最近老是和ListActivity過意不去，碰到了它的幾個問題，乾脆讀下它的源碼吧，搞清楚它的內部機制，有利於問題的解決。
Android中的ListActivity其實就是一個自帶ListView的Activity,ListActivity它位於命名空間：android.app之下，從它的源代碼就可以清楚的看到，ListActivity繼承了Activity，它的基本用法網上有很多資料，隨便GOOGLE一下，到處都是，下面主要是分析一下它的Java源代碼.
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);
}
};
}
代碼如上，在使用ListActivity時，一般用 this.setListAdapter()傳入一個ListAdapter,用來綁定ListActivity中的ListView,當調用setListAdapter()時，在ListActivity內部，會調用如下方法：
/**
* Provide the cursor for the list view.
*/
public void setListAdapter(ListAdapter adapter) {
synchronized (this) {
ensureList();
mAdapter = adapter;
mList.setAdapter(adapter);
}
}
其中ensureList();是一個比較重要的方法，可以查看源代碼知道，mList其實就是ListActivity中的ListView了，
接下來會調用ensureList(),先看下它是怎麼弄的吧：
private void ensureList() {
if (mList != null) {
return;
}
setContentView(com.android.internal.R.layout.list_content_simple);
}
可以看到，原來，就是在這裏，setContentView（）引入了一個名叫com.android.internal.R.layout.list_content_simple的佈局資源，這個資源，就位於我們的d:\AndroidSDK\platforms\android-16\data\res\layout下的list_content_simple.xml佈局文件，用ECLIPSE打開這個佈局文件看下吧。
<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"
/>
原來這個ListView的ID就是android:id="@android:id/list"，所以在這裏如果我們要自定義LISTVIEW的佈局文件時，這個LISTVIEW的ID就必須取名爲：android:id="@android:id/list"啦，呵呵。
繼續往下分析，當調用 setContentView(com.android.internal.R.layout.list_content_simple)這段代碼後，接下來會調用onContentChanged（），這個方法是用來幹嘛的呢，看下它的英文註釋：
/**
* Updates the screen state (current list and other views) when the
* content changes.
*
* @see Activity#onContentChanged()
*/
原來就是當這個Activity內容發生改變時，就會調用。
@Override
public void onContentChanged() {
super.onContentChanged(); //調用基類的onContentChanged
View emptyView = findViewById(com.android.internal.R.id.empty); //查找ID爲com.android.internal.R.id.empty的VIEW，這個VIEW一般用來當LISTVIEW沒有任何ITEM時，顯示一條信息。
mList = (ListView)findViewById(com.android.internal.R.id.list);//查找ID爲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()這個方法不是LISTACTIVITY的，而是它的父父類AdapterView的，這裏的AdapterView是一個抽象類，其官方的定義是：“An AdapterView is a view whose children are determined by an Adapter.”，意思爲：她是一個VIEW，什麼VIEW呢？是它的子項需要一個適配器ADAPTER來填充的VIEW.
}
mList.setOnItemClickListener(mOnClickListener);//LISTVIEW的ITEM選中的回調方法
if (mFinishedStart) {
setListAdapter(mAdapter);
}
mHandler.post(mRequestFocus); //Handler,這裏把一個類型爲Runnable的玩意兒投放到了消息隊列裏面，待主線程來處理。其中mRequestFocus是用來使ListView獲取焦點的。
mFinishedStart = true;
}
好了，作爲一名應用程序員，對LISTACTIVITY有個這麼多認識就差不多了，請“專家”和“磚家”們批評，指正，謝謝！