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 ，

实现MyWakefulReceiver

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

    public class MyWakefulReceiver extends WakefulBroadcastReceiver {

        @Override
        public 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");
    }

    @Override
    protected 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;
        }
    }