在上一篇文章Android获取第三方音乐播放器的音乐信息的末尾，提到可以通过捕获第三方音乐发送的的Notification，来获取到当前播放的歌曲的信息。先分析下Notification的更新机制。
涉及Service:
StatusBarManagerService
NotificationManagerService
这个两个service都会在frameworks/base/services/java/com/android/server/SystemServer.java文件里面进行启动的
class ServerThread extends Thread {
public void run() {
......
StatusBarManagerService statusBar = null;
NotificationManagerService notification = null;
......
statusBar = new StatusBarManagerService(context, wm);
ServiceManager.addService(Context.STATUS_BAR_SERVICE, statusBar);
......
notification = new NotificationManagerService(context, statusBar, lights);
ServiceManager.addService(Context.NOTIFICATION_SERVICE, notification);
......
}
}
下面开始跟踪Notification的Notify流程。
1.
当在Activity里面创建一个Notification，并Notify
NotificationManager nm = (NotificationManager)getSystemService(Context.NOTIFICATION_SERVICE);
Notification n = new Notification(R.drawable.chat, "Hello,there!", System.currentTimeMillis());
n.flags = Notification.FLAG_AUTO_CANCEL;
Intent i = new Intent(arg0.getContext(), NotificationShow.class);
i.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP|Intent.FLAG_ACTIVITY_NEW_TASK);
//PendingIntent
PendingIntent contentIntent = PendingIntent.getActivity(
arg0.getContext(),
R.string.app_name,
i,
PendingIntent.FLAG_UPDATE_CURRENT);
n.setLatestEventInfo(
arg0.getContext(),
"Hello,there!",
"Hello,there,I'm john.",
contentIntent);
nm.notify(R.string.app_name, n);
2.调用NotificationManager.notify,进入notify 方法
public void notify(int id, Notification notification)
{
notify(null, id, notification);
}
public void notify(String tag, int id, Notification notification)
{
......
INotificationManager service = getService();
......
service.enqueueNotificationWithTag(pkg, mContext.getOpPackageName(), tag, id,
notification, idOut, UserHandle.myUserId());
......
}
3.在调用notify(String tag, int id, Notification notification)时，会调用getService()
static public INotificationManager getService()
78 {
79 if (sService != null) {
80 return sService;
81 }
82 IBinder b = ServiceManager.getService("notification");
83 sService = INotificationManager.Stub.asInterface(b);
84 return sService;
85 }这里发现，会调用ServiceManager.getService("notification"), 这样就会获取到文章刚开始讲的NotificationManagerService 的一个binder对象。获取回到第二步调用service.enqueueNotificationWithTAG.
4.进入NotificationManagerService的enqueueNotificationWithTAG
public void enqueueNotificationWithTag(String pkg, String basePkg, String tag, int id,
1621 Notification notification, int[] idOut, int userId)
1622 {
1623 enqueueNotificationInternal(pkg, basePkg, Binder.getCallingUid(), Binder.getCallingPid(),
1624 tag, id, notification, idOut, userId);
1625 }
1634 public void enqueueNotificationInternal(final String pkg, String basePkg, final int callingUid,
1635 final int callingPid, final String tag, final int id, final Notification notification,
1636 int[] idOut, int incomingUserId)
1637 {
......
final StatusBarNotification n = new StatusBarNotification(
1755 pkg, id, tag, callingUid, callingPid, score, notification, user);
1792 mStatusBar.updateNotification(r.statusBarKey, n);
1793
......
}在这个方法里，会将前面传递进来的Notification封装成一个StatusBarNotification对象，然后调用mStatusBar.updateNotification去更新,这个mStatusBar是什么？
5. 文章最开始提到会在ServerThread里面添加NotificationManagerService，在创建的NotificationManagerService的时候，会调用它的构造函数
NotificationManagerService(Context context, StatusBarManagerService statusBar,
1291 LightsService lights)
1292 {
.......
}这里面就会传递进来，在ServerThread里刚刚创建的StatusBarManagerService. 所以在第4步的时候我们能够调用mStatusBar.updateNotification（）。
6.
StatusBarManagerService 中mStatusBar.updateNotification()方法
public void updateNotification(IBinder key, StatusBarNotification notification) {
512 synchronized (mNotifications) {
516 mNotifications.put(key, notification);
......
519 mBar.updateNotification(key, notification);
......
524 }这里会调用mBar.updateNotification(),mBar是什么？
7. StatusBarManagerService 中 mBar对象
volatile IStatusBar mBar;
// Callbacks from the status bar service.
428 // ================================================================================
429 public void registerStatusBar(IStatusBar bar, StatusBarIconList iconList,
430 List notificationKeys, List notifications,
431 int switches[], List binders) {
......
上面发现有个registerStatusBar方法，会传递进来一个IStatusBar 对象，那这个方法会在哪里调用呢？
8. 在StatusBarManagerService中，我们发现调用mBar对象都是去处理StatusBar的UI，那这个StatusBar是哪里的？SystemUI里面的。检索下SystemUI的代码，果然发现在
BaseStatusBar里面：
public void start() {
mBarService.registerStatusBar(mCommandQueue, iconList, notificationKeys, notifications,
250 switches, binders);
}在这里registerStatusBar, 而这个mCommandQueue参数就是前面IStatusBar对象。
9. mCommanQueue变量
protected CommandQueue mCommandQueue;
28/**
29 * This class takes the functions from IStatusBar that come in on
30 * binder pool threads and posts messages to get them onto the main
31 * thread, and calls onto Callbacks. It also takes care of
32 * coalescing these calls so they don't stack up. For the calls
33 * are coalesced, note that they are all idempotent.
34 */
35public class CommandQueue extends IStatusBar.Stub{
/**
78 * These methods are called back on the main thread.
79 */
80 public interface Callbacks {
81 public void addIcon(String slot, int index, int viewIndex, StatusBarIcon icon);
82 public void updateIcon(String slot, int index, int viewIndex,
83 StatusBarIcon old, StatusBarIcon icon);
84 public void removeIcon(String slot, int index, int viewIndex);
85 public void addNotification(IBinder key, StatusBarNotification notification);
86 public void updateNotification(IBinder key, StatusBarNotification notification);
87 public void removeNotification(IBinder key);
88 public void disable(int state);
89 public void animateExpandNotificationsPanel();
90 public void animateCollapsePanels(int flags);
91 public void animateExpandSettingsPanel();
92 public void setSystemUiVisibility(int vis, int mask);
93 public void topAppWindowChanged(boolean visible);
94 public void setImeWindowStatus(IBinder token, int vis, int backDisposition);
95 public void setHardKeyboardStatus(boolean available, boolean enabled);
96 public void toggleRecentApps();
97 public void preloadRecentApps();
98 public void showSearchPanel();
99 public void hideSearchPanel();
100 public void cancelPreloadRecentApps();
101 public void setNavigationIconHints(int hints);
102 public void setWindowState(int window, int state);
103 }
}
发现这个方法继承IStatusBar.stub,里面有一连串的回调函数。这些回调函数在BaseStatusBar里面实现的
public abstract class BaseStatusBar extends SystemUI implements
79 CommandQueue.Callbacks {
10. 回到第6步，当在StatusBarManagerService中调用mBar.updateNotification时，根据7，8，9的分析，最终会调用在BaseStatusBar里面实现的UpdateNotification方法
updateNotification(IBinder key, StatusBarNotification notification) {
......
}在这里会将Notification显 示在StatusBar上面。
这里面主要涉及到两个NotificationManagerService和StatusBarManager的交互。
第一步。
系统启动时在ServerThread里生成StatusBarManagerService和NotificationManagerService，并将StatusBarManagerService对象作为一个参数传递到NotificationManagerService的构造方法中。
系统加载应用，SystemUI中， BaseStatusBar实现回调函数CommandQueue.Callbacks, 并调用StatusBarManagerService的registerStatusBar方法，将CommandQueue对象传递进去
第二步
Activity里调用notify方法，在NotificationManagerService将Notification封装成StatusBarNotification。
NotificationManagerService调用statusBarManagerService的updateNotification()。
第三步
StatusBarManagerService调用CommandQueue实现的回调函数，最终将Notification显示在SystemUI上面。
下一篇文章会讲怎么实现捕获Notification的内容，从而实现采集第三方播放器播放音乐的信息
