Android 的广播发送跟接收（二）

1、简介

广播的发送者将广播发送到ActivityManagerService，ActivityManagerService接收到这个广播以后，就会在自己的注册中心查看有哪些广播接收器订阅了该广播，然后把这个广播逐一发送到这些广播接收器中，但是ActivityManagerService并不等待广播接收器处理这些广播就返回了，因此，广播的发送和处理是异步的。广播的发送路径就是从发送者到ActivityManagerService，再从ActivityManagerService 到接收者，这中间的两个过程都是通过Binder进程间通信机制来完成的。

image.png

2、源码分析：

step1、先从调用的地方开始：Activity 中调用sendBroadcast方法，再调用ContextImpl 的sendBroadcast

@Override
public void sendBroadcast(Intent intent) {
    warnIfCallingFromSystemProcess();
    String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());
    try {
        intent.prepareToLeaveProcess(this);
        ActivityManager.getService().broadcastIntent(
                mMainThread.getApplicationThread(), intent, resolvedType, null,
                Activity.RESULT_OK, null, null, null, AppOpsManager.OP_NONE, null, false, false,
                getUserId());
    } catch (RemoteException e) {
        throw e.rethrowFromSystemServer();
    }
}

ActivityManager.getService()获取AMS 调用broadcastIntent 方法

final int broadcastIntentLocked(ProcessRecord callerApp,
        String callerPackage, Intent intent, String resolvedType,
        IIntentReceiver resultTo, int resultCode, String resultData,
        Bundle resultExtras, String[] requiredPermissions, int appOp, Bundle bOptions,
        boolean ordered, boolean sticky, int callingPid, int callingUid, int userId) {
    intent = new Intent(intent);

   
    ........

    // Figure out who all will receive this broadcast.
    List receivers = null;
    List registeredReceivers = null;

    if (intent.getComponent() == null) {

       
        } else {
            registeredReceivers = mReceiverResolver.queryIntent(intent,resolvedType, false
             /*defaultOnly*/, userId);//    
        }
    }
    ..........

    final boolean replacePending =
            (intent.getFlags()&Intent.FLAG_RECEIVER_REPLACE_PENDING) != 0;

    if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST, "Enqueueing broadcast: " + intent.getAction()
            + " replacePending=" + replacePending);

    int NR = registeredReceivers != null ? registeredReceivers.size() : 0;
   

    // Merge into one list.
    int ir = 0;
    if (receivers != null) {
        // A special case for PACKAGE_ADDED: do not allow the package
        // being added to see this broadcast.  This prevents them from
        // using this as a back door to get run as soon as they are
        // installed.  Maybe in the future we want to have a special install
        // broadcast or such for apps, but we'd like to deliberately make
        // this decision.
        String skipPackages[] = null;
        if (Intent.ACTION_PACKAGE_ADDED.equals(intent.getAction())
                || Intent.ACTION_PACKAGE_RESTARTED.equals(intent.getAction())
                || Intent.ACTION_PACKAGE_DATA_CLEARED.equals(intent.getAction())) {
            Uri data = intent.getData();
            if (data != null) {
                String pkgName = data.getSchemeSpecificPart();
                if (pkgName != null) {
                    skipPackages = new String[] { pkgName };
                }
            }
        } else if (Intent.ACTION_EXTERNAL_APPLICATIONS_AVAILABLE.equals(intent.getAction())) {
            skipPackages = intent.getStringArrayExtra(Intent.EXTRA_CHANGED_PACKAGE_LIST);
        }
        if (skipPackages != null && (skipPackages.length > 0)) {
            for (String skipPackage : skipPackages) {
                if (skipPackage != null) {
                    int NT = receivers.size();
                    for (int it=0; it= curt.priority) {
                // Insert this broadcast record into the final list.
                receivers.add(it, curr);
                ir++;
                curr = null;
                it++;
                NT++;
            } else {
                // Skip to the next ResolveInfo in the final list.
                it++;
                curt = null;
            }
        }
    }
    while (ir < NR) {
        if (receivers == null) {
            receivers = new ArrayList();
        }
        receivers.add(registeredReceivers.get(ir));
        ir++;
    }

    if (isCallerSystem) {
        checkBroadcastFromSystem(intent, callerApp, callerPackage, callingUid,
                isProtectedBroadcast, receivers);
    }

    if ((receivers != null && receivers.size() > 0)
            || resultTo != null) {
        BroadcastQueue queue = broadcastQueueForIntent(intent);
        BroadcastRecord r = new BroadcastRecord(queue, intent, callerApp,
                callerPackage, callingPid, callingUid, callerInstantApp, resolvedType,
                requiredPermissions, appOp, brOptions, receivers, resultTo, resultCode,
                resultData, resultExtras, ordered, sticky, false, userId);

        if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST, "Enqueueing ordered broadcast " + r
                + ": prev had " + queue.mOrderedBroadcasts.size());
        if (DEBUG_BROADCAST) Slog.i(TAG_BROADCAST,
                "Enqueueing broadcast " + r.intent.getAction());

        final BroadcastRecord oldRecord =
                replacePending ? queue.replaceOrderedBroadcastLocked(r) : null;
        if (oldRecord != null) {
            // Replaced, fire the result-to receiver.
            if (oldRecord.resultTo != null) {
                final BroadcastQueue oldQueue = broadcastQueueForIntent(oldRecord.intent);
                try {
                    oldQueue.performReceiveLocked(oldRecord.callerApp, oldRecord.resultTo,
                            oldRecord.intent,
                            Activity.RESULT_CANCELED, null, null,
                            false, false, oldRecord.userId);
                } catch (RemoteException e) {
                    Slog.w(TAG, "Failure ["
                            + queue.mQueueName + "] sending broadcast result of "
                            + intent, e);

                }
            }
        } else {
            queue.enqueueOrderedBroadcastLocked(r);
            queue.scheduleBroadcastsLocked();
        }
    } else {
        // There was nobody interested in the broadcast, but we still want to record
        // that it happened.
        if (intent.getComponent() == null && intent.getPackage() == null
                && (intent.getFlags()&Intent.FLAG_RECEIVER_REGISTERED_ONLY) == 0) {
            // This was an implicit broadcast... let's record it for posterity.
            addBroadcastStatLocked(intent.getAction(), callerPackage, 0, 0, 0);
        }
    }

    return ActivityManager.BROADCAST_SUCCESS;
}

解读一下：1、这里就通过mReceiverResolver.queryIntent函数将这个BroadcastFilter实例取回来。由于注册一个广播类型的接收器可能有多个，所以这里把所有符合条件的的BroadcastFilter实例放在一个List中，然后返回来。在我们这个场景中，这个List就只有一个BroadcastFilter实例。

2、final boolean replacePending = (intent.getFlags()&Intent.FLAG_RECEIVER_REPLACE_PENDING) != 0;对这个方法是这里是查看一下这个intent的Intent.FLAG_RECEIVER_REPLACE_PENDING位有没有设置，如果设置了的话，ActivityManagerService就会在当前的系统中查看有没有相同的intent还未被处理，如果有的话，就有当前这个新的intent来替换旧的intent。这里，我们没有设置intent的Intent.FLAG_RECEIVER_REPLACE_PENDING位，因此，这里的replacePending变量为false
3、最后会调用 queue.enqueueOrderedBroadcastLocked(r);
queue.scheduleBroadcastsLocked();这个queue是BroadcastQueue类。将BroadcastRecord 加入到队列中。
4、看看scheduleBroadcastsLocked方法。
public void scheduleBroadcastsLocked() {
if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST, "Schedule broadcasts ["
+ mQueueName + "]: current="
+ mBroadcastsScheduled);

    if (mBroadcastsScheduled) {
        return;
    }
    mHandler.sendMessage(mHandler.obtainMessage(BROADCAST_INTENT_MSG, this));
    mBroadcastsScheduled = true;
}

private final class BroadcastHandler extends Handler {
    public BroadcastHandler(Looper looper) {
        super(looper, null, true);
    }

    @Override
    public void handleMessage(Message msg) {
        switch (msg.what) {
            case BROADCAST_INTENT_MSG: {
                if (DEBUG_BROADCAST) Slog.v(
                        TAG_BROADCAST, "Received BROADCAST_INTENT_MSG");
                processNextBroadcast(true);
            } break;
            case BROADCAST_TIMEOUT_MSG: {
                synchronized (mService) {
                    broadcastTimeoutLocked(true);
                }
            } break;
        }
    }
}

然后调用processNextBroadcast 来send 广播消息，具体看这个方法
public final class ActivityManagerService extends ActivityManagerNative
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
......

private final void processNextBroadcast(boolean fromMsg) {
    synchronized(this) {
        BroadcastRecord r;

        ......

        if (fromMsg) {
            mBroadcastsScheduled = false;
        }

        // First, deliver any non-serialized broadcasts right away.
        while (mParallelBroadcasts.size() > 0) {
            r = mParallelBroadcasts.remove(0);
            ......
            final int N = r.receivers.size();
            ......
            for (int i=0; i

}

这里传进来的参数fromMsg为true，于是把mBroadcastScheduled重新设为false，这样，下一个广播就能进入到消息队列中进行处理了。前面我们在Step 5中，把一个广播记录块BroadcastRecord放在了mParallelBroadcasts中，因此，这里就把它取出来进行处理了。广播记录块BroadcastRecord的receivers列表中包含了要接收这个广播的目标列表，即前面我们注册的广播接收器，用BroadcastFilter来表示，这里while循环中的for循环就是把这个广播发送给每一个订阅了该广播的接收器了，通过deliverToRegisteredReceiverLocked函数执行。

image.png

然后调用performReceiveLocked方法，当看到截图中的scheduleRegisteredReceiver，回到ActivityThread中

image.png

在上一节我分享过这个receiver.performReceive，这个receiver实际上是LoadedApk.ReceiverDispatcher

image.png

然后调用LoadedApk.ReceiverDispatcher.中performReceive方法

1

图中截图就是收到消息地方
public final Runnable getRunnable() {
return () -> {
final BroadcastReceiver receiver = mReceiver;
final boolean ordered = mOrdered;

                .........
                try {
                    ClassLoader cl = mReceiver.getClass().getClassLoader();
                    intent.setExtrasClassLoader(cl);
                    intent.prepareToEnterProcess();
                    setExtrasClassLoader(cl);
                    receiver.setPendingResult(this);
                    //回调接收消息的地方
                    receiver.onReceive(mContext, intent);
                } catch (Exception e) {
                    if (mRegistered && ordered) {
                        if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,
                                "Finishing failed broadcast to " + mReceiver);
                        sendFinished(mgr);
                    }
                    if (mInstrumentation == null ||
                            !mInstrumentation.onException(mReceiver, e)) {
                        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                        throw new RuntimeException(
                                "Error receiving broadcast " + intent
                                        + " in " + mReceiver, e);
                    }
                }

                if (receiver.getPendingResult() != null) {
                    finish();
                }
                Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            };
        }

到此处消息发送到接收完成
最后，我们总结一下这个Android应用程序发送广播的过程：

       1. ，计数器服务CounterService通过sendBroadcast把一个广播通过Binder进程间通信机制发送给ActivityManagerService，ActivityManagerService根据这个广播的Action类型找到相应的广播接收器，然后把这个广播放进自己的消息队列中去，就完成第一阶段对这个广播的异步分发了；

    2. ，ActivityManagerService在消息循环中处理这个广播，并通过Binder进程间通信机制把这个广播分发给注册的广播接收分发器ReceiverDispatcher，ReceiverDispatcher把这个广播放进MainActivity所在的线程的消息队列中去，就完成第二阶段对这个广播的异步分发了；

    3. ， ReceiverDispatcher的内部类Args在MainActivity所在的线程消息循环中处理这个广播，最终是将这个广播分发给所注册的BroadcastReceiver实例的onReceive函数进行处理。

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作者：罗升阳
来源：CSDN
原文：https://blog.csdn.net/luoshengyang/article/details/6744448
版权声明：本文为博主原创文章，转载请附上博文链接！