Android_ListView_有Header或Footer时onItemClick里的position的问题
当ListView有Header或者Footer时，onItemClick方法的position会包含Header和Footer。
例如：一个ListView有10个item。如果不加Header和Footer，那么positon会是0～9。如果加了Header和Footer，那么positon就回变成0～10（其中0是header，10是footer）。
如果没有意识到这一点，那么很有可能会出现问题。
final MyListAdapter adapter = new MyListAdapter(this);
listView.setAdapter(adapter);
listView.setHeaderDividersEnabled(true);
listView.setFooterDividersEnabled(true);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Log.i(getPackageName(), position + " : " + adapter.getItem(position));
}
});上面的代码会在点击footer和最后一个item的时候crush掉。
查看ListView的源代码，会发现问题所在。下面是addHeaderView和setAdapter方法。
/**
* Add a fixed view to appear at the top of the list. If addHeaderView is
* called more than once, the views will appear in the order they were
* added. Views added using this call can take focus if they want.
* <p>
* NOTE: Call this before calling setAdapter. This is so ListView can wrap
* the supplied cursor with one that that will also account for header
* views.
*
* @param v The view to add.
* @param data Data to associate with this view
* @param isSelectable whether the item is selectable
*/
public void addHeaderView(View v, Object data, boolean isSelectable) {
if (mAdapter != null) {
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);
}
/**
* Sets the data behind this ListView.
*
* The adapter passed to this method may be wrapped by a {@link WrapperListAdapter},
* depending on the ListView features currently in use. For instance, adding
* headers and/or footers will cause the adapter to be wrapped.
*
* @param adapter The ListAdapter which is responsible for maintaining the
* data backing this list and for producing a view to represent an
* item in that data set.
*
* @see #getAdapter()
*/
@Override
public void setAdapter(ListAdapter adapter) {
if (null != mAdapter) {
mAdapter.unregisterDataSetObserver(mDataSetObserver);
}
resetList();
mRecycler.clear();
if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter);
} else {
mAdapter = adapter;
}
//其它的一些代码这里省略之...
}从代码和注释里都可以很清楚的得知，addHeaderView一定要在setAdapter之前调用，如果不这样做，addHeaderView会抛出一个异常。Android为什么要这样？这是因为，在setAdapter的时候，会针对我遇到的这种情况（也就是添加Header后position不正确的这种情况）做些特殊的处理。setAdapter在内部判断了当前ListView是否有Header或者Footer，如果没有，就直接使用参数传进来的adapter；如果有，则用一个decorated的HeaderViewListAdapter来替换参数。这个HeaderViewListAdapter的使命，就是排除Header和Footer，让position（当然也包括getItem, getItemId）等方法的position参数）正确返回。
分析到这里，解决方案就出来了：在onItemClick不要直接使用我们声明的adapter，而是用ListView里的那个decorated adapter。获取它的方法就是调用parent.getAdapter()。当然，如果ListView没有Header和Footer，直接使用声明的adapter也没有问题，不过为了避免出错，还是统一使用decorated adapter比较好。
把onItemClick改成下面这样，就可以了。
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Log.i(getPackageName(), position + " : " + parent.getAdapter().getItem(position));
}