Override ListView getAdapter造成的後果

Override ListView getAdapter造成的後果

最近工作中，發現了一個bug，是和ListView Adapter有關的。產生了FC，描述資訊大約是

"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內的資料發生了變化，但是UI卻沒有更新，您是否忘記調用了notifyDataSetChanged？

這實際上是一個非常有誤導的資訊。一般情況下，我們不會忘記調用該函數的。但是如果我們不小心，從listview繼承一個新的類，並override它的getAdapter方法，就可能會出問題了。

ListView是支援HeaderView和footerView的，即在listview的最初和最末尾的位置添加一些特殊的view。它的實現方法，就是通過一個HeaderViewListAdapter。

HeaderViewListAdapter會封裝一個Adapter，這個是由使用者自己設定的。ListView中對應的代碼是

    @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，即可能是一個HeaderViewListAdapter.

如果override getAdapter，並返回HeaderViewListAdapter內部封裝的Adapter，就會出問題。也就是上面提到的FC.

這種問題是怎麼出現呢？

首先，這個異常拋出的位置，是在函數layoutChildren中，拋出的條件是mItemCount != mAdapter.getCount()，代碼如下：

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() + ")]");            }

那麼mItemCount的值是在哪裡賦值呢？mItemCount不是ListView的成員，而是ListView的超超類：AdapterView的成員，這個值也是在DataObserver.onChanged中設定的，您可參考AdapterView的源碼：

class AdapterDataSetObserver extends DataSetObserver {        private Parcelable mInstanceState = null;        @Override        public void onChanged() {            mDataChanged = true;            mOldItemCount = mItemCount;            mItemCount = getAdapter().getCount(); //這裡！注意用法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();        }

如果 getAdapter() != mAdapter就會發生問題：getAdatper返回的是mAdapter(即HeaderListViewAdapter)，那麼，mAdapter.getCount() == getAdapter().getCount() + header view count + footer view count.

出現上面的問題就在所難免了。