當ListView有Header時，onItemClick里的position不正確

今天在做項目的時候，遇到一個問題，記錄下來。

當給ListView加了一個HeaderView后（代碼如下），我們發現，onItemClick方法里的position參數的值不是我們所期望的，比如點擊ListView的第一行，我們期望的position是0，可是實際上卻是1，也就是說，它是從Header而不是從第一行開始計數的。

@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，發現有個老外issue過一個bug，和我遇到的問題一樣，不過這個bug被RomainGuy reject掉了，理由是，你用錯了，請用getAdapter。這回答的太簡潔了，完全沒法理解，所以只好又去仔細研究ListView的代碼，終於領會他的意思了。把其中addHeaderView和setAdapter方法貼下來

/**
 * 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;
    }

    //其它的一些代碼這里省略之...
}

 

 

從代碼和注釋里都可以很清楚的得知，addHeaderView一定要在setAdapter之前調用，如果不這樣做，addHeaderView會拋出一個異常。Android為什么要這樣？這是因為，在setAdapter的時候，會針對我遇到的這種情況（也就是添加Header后position不正確的這種情況）做些特殊的處理。setAdapter在內部判斷了當前ListView是否有Header或者Footer，如果沒有，就直接使用參數傳進來的adapter；如果有，則用一個decorated的HeaderViewListAdapter來替換參數。這個HeaderViewListAdapter的使命，就是排除Header和Footer，讓position（當然也包括getItem, getItemId）等方法的position參數）正確返回。

分析到這里，解決方案就出來了：在onItemClick不要直接使用我們聲明的adapter，而是用ListView里的那個decorated adapter。獲取它的方法就是調用parent.getAdapter()。當然，如果ListView沒有Header和Footer，直接使用聲明的adapter也沒有問題，不過為了避免出錯，還是統一使用decorated adapter比較好。

把onItemClick改成下面這樣，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最初發表於：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聲明的情況下轉帖、分發和印刷等。