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  众所周知，notification是在状态栏上显示的可以定制声音，震动，Led灯，单击跳转，显示内容的通知。通常应用中要发送一个notification都是通过以下方式：

NotificationManager manager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE); ……//创建notificationmanager.notify(1, notification);

我们可以在创建notification时指定notification的声音，震动，Led灯，单击跳转，显示内容，通过NotificationManager发送出去，最终在状态栏上显示，在显示过程中对声音，震动，Led灯做同步处理。

  先来看看NotificationManager发送通知后的显示时序图：

   从时序图上看出，同android所有的Manager类一样，最终都是通过调用Framework中的Service类实现所要完成的操作。下面详细描述每一步所完成的操作：

   1，在调用NotificationManager.notify后，NotificationManager通过内部的NotificationManagerService代理处理后续的发送任务，这里调用的是NotificationManagerService.enqueueNotification.

2，NotificationManagerService.enqueueNotification中反复调用了自己的几个方法，最终调用的是enqueueNotificationInternal发送，这个方法里完成了所有对notification发送的处理，也包括了对notification的更新。此类在Frameworks的services目录下面，方法如下：

public void enqueueNotificationInternal(String pkg, int callingUid, int callingPid,            String tag, int id, int priority, Notification notification, int[] idOut)    {        checkIncomingCall(pkg);        // Limit the number of notifications that any given package except the android        // package can enqueue.  Prevents DOS attacks and deals with leaks.        if (!"android".equals(pkg)) {            synchronized (mNotificationList) {                int count = 0;                final int N = mNotificationList.size();                for (int i=0; i
   
    = MAX_PACKAGE_NOTIFICATIONS) {                            Slog.e(TAG, "Package has already posted " + count                                    + " notifications.  Not showing more.  package=" + pkg);                            return;                        }                    }                }            }        }        // This conditional is a dirty hack to limit the logging done on        //     behalf of the download manager without affecting other apps.        if (!pkg.equals("com.android.providers.downloads")                || Log.isLoggable("DownloadManager", Log.VERBOSE)) {            EventLog.writeEvent(EventLogTags.NOTIFICATION_ENQUEUE, pkg, id, tag,                    notification.toString());        }        if (pkg == null || notification == null) {            throw new IllegalArgumentException("null not allowed: pkg=" + pkg                    + " id=" + id + " notification=" + notification);        }        if (notification.icon != 0) {            if (notification.contentView == null) {                throw new IllegalArgumentException("contentView required: pkg=" + pkg                        + " id=" + id + " notification=" + notification);            }        }        synchronized (mNotificationList) {            NotificationRecord r = new NotificationRecord(pkg, tag, id,                     callingUid, callingPid,                     priority,                    notification);            NotificationRecord old = null;            int index = indexOfNotificationLocked(pkg, tag, id);            if (index < 0) {                mNotificationList.add(r);            } else {                old = mNotificationList.remove(index);                mNotificationList.add(index, r);                // Make sure we don't lose the foreground service state.                if (old != null) {                    notification.flags |=                        old.notification.flags&Notification.FLAG_FOREGROUND_SERVICE;                }            }            // Ensure if this is a foreground service that the proper additional            // flags are set.            if ((notification.flags&Notification.FLAG_FOREGROUND_SERVICE) != 0) {                notification.flags |= Notification.FLAG_ONGOING_EVENT                        | Notification.FLAG_NO_CLEAR;            }            if (notification.icon != 0) {                StatusBarNotification n = new StatusBarNotification(pkg, id, tag,                        r.uid, r.initialPid, notification);                n.priority = r.priority;                if (old != null && old.statusBarKey != null) {                    r.statusBarKey = old.statusBarKey;                    long identity = Binder.clearCallingIdentity();                    try {                        mStatusBar.updateNotification(r.statusBarKey, n);                    }                    finally {                        Binder.restoreCallingIdentity(identity);                    }                } else {                    long identity = Binder.clearCallingIdentity();                    try {                        r.statusBarKey = mStatusBar.addNotification(n);                        mAttentionLight.pulse();                    }                    finally {                        Binder.restoreCallingIdentity(identity);                    }                }                sendAccessibilityEvent(notification, pkg);            } else {                Slog.e(TAG, "Ignoring notification with icon==0: " + notification);                if (old != null && old.statusBarKey != null) {                    long identity = Binder.clearCallingIdentity();                    try {                        mStatusBar.removeNotification(old.statusBarKey);                    }                    finally {                        Binder.restoreCallingIdentity(identity);                    }                }            }            // If we're not supposed to beep, vibrate, etc. then don't.            if (((mDisabledNotifications & StatusBarManager.DISABLE_NOTIFICATION_ALERTS) == 0)                    && (!(old != null                        && (notification.flags & Notification.FLAG_ONLY_ALERT_ONCE) != 0 ))                    && mSystemReady) {                final AudioManager audioManager = (AudioManager) mContext                .getSystemService(Context.AUDIO_SERVICE);                // sound                final boolean useDefaultSound =                    (notification.defaults & Notification.DEFAULT_SOUND) != 0;                if (useDefaultSound || notification.sound != null) {                    Uri uri;                    if (useDefaultSound) {                        uri = Settings.System.DEFAULT_NOTIFICATION_URI;                    } else {                        uri = notification.sound;                    }                    boolean looping = (notification.flags & Notification.FLAG_INSISTENT) != 0;                    int audioStreamType;                    if (notification.audioStreamType >= 0) {                        audioStreamType = notification.audioStreamType;                    } else {                        audioStreamType = DEFAULT_STREAM_TYPE;                    }                    mSoundNotification = r;                    // do not play notifications if stream volume is 0                    // (typically because ringer mode is silent).                    if (audioManager.getStreamVolume(audioStreamType) != 0) {                        long identity = Binder.clearCallingIdentity();                        try {                            mSound.play(mContext, uri, looping, audioStreamType);                        }                        finally {                            Binder.restoreCallingIdentity(identity);                        }                    }                }                // vibrate                final boolean useDefaultVibrate =                    (notification.defaults & Notification.DEFAULT_VIBRATE) != 0;                if ((useDefaultVibrate || notification.vibrate != null)                        && audioManager.shouldVibrate(AudioManager.VIBRATE_TYPE_NOTIFICATION)) {                    mVibrateNotification = r;                    mVibrator.vibrate(useDefaultVibrate ? DEFAULT_VIBRATE_PATTERN                                                        : notification.vibrate,                              ((notification.flags & Notification.FLAG_INSISTENT) != 0) ? 0: -1);                }            }            // this option doesn't shut off the lights            // light            // the most recent thing gets the light            mLights.remove(old);            if (mLedNotification == old) {                mLedNotification = null;            }            //Slog.i(TAG, "notification.lights="            //        + ((old.notification.lights.flags & Notification.FLAG_SHOW_LIGHTS) != 0));            if ((notification.flags & Notification.FLAG_SHOW_LIGHTS) != 0) {                mLights.add(r);                updateLightsLocked();            } else {                if (old != null                        && ((old.notification.flags & Notification.FLAG_SHOW_LIGHTS) != 0)) {                    updateLightsLocked();                }            }        }        idOut[0] = id;    }
   

3，首先是对Notification的属性做了验证，同一个应用发送notification数量不能超过MAX_PACKAGE_NOTIFICATIONS的值99，其余的验证包括notification不为空，contentView 不为空的验证，条件都满足后先创建一个NotificationRecord

NotificationRecord r = new NotificationRecord(pkg, tag, id,                     callingUid, callingPid,                     priority,                    notification);

if (notification.icon != 0) {                StatusBarNotification n = new StatusBarNotification(pkg, id, tag,                        r.uid, r.initialPid, notification);                n.priority = r.priority;                if (old != null && old.statusBarKey != null) {                    r.statusBarKey = old.statusBarKey;                    long identity = Binder.clearCallingIdentity();                    try {                        mStatusBar.updateNotification(r.statusBarKey, n);                    }                    finally {                        Binder.restoreCallingIdentity(identity);                    }                } else {                    long identity = Binder.clearCallingIdentity();                    try {                        r.statusBarKey = mStatusBar.addNotification(n);                        mAttentionLight.pulse();                    }                    finally {                        Binder.restoreCallingIdentity(identity);                    }                }                sendAccessibilityEvent(notification, pkg);            } else {                Slog.e(TAG, "Ignoring notification with icon==0: " + notification);                if (old != null && old.statusBarKey != null) {                    long identity = Binder.clearCallingIdentity();                    try {                        mStatusBar.removeNotification(old.statusBarKey);                    }                    finally {                        Binder.restoreCallingIdentity(identity);                    }                }            }

4，PhoneStatusBar在SystemUI的包里面，在它的addNotification方法中完成了所有Notification在状态栏上的显示操作。对PhoneStatusBar.addNotification的调用是采用Handle的方式，此Handle的Message成功发送后，StatusBarManagerService的addNotification就返回了， 然后再返回到NotificationManagerService的调用处，接着处理后续的声音播放NotificationPlayer.play、震动Vibrator.vibrator、Led指示灯updateLightsLocked，这些都是和PhoneStatusBar的addNotification同时在进行，当Notification在显示的时候，声音，震动，Led的处理就同步完成了。

5，在PhoneStatusBar.addNotification中调用addNotificationView将notification的icon添加到statusBar，还有notification在ExpandedView（状态栏拉开的那个界面）中显示的View也是在此方法中创建。

接着调用tick方法，显示Notification的tick提示，就是notification显示时在状态栏上滚动的那个效果。

至此，Notification的显示就完成了。StatusBar上除了Notification的显示还有StatusIcon（状态图标），状态图标的显示比较简单。系统的状态图标都是在SystemUI中控制显示，通过注册监听，接收系统广播控制图标的显示。SystemUI在启动时系统设置的状态图标都会加入到状态栏的view中，但不会设置显示，当相应事件发生后，注册的监听或广播就会设置相应图标显示。要显示的状态图标都需要现在/frameworks/base/core/values/config.xml中设置名称，否则系统无法显示状态图标。

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