but ListView did not receive a notification. Make sure the content of  your adapter is not modified from a background thread, but only from the UI thread. Make sure your adapter calls notifyDataSetChanged() when its content changes. [in ListView(xxx) with Adapter(HeaderViewListAdapter)]"

它的大意是，Adapter内的数据发生了变化，可是UI却没有更新，您是否忘记调用了notifyDataSetChanged？

这实际上是一个很有误导的信息。普通情况下，我们不会忘记调用该函数的。可是假设我们不小心，从listview继承一个新的类，并override它的getAdapter方法，就可能会出问题了。

ListView是支持HeaderView和footerView的，即在listview的最初和最末尾的位置加入�一些特殊的view。它的实现方法，就是通过一个HeaderViewListAdapter。

HeaderViewListAdapter会包装一个Adapter，这个是由用户自己设置的。ListView中相应的代码是

    @Override
    public void setAdapter(ListAdapter adapter) {
        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;
        }

ListView的getAdapter返回的是mAdapter，就可以能是一个HeaderViewListAdapter.

假设override getAdapter，并返回HeaderViewListAdapter内部包装的Adapter，就会出问题。也就是上面提到的FC.

这样的问题是怎么出现呢？

首先，这个异常抛出的位置，是在函数layoutChildren中，抛出的条件是mItemCount != mAdapter.getCount()，代码例如以下：

else if (mItemCount != mAdapter.getCount()) {
                throw new IllegalStateException("The content of the adapter has changed but "
                        + "ListView did not receive a notification. Make sure the content of "
                        + "your adapter is not modified from a background thread, but only from "
                        + "the UI thread. Make sure your adapter calls notifyDataSetChanged() "
                        + "when its content changes. [in ListView(" + getId() + ", " + getClass()
                        + ") with Adapter(" + mAdapter.getClass() + ")]");
            }

那么mItemCount的值是在哪里赋值呢？mItemCount不是ListView的成员，而是ListView的超超类：AdapterView的成员，这个值也是在DataObserver.onChanged中设置的，您可參考AdapterView的源代码：

class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;

        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = getAdapter().getCount(); //这里！注意使用方法getAdapter()

            // 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();
        }

假设 getAdapter() != mAdapter就会发生故障：getAdatper返回的是mAdapter(即HeaderListViewAdapter)，那么，mAdapter.getCount() == getAdapter().getCount() + header view count + footer view count.

出现上面的问题就在所难免了。