Override ListView getAdapter造成的后果

时间:2023-11-28 12:08:08

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。