Android中的AppWidget
Android中的AppWidget包括以下几个部分：
AppWidgetProvider
AppWidgetProvider是AppWidget提供者需要实现的接口，它实际上是一个BroadcastReceiver。只不过子类要实现的不再是onReceive，而是转换成了几个新的函数：
1 public void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds)
2 public void onDeleted(Context context, int[] appWidgetIds)
3 public void onEnabled(Context context)
4 public void onDisabled(Context context)
这几个函数用来响应AppWidgetService发出的相应的广播消息。
AppWidgetProvider的实现者
作为AppWidgetProvider的实现者，一定要实现onUpdate函数，因为这个函数决定widget的显示方式，如果没有这个函数widget根本没办法出现。
1 void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds)
onUpdate的实现基本上遵循下面的流程：
o 创建RemoteViews
o 调用AppWidgetManager的updateAppWidget去更新widget.
现在我们看下Music里的MediaAppWidgetProvider实现：
1 public void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds) {
2 defaultAppWidget(context, appWidgetIds);
3
4 // Send broadcast intent to any running MediaPlaybackService so it can
5 // wrap around with an immediate update.
6 Intent updateIntent = new Intent(MediaPlaybackService.SERVICECMD);
7 updateIntent.putExtra(MediaPlaybackService.CMDNAME,
8 MediaAppWidgetProvider.CMDAPPWIDGETUPDATE);
9 updateIntent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_IDS, appWidgetIds);
10 updateIntent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY);
11 context.sendBroadcast(updateIntent);
12 }
在defaultAppWidget里面：
o 创建RemoteViews，并设置相应的属性。
1 final Resources res = context.getResources();
2 final RemoteViews views = new RemoteViews(context.getPackageName(), R.layout.album_appwidget);
3
4 views.setViewVisibility(R.id.title, View.GONE);
5 views.setTextViewText(R.id.artist, res.getText(R.string.emptyplaylist));
o 为View上的控制设置事件处理方法。
1 linkButtons(context, views, false /* not playing */);
2
3 private void linkButtons(Context context, RemoteViews views, boolean playerActive) {
4 // Connect up various buttons and touch events
5 Intent intent;
6 PendingIntent pendingIntent;
7
8 final ComponentName serviceName = new ComponentName(context, MediaPlaybackService.class);
9
10 if (playerActive) {
11 intent = new Intent(context, MediaPlaybackActivity.class);
12 pendingIntent = PendingIntent.getActivity(context,
13 0 /* no requestCode */, intent, 0 /* no flags */);
14 views.setOnClickPendingIntent(R.id.album_appwidget, pendingIntent);
15 } else {
16 intent = new Intent(context, MusicBrowserActivity.class);
17 pendingIntent = PendingIntent.getActivity(context,
18 0 /* no requestCode */, intent, 0 /* no flags */);
19 views.setOnClickPendingIntent(R.id.album_appwidget, pendingIntent);
20 }
21
22 intent = new Intent(MediaPlaybackService.TOGGLEPAUSE_ACTION);
23 intent.setComponent(serviceName);
24 pendingIntent = PendingIntent.getService(context,
25 0 /* no requestCode */, intent, 0 /* no flags */);
26 views.setOnClickPendingIntent(R.id.control_play, pendingIntent);
27
28 intent = new Intent(MediaPlaybackService.NEXT_ACTION);
29 intent.setComponent(serviceName);
30 pendingIntent = PendingIntent.getService(context,
31 0 /* no requestCode */, intent, 0 /* no flags */);
32 views.setOnClickPendingIntent(R.id.control_next, pendingIntent);
33 }
o 更新widget
1 pushUpdate(service, appWidgetIds, views);
2 private void pushUpdate(Context context, int[] appWidgetIds, RemoteViews views) {
3 // Update specific list of appWidgetIds if given, otherwise default to all
4 final AppWidgetManager gm = AppWidgetManager.getInstance(context);
5 if (appWidgetIds != null) {
6 gm.updateAppWidget(appWidgetIds, views);
7 } else {
8 gm.updateAppWidget(THIS_APPWIDGET, views);
9 }
10 }
RemoteViews
RemoteViews并不是一个真正的View，它没有实现View的接口，而只是一个用于描述View的实体。比如：创建View需要的资源ID和各个控件的事件响应方法。RemoteViews会通过进程间通信机制传递给AppWidgetHost。
现在我们可以看出，Android中的AppWidget与google widget和中移动的widget并不是一个概念，这里的AppWidget只是把一个进程的控件嵌入到别外一个进程的窗口里的一种方法。View在另 外一个进程里显示，但事件的处理方法还是在原来的进程里。这有点像 X Window中的嵌入式窗口。
AppWidgetHost
AppWidgetHost是真正容纳AppWidget的地方，它的主要功能有两个：
o 监听来自AppWidgetService的事件：
1 class Callbacks extends IAppWidgetHost.Stub {
2 public void updateAppWidget(int appWidgetId, RemoteViews views) {
3 Message msg = mHandler.obtainMessage(HANDLE_UPDATE);
4 msg.arg1 = appWidgetId;
5 msg.obj = views;
6 msg.sendToTarget();
7 }
8
9 public void providerChanged(int appWidgetId, AppWidgetProviderInfo info) {
10 Message msg = mHandler.obtainMessage(HANDLE_PROVIDER_CHANGED);
11 msg.arg1 = appWidgetId;
12 msg.obj = info;
13 msg.sendToTarget();
14 }
15 }
这是主要处理update和provider_changed两个事件,根据这两个事件更新widget。
1 class UpdateHandler extends Handler {
2 public UpdateHandler(Looper looper) {
3 super(looper);
4 }
5
6 public void handleMessage(Message msg) {
7 switch (msg.what) {
8 case HANDLE_UPDATE: {
9 updateAppWidgetView(msg.arg1, (RemoteViews)msg.obj);
10 break;
11 }
12 case HANDLE_PROVIDER_CHANGED: {
13 onProviderChanged(msg.arg1, (AppWidgetProviderInfo)msg.obj);
14 break;
15 }
16 }
17 }
18 }
o 另外一个功能就是创建AppWidgetHostView。前面我们说过RemoteViews不是真正的View，只是View的描述，而 AppWidgetHostView才是真正的View。这里先创建AppWidgetHostView，然后通过AppWidgetService查询 appWidgetId对应的RemoteViews，最后把RemoteViews传递给AppWidgetHostView去 updateAppWidget。
1 public final AppWidgetHostView createView(Context context, int appWidgetId,
2 AppWidgetProviderInfo appWidget) {
3 AppWidgetHostView view = onCreateView(context, appWidgetId, appWidget);
4 view.setAppWidget(appWidgetId, appWidget);
5 synchronized (mViews) {
6 mViews.put(appWidgetId, view);
7 }
8 RemoteViews views = null;
9 try {
10 views = sService.getAppWidgetViews(appWidgetId);
11 } catch (RemoteException e) {
12 throw new RuntimeException("system server dead?", e);
13 }
14 view.updateAppWidget(views);
15 return view;
16 }
AppWidgetHostView
AppWidgetHostView是真正的View，但它只是一个容器，用来容纳实际的AppWidget的View。这个AppWidget的View是根据RemoteViews的描述来创建。这是在updateAppWidget里做的：
1 public void updateAppWidget(RemoteViews remoteViews) {
2 ...
3 if (content == null && layoutId == mLayoutId) {
4 try {
5 remoteViews.reapply(mContext, mView);
6 content = mView;
7 recycled = true;
8 if (LOGD) Log.d(TAG, "was able to recycled existing layout");
9 } catch (RuntimeException e) {
10 exception = e;
11 }
12 }
13
14 // Try normal RemoteView inflation
15 if (content == null) {
16 try {
17 content = remoteViews.apply(mContext, this);
18 if (LOGD) Log.d(TAG, "had to inflate new layout");
19 } catch (RuntimeException e) {
20 exception = e;
21 }
22 }
23 ...
24 if (!recycled) {
25 prepareView(content);
26 addView(content);
27 }
28
29 if (mView != content) {
30 removeView(mView);
31 mView = content;
32 }
33 ...
34 }
remoteViews.apply创建了实际的View，下面代码可以看出:
1 public View apply(Context context, ViewGroup parent) {
2 View result = null;
3
4 Context c = prepareContext(context);
5
6 Resources r = c.getResources();
7 LayoutInflater inflater = (LayoutInflater) c
8 .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
9
10 inflater = inflater.cloneInContext(c);
11 inflater.setFilter(this);
12
13 result = inflater.inflate(mLayoutId, parent, false);
14
15 performApply(result);
16
17 return result;
18 }
Host的实现者
AppWidgetHost和AppWidgetHostView是在框架中定义的两个基类。应用程序可以利用这两个类来实现自己的Host。Launcher是缺省的桌面，它是一个Host的实现者。
LauncherAppWidgetHostView扩展了AppWidgetHostView，实现了对长按事件的处理。
LauncherAppWidgetHost扩展了AppWidgetHost，这里只是重载了onCreateView，创建LauncherAppWidgetHostView的实例。
AppWidgetService
AppWidgetService存在的目的主要是解开AppWidgetProvider和AppWidgetHost之间的耦合。如果 AppWidgetProvider和AppWidgetHost的关系固定死了，AppWidget就无法在任意进程里显示了。而有了 AppWidgetService，AppWidgetProvider根本不需要知道自己的AppWidget在哪里显示了。