当ListView有Header时，onItemClick里的position不正确

Fri, Mar 9, 2012

今天在做项目的时候，遇到一个问题，记录下来。

当给ListView加了一个HeaderView后（代码如下），我们发现，onItemClick方法里的position参数的值不是我们所期望的，比如点击ListView的第一行，我们期望的position是0，可是实际上却是1，也就是说，它是从Header而不是从第一行开始计数的。

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.home);

    mAdapter = new MyAdapter(this);

    mListView = (ListView) findViewById(R.id.list);
    mListView.addHeaderView(getLayoutInflater().inflate(R.layout.list_header));
    mListView.setAdapter(mAdapter);
    mListView.setOnClickListener(this);
}

@Override
public void onItemClick(AdapterView parent, View v, int position, long id) {
    doSomething(mAdapter.getItem(position));
}

Google了下，发现有个老外issue过一个bug，和我遇到的问题一样，不过这个bug被RomainGuy reject掉了，理由是，你用错了，请用getAdapter。这回答的太简洁了，完全没法理解，所以只好又去仔细研究ListView的代码，终于领会他的意思了。把其中addHeaderView和setAdapter方法贴下来

/**
 * 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.
 * 

 * 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

@Override
public void onItemClick(AdapterView parent, View v, int position, long id) {
    doSomething(parent.getAdapter().getItem(position));
}

本文由Roy最初发表于：http://blog.chengbo.net/2012/03/09/onitemclick-return-wrong-position-when-listview-has-headerview.html，你可以在保持文章完整和保留本声明的情况下转帖、分发和印刷等。