当ListView有Header时，onItemClick里的position不对

当给ListView加了一个HeaderView后（代码例如以下），我们发现，onItemClick方法里的position參数的值不是我们所期望的。比方点击ListView的第一行，我们期望的position是0，但是实际上却是1。也就是说，它是从Header而不是从第一行開始计数的。

@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，只是这个bug被RomainGuy reject掉了，理由是，你用错了。请用getAdapter。

这回答的太简洁了，全然没法理解，所以仅仅好又去细致研究ListView的代码。最终领会他的意思了。

把当中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.
 * <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;
    }

    //其他的一些代码这里省略之...
}

从代码和凝视里都能够非常清楚的得知。addHeaderView一定要在setAdapter之前调用，假设不这样做，addHeaderView会抛出一个异常。

Android为什么要这样？这是由于。在setAdapter的时候，会针对我遇到的这样的情况（也就是加入Header后position不对的这样的情况）做些特殊的处理。

setAdapter在内部推断了当前ListView是否有Header或者Footer，假设没有，就直接使用參数传进来的adapter；假设有。则用一个decorated的HeaderViewListAdapter来替换參数。这个HeaderViewListAdapter的使命。就是排除Header和Footer，让position（当然也包含getItem, getItemId）等方法的position參数）正确返回。

分析到这里，解决方式就出来了：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，假设ListView没有Header和Footer。直接使用声明的adapter也没有问题，只是为了避免出错，还是统一使用decorated adapter比較好。

把onItemClick改成以下这样，就能够了

@Override
public void onItemClick(AdapterView<?
> parent, View v, int position, long id) {
    doSomething(parent.getAdapter().getItem(position));
    //或者 doSomething( parent.getItemAtPosition(position) );
}

错误使用方法一：
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int position, long id) {
   String item = (String) mDataList1.get(position);
}

错误使用方法二：
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int position, long id) {
   String item = (String) mAdapter.getItem(position);
}