Android 4.4以下ListView addHeader和addFooter放在setAdapter之后导致异常的原因分析和解决

问题重现：

在Android 19之前，ListView的addHeader和addFooter必须在setAdapter之前调用，否则就会有异常，Android 19以及之后是没有这个问题的。异常的具体表现为：

1.在setAdapter之后调用mListView.addHeaderView(mHeader);会抛出异常：

Cannot add header view to list -- setAdapter has already been called.

2.在setAdapter之后调用mListView.addFooterView(mFooter);虽然不会抛出异常，但是却看不到添加的footer。

问题解决：

在Android 19之前，如果想在setAdapter之后添加header和footer，可以这样做：

//设置adapter之后设置header和footer
ListAdapter originalAdapter; //得到之前给listview设置的adapter，如果已经知道，可以不用
if (mListView.getAdapter() instanceof HeaderViewListAdapter) {
HeaderViewListAdapter headerViewListAdapter = (HeaderViewListAdapter) mListView.getAdapter();
originalAdapter = headerViewListAdapter.getWrappedAdapter();
} else {
originalAdapter = mListView.getAdapter();
}
mListView.setAdapter(null);
mListView.addHeaderView(mHeader);
mListView.addFooterView(mFooter);
mListView.setAdapter(originalAdapter);

问题原因

我们可以对比Android 18和19的ListView的源码，在addHeaderView的改变：

API 18

public void addHeaderView(View v, Object data, boolean isSelectable) {

if (mAdapter != null && ! (mAdapter instanceof HeaderViewListAdapter)) {
throw new IllegalStateException(
"Cannot add header view to list -- setAdapter has already been called.");
}

FixedViewInfo info = new FixedViewInfo();
info.view = v;
info.data = data;
info.isSelectable = isSelectable;
mHeaderViewInfos.add(info);

// in the case of re-adding a header view, or adding one later on,
// we need to notify the observer
if (mAdapter != null && mDataSetObserver != null) {
mDataSetObserver.onChanged();
}
}

API 19

public void addHeaderView(View v, Object data, boolean isSelectable) {
final FixedViewInfo info = new FixedViewInfo();
info.view = v;
info.data = data;
info.isSelectable = isSelectable;
mHeaderViewInfos.add(info);

// Wrap the adapter if it wasn't already wrapped.
if (mAdapter != null) {
if (!(mAdapter instanceof HeaderViewListAdapter)) {
mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, mAdapter);
}

// In the case of re-adding a header view, or adding one later on,
// we need to notify the observer.
if (mDataSetObserver != null) {
mDataSetObserver.onChanged();
}
}
}

可以看到，在Android 19之前，如果ListView设置过Adapter，会抛出异常，在Android 19中，google做了调整。