Android WakefulBroadcastReceiver的使用

WakefulBroadcastReceiver 是一种特殊类型的广播接收器，为应用创建和管理 PARTIAL_WAKE_LOCK 。

简单来说，
WakefulBroadcastReceiver 是持有系统唤醒锁的 BroadcastReceiver ，用于执行需要保持CPU运转的场景。

注册

注册 Receiver ，

<receiver android:name=".MyWakefulReceiver"></receiver>

实现MyWakefulReceiver

重写 onReceive() 方法，使用 startWakefulService(Context context, Intent intent) 启动 Service ，

    public class MyWakefulReceiver extends WakefulBroadcastReceiver {@Overridepublic void onReceive(Context context, Intent intent) {// Start the service, keeping the device awake while the service is// launching. This is the Intent to deliver to the service.Intent service = new Intent(context, MyIntentService.class);startWakefulService(context, service);}}

Service

Service 里执行正常的逻辑，实行结束后调用 MyWakefulReceiver.completeWakefulIntent(Intent intent) 方法。

public class MyIntentService extends IntentService {public MyIntentService() {super("MyIntentService");}@Overrideprotected void onHandleIntent(Intent intent) {Bundle extras = intent.getExtras();// Do the work that requires your app to keep the CPU running.// ...// Release the wake lock provided by the WakefulBroadcastReceiver.MyWakefulReceiver.completeWakefulIntent(intent);}
}

源码分析

在 startWakefulService(Context context, Intent intent) 方法中，通过 PowerManager.WakeLock 持有了系统锁，并为每个 intent 设置了一个 id 用于标识，存储在 sActiveWakeLocks 数组中，

/*** Do a {@link android.content.Context#startService(android.content.Intent)* Context.startService}, but holding a wake lock while the service starts.* This will modify the Intent to hold an extra identifying the wake lock;* when the service receives it in {@link android.app.Service#onStartCommand* Service.onStartCommand}, it should pass back the Intent it receives there to* {@link #completeWakefulIntent(android.content.Intent)} in order to release* the wake lock.** @param context The Context in which it operate.* @param intent The Intent with which to start the service, as per* {@link android.content.Context#startService(android.content.Intent)* Context.startService}.*/public static ComponentName startWakefulService(Context context, Intent intent) {synchronized (sActiveWakeLocks) {int id = mNextId;mNextId++;if (mNextId <= 0) {mNextId = 1;}intent.putExtra(EXTRA_WAKE_LOCK_ID, id);ComponentName comp = context.startService(intent);if (comp == null) {return null;}PowerManager pm = (PowerManager)context.getSystemService(Context.POWER_SERVICE);PowerManager.WakeLock wl = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,"androidx.core:wake:" + comp.flattenToShortString());wl.setReferenceCounted(false);wl.acquire(60 * 1000);sActiveWakeLocks.put(id, wl);return comp;}}

当 Service 调用 MyWakefulReceiver.completeWakefulIntent(Intent intent) 方法时，释放系统锁，并根据 id 将此锁从数组中移除，

/*** Finish the execution from a previous {@link #startWakefulService}.  Any wake lock* that was being held will now be released.** @param intent The Intent as originally generated by {@link #startWakefulService}.* @return Returns true if the intent is associated with a wake lock that is* now released; returns false if there was no wake lock specified for it.*/public static boolean completeWakefulIntent(Intent intent) {final int id = intent.getIntExtra(EXTRA_WAKE_LOCK_ID, 0);if (id == 0) {return false;}synchronized (sActiveWakeLocks) {PowerManager.WakeLock wl = sActiveWakeLocks.get(id);if (wl != null) {wl.release();sActiveWakeLocks.remove(id);return true;}// We return true whether or not we actually found the wake lock// the return code is defined to indicate whether the Intent contained// an identifier for a wake lock that it was supposed to match.// We just log a warning here if there is no wake lock found, which could// happen for example if this function is called twice on the same// intent or the process is killed and restarted before processing the intent.Log.w("WakefulBroadcastReceiv.", "No active wake lock id #" + id);return true;}}