ListActivity源码分析

　　最近老是和ListActivity过意不去，碰到了它的几个问题，干脆读下它的源码吧，搞清楚它的内部机制，有利于问题的解决。

    Android中的ListActivity其实就是一个自带ListView的Activity,ListActivity它位于命名空间：android.app之下，从它的源代码就可以清楚的看到，ListActivity继承了Activity，它的基本用法网上有很多资料，随便GOOGLE一下，到处都是，下面主要是分析一下它的Java源代码.（转自王三丰）

public class ListActivity extends Activity {
    /**
     * This field should be made private, so it is hidden from the SDK.
     * {@hide}
     */
    protected ListAdapter mAdapter;
    /**
     * This field should be made private, so it is hidden from the SDK.
     * {@hide}
     */
    protected ListView mList;
 
    private Handler mHandler = new Handler();
    private boolean mFinishedStart = false;
 
    private Runnable mRequestFocus = new Runnable() {
        public void run() {
            mList.focusableViewAvailable(mList);
        }
    };
 
    /**
     * This method will be called when an item in the list is selected.
     * Subclasses should override. Subclasses can call
     * getListView().getItemAtPosition(position) if they need to access the
     * data associated with the selected item.
     *
     * @param l The ListView where the click happened
     * @param v The view that was clicked within the ListView
     * @param position The position of the view in the list
     * @param id The row id of the item that was clicked
     */
    protected void onListItemClick(ListView l, View v, int position, long id) {
    }
 
    /**
     * Ensures the list view has been created before Activity restores all
     * of the view states.
     *
     *@see Activity#onRestoreInstanceState(Bundle)
     */
    @Override
    protected void onRestoreInstanceState(Bundle state) {
        ensureList();
        super.onRestoreInstanceState(state);
    }
 
    /**
     * @see Activity#onDestroy()
     */
    @Override
    protected void onDestroy() {
        mHandler.removeCallbacks(mRequestFocus);
        super.onDestroy();
    }
 
    /**
     * Updates the screen state (current list and other views) when the
     * content changes.
     *
     * @see Activity#onContentChanged()
     */
    @Override
    public void onContentChanged() {
        super.onContentChanged();
        View emptyView = findViewById(com.android.internal.R.id.empty);
        mList = (ListView)findViewById(com.android.internal.R.id.list);
        if (mList == null) {
            throw new RuntimeException(
                    "Your content must have a ListView whose id attribute is " +
                    "'android.R.id.list'");
        }
        if (emptyView != null) {
            mList.setEmptyView(emptyView);
        }
        mList.setOnItemClickListener(mOnClickListener);
        if (mFinishedStart) {
            setListAdapter(mAdapter);
        }
        mHandler.post(mRequestFocus);
        mFinishedStart = true;
    }
 
    /**
     * Provide the cursor for the list view.
     */
    public void setListAdapter(ListAdapter adapter) {
        synchronized (this) {
            ensureList();
            mAdapter = adapter;
            mList.setAdapter(adapter);
        }
    }
 
    /**
     * Set the currently selected list item to the specified
     * position with the adapter's data
     *
     * @param position
     */
    public void setSelection(int position) {
        mList.setSelection(position);
    }
 
    /**
     * Get the position of the currently selected list item.
     */
    public int getSelectedItemPosition() {
        return mList.getSelectedItemPosition();
    }
 
    /**
     * Get the cursor row ID of the currently selected list item.
     */
    public long getSelectedItemId() {
        return mList.getSelectedItemId();
    }
 
    /**
     * Get the activity's list view widget.
     */
    public ListView getListView() {
        ensureList();
        return mList;
    }
 
    /**
     * Get the ListAdapter associated with this activity's ListView.
     */
    public ListAdapter getListAdapter() {
        return mAdapter;
    }
 
    private void ensureList() {
        if (mList != null) {
            return;
        }
        setContentView(com.android.internal.R.layout.list_content_simple);
 
    }
 
    private AdapterView.OnItemClickListener mOnClickListener = new AdapterView.OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v, int position, long id)
        {
            onListItemClick((ListView)parent, v, position, id);
        }
    };
}

代码如上，在使用ListActivity时，一般用 this.setListAdapter()传入一个ListAdapter,用来绑定ListActivity中的ListView,当调用setListAdapter()时，在ListActivity内部，会调用如下方法：

      /**
     * Provide the cursor for the list view.
     */
    public void setListAdapter(ListAdapter adapter) {
        synchronized (this) {
            ensureList();
            mAdapter = adapter;
            mList.setAdapter(adapter);
        }
    }

  其中ensureList();是一个比较重要的方法，可以查看源代码知道，mList其实就是ListActivity中的ListView了，

  接下来会调用ensureList(),先看下它是怎么弄的吧：

    private void ensureList() {
        if (mList != null) {
            return;
        }
        setContentView(com.android.internal.R.layout.list_content_simple);

    }

    可以看到，原来，就是在这里，setContentView（）引入了一个名叫com.android.internal.R.layout.list_content_simple的布局资源，这个资源，就位于我们的d:\AndroidSDK\platforms\android-16\data\res\layout下的list_content_simple.xml布局文件，用ECLIPSE打开这个布局文件看下吧。

   <ListView xmlns:android="http://schemas.android.com/apk/res/android" android:id="@android:id/list"
    android:layout_width="match_parent" 
    android:layout_height="match_parent"
    android:drawSelectorOnTop="false"
    />

 原来这个ListView的ID就是android:id="@android:id/list"，所以在这里如果我们要自定义LISTVIEW的布局文件时，这个LISTVIEW的ID就必须取名为：android:id="@android:id/list"啦，呵呵。

 继续往下分析，当调用   setContentView(com.android.internal.R.layout.list_content_simple)这段代码后，接下来会调用onContentChanged（），这个方法是用来干嘛的呢，看下它的英文注释： 

     /**
     * Updates the screen state (current list and other views) when the
     * content changes.
     *
     * @see Activity#onContentChanged()
     */

    原来就是当这个Activity内容发生改变时，就会调用。

   @Override
    public void onContentChanged() {
        super.onContentChanged();  //调用基类的onContentChanged
        View emptyView = findViewById(com.android.internal.R.id.empty); //查找ID为com.android.internal.R.id.empty的VIEW，这个VIEW一般用来当LISTVIEW没有任何ITEM时，显示一条信息。
        mList = (ListView)findViewById(com.android.internal.R.id.list);//查找ID为com.android.internal.R.id.list的LISTVIEW
        if (mList == null) {
            throw new RuntimeException(
                    "Your content must have a ListView whose id attribute is " +
                    "'android.R.id.list'");
        }
        if (emptyView != null) {
            mList.setEmptyView(emptyView); //setEmptyView()这个方法不是LISTACTIVITY的，而是它的父父类AdapterView的，这里的AdapterView是一个抽象类，其官方的定义是：“An AdapterView is a view whose children are determined by an Adapter.”，意思为：她是一个VIEW，什么VIEW呢？是它的子项需要一个适配器ADAPTER来填充的VIEW.

        }
        mList.setOnItemClickListener(mOnClickListener);//LISTVIEW的ITEM选中的回调方法
        if (mFinishedStart) {
            setListAdapter(mAdapter); 
        }
        mHandler.post(mRequestFocus); //Handler,这里把一个类型为Runnable的玩意儿投放到了消息队列里面，待主线程来处理。其中mRequestFocus是用来使ListView获取焦点的。
        mFinishedStart = true;
    }

  好了，作为一名应用程序员，对LISTACTIVITY有个这么多认识就差不多了，请“专家”和“砖家”们批评，指正，谢谢！