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.
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！