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
代码语言：javascript复制"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
代码语言：javascript复制 @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：
代码语言：javascript复制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：
代码语言：javascript复制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。
