安卓broadcastreceiver_Android手电筒原理

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目录

• BroadcastReceiver概述
• BroadcastReceiver分类
• registerReceiver流程图
• 源码解析
• 总结

1. BroadcastReceiver概述

广播作为四大组件之一，在平时开发过程中会大量使用到，使用方式也是多种多样的，既可以自己在manifest中注册，也可以在java代码中动态注册，既可以接收由系统发出的广播，也可以接受自己定义并发送的广播。广播可以实现进程内以及跨进程之间的通信。从本文开始将分别介绍广播的注册，广播的派发，本地广播（LocalBroadcast）以及Android O上对广播的限制，本文主要介绍广播动态注册。

2. BroadcastReceiver分类

从注册方式上区分：动态注册以及静态注册（显示广播和隐式广播）
从发送方式上区分：无序广播和有序广播
从处理类型上区分：前台广播和后台广播
从运行方式上区分：普通广播和Sticky广播（已弃用）
从发送者区分：系统广播和自定义广播
此外还有protect broadcast（只允许指定应用可以发送）
sticky广播：系统保存了一部分广播的状态，当你注册的时候，不需要等到下次这个广播发出来，直接把最近上一次发送的这个广播返回给你

以上的这些概念在接下来的介绍中都会逐个涉及。

3. registerReceiver流程图

其中的APP，ContextImpl，LoadedApk，ActivityManagerProxy都在APP本身的进程中，ActivityManagerService在system_server进程中。

1. 首先在APP的进程中发起广播的注册，通过registerReceiver接口，这个方法有很多重载方法，但是最终的入口都是在ContextImpl中，后面会详细介绍
2. 从之前的Context的学习可以知道，registerReceiver最终调用的实现在ContextImpl
3. 如果没有指定处理广播的handler，则默认使用主线程的handler
4. 获取要注册的ReceiverDispatcher，在注册的Context相同的情况下，每个Receiver对应一个ReceiverDispatcher
5. 通过binder call到systemserver进行广播注册

4. 源码解析

4.1 ContextImpl.registerReceiverInternal

    private Intent registerReceiverInternal(BroadcastReceiver receiver, int userId,
            IntentFilter filter, String broadcastPermission,
            Handler scheduler, Context context, int flags) {
        IIntentReceiver rd = null;
        if (receiver != null) {
            if (mPackageInfo != null && context != null) {
                if (scheduler == null) {
                    // 注册receiver的时候可以指定接受recover的Handler
                    // 如果没有指定，则默认用主线程的handler处理
                    scheduler = mMainThread.getHandler();
                }
                // 获取IIntentReceiver
                // 这个是一个Binder对象，当广播来临时，用于AMS向客户端发起回调
                rd = mPackageInfo.getReceiverDispatcher(
                    receiver, context, scheduler,
                    mMainThread.getInstrumentation(), true);
            } else {
                if (scheduler == null) {
                    scheduler = mMainThread.getHandler();
                }
                rd = new LoadedApk.ReceiverDispatcher(
                        receiver, context, scheduler, null, true).getIIntentReceiver();
            }
        }
        try {
            // binder call至AMS，进行广播注册
            final Intent intent = ActivityManager.getService().registerReceiver(
                    mMainThread.getApplicationThread(), mBasePackageName, rd, filter,
                    broadcastPermission, userId, flags);
            if (intent != null) {
                intent.setExtrasClassLoader(getClassLoader());
                intent.prepareToEnterProcess();
            }
            return intent;
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

参数解析：
receiver：将要注册的receiver
userId：用户空间标志，默认情况下我们都只有一个user，现在一些手机推出的分身，其实就是用的第二个user，这种情况下userid会变，否则默认主空间的都是0
IntentFilter：要注册的广播的filter
broadcastPermission：指定要注册的广播的权限
scheduler：指定广播接受（也就是onReceive）所在的线程，也就是说注册的时候就可以指定好广播处理放在哪个线程，如果receiver中事情太多，可以放在另外一个线程，这样可以避免主线程被卡住
context：通过getOuterContext获取到，前面在介绍context的时候有提到，application/service/activity中获取到的是不一样的
flags:注册广播所携带的flag

4.2 LoadedApk.getReceiverDispatcher

    public IIntentReceiver getReceiverDispatcher(BroadcastReceiver r,
            Context context, Handler handler,
            Instrumentation instrumentation, boolean registered) {
        synchronized (mReceivers) {
            // 如果Context相同，每个receiver对应一个ReceiverDispatcher
            LoadedApk.ReceiverDispatcher rd = null;
            ArrayMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher> map = null;
            if (registered) {
                map = mReceivers.get(context);
                if (map != null) {
                    rd = map.get(r);
                }
            }
            if (rd == null) {
                rd = new ReceiverDispatcher(r, context, handler,
                        instrumentation, registered);
                if (registered) {
                    if (map == null) {
                        map = new ArrayMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher>();
                        mReceivers.put(context, map);
                    }
                    map.put(r, rd);
                }
            } else {
                rd.validate(context, handler);
            }
            rd.mForgotten = false;
            return rd.getIIntentReceiver();
        }
    }

mReceivers是一个二级map，一级key是context，二级key是BroadcastReceiver，value是ReceiverDispatcher。
这里的ReceiverDispatcher又是什么呢？
它是LoadedApk中的一个内部类，保存了这个receiver的信息，用于在广播派发到本进程的时候执行，上面这方法最重要的是getIIntentReceiver，这个就非常重要了，它是一个Binder对象，说在广播注册的时候将这个binder对象传递到了AMS，然后当广播派发到本进程的时候，通过这个binder对象再会调回来，它在ReceiverDispatcher创建的时候创建。

    static final class ReceiverDispatcher {
        // 是一个binder对象
        final static class InnerReceiver extends IIntentReceiver.Stub {
            final WeakReference<LoadedApk.ReceiverDispatcher> mDispatcher;
            final LoadedApk.ReceiverDispatcher mStrongRef;

            InnerReceiver(LoadedApk.ReceiverDispatcher rd, boolean strong) {
                mDispatcher = new WeakReference<LoadedApk.ReceiverDispatcher>(rd);
                mStrongRef = strong ? rd : null;
            }

            @Override
            public void performReceive(Intent intent, int resultCode, String data,
                    Bundle extras, boolean ordered, boolean sticky, int sendingUser) {
                    // 这里就是广播真正派发到本进程的时候从systemserver binder call过来执行的
                ...
        }

        final IIntentReceiver.Stub mIIntentReceiver;
        final BroadcastReceiver mReceiver;
        final Context mContext;
        final Handler mActivityThread;
        final Instrumentation mInstrumentation;
        final boolean mRegistered;
        final IntentReceiverLeaked mLocation;
        RuntimeException mUnregisterLocation;
        boolean mForgotten;
        ...
    }

到这里，广播注册在APP进程的流程就走完了，主要做了几件事：

1. 获取handler
2. 获取ReceiverDispatcher
3. 获取InnerReceiver
4. 将上面这些连带receiver的相关信息，发起binder call到ActivityManagerService

4.3 ActivityManagerService.registerReceiver

    public Intent registerReceiver(IApplicationThread caller, String callerPackage,
            IIntentReceiver receiver, IntentFilter filter, String permission, int userId,
            int flags) {
        enforceNotIsolatedCaller("registerReceiver");
        ArrayList<Intent> stickyIntents = null;
        ProcessRecord callerApp = null;
        final boolean visibleToInstantApps
                = (flags & Context.RECEIVER_VISIBLE_TO_INSTANT_APPS) != 0;
        int callingUid;
        int callingPid;
        boolean instantApp;
        synchronized(this) {
            if (caller != null) {
                // 正常来讲caller是发起binder call的客户端进程对应的ApplicationThread对象
                // 如果为null则抛异常
                callerApp = getRecordForAppLocked(caller);
                if (callerApp == null) {
                    throw new SecurityException(
                            "Unable to find app for caller " + caller
                            + " (pid=" + Binder.getCallingPid()
                            + ") when registering receiver " + receiver);
                }
                if (callerApp.info.uid != SYSTEM_UID &&
                        !callerApp.pkgList.containsKey(callerPackage) &&
                        !"android".equals(callerPackage)) {
                    throw new SecurityException("Given caller package " + callerPackage
                            + " is not running in process " + callerApp);
                }
                callingUid = callerApp.info.uid;
                callingPid = callerApp.pid;
            } else {
                callerPackage = null;
                callingUid = Binder.getCallingUid();
                callingPid = Binder.getCallingPid();
            }

            // 判断caller是否为instant app
            instantApp = isInstantApp(callerApp, callerPackage, callingUid);
            userId = mUserController.handleIncomingUser(callingPid, callingUid, userId, true,
                    ALLOW_FULL_ONLY, "registerReceiver", callerPackage);

            // 获取广播注册的filter中的action封装到list中
            Iterator<String> actions = filter.actionsIterator();
            if (actions == null) {
                ArrayList<String> noAction = new ArrayList<String>(1);
                noAction.add(null);
                actions = noAction.iterator();
            }

            // mStickyBroadcasts是一个二级map
            // 一级key是userId，二级key是广播对应的action，value是广播对应intent的list（一般只有一个intent）
            // 这里是为了查询对于当前user，本次注册的所有action对应的sticky广播的intent
            int[] userIds = { UserHandle.USER_ALL, UserHandle.getUserId(callingUid) };
            while (actions.hasNext()) {
                String action = actions.next();
                for (int id : userIds) {
                    ArrayMap<String, ArrayList<Intent>> stickies = mStickyBroadcasts.get(id);
                    if (stickies != null) {
                        ArrayList<Intent> intents = stickies.get(action);
                        if (intents != null) {
                            if (stickyIntents == null) {
                                stickyIntents = new ArrayList<Intent>();
                            }
                            stickyIntents.addAll(intents);
                        }
                    }
                }
            }
        }

        ArrayList<Intent> allSticky = null;
        // 这里不为null表示本次注册的广播中有sticky广播
        if (stickyIntents != null) {
            final ContentResolver resolver = mContext.getContentResolver();
            // 查找匹配的sticky广播
            for (int i = 0, N = stickyIntents.size(); i < N; i++) {
                Intent intent = stickyIntents.get(i);
                // 如果caller是instant app，且intent的flag不允许对instant可见，则跳过
                if (instantApp &&
                        (intent.getFlags() & Intent.FLAG_RECEIVER_VISIBLE_TO_INSTANT_APPS) == 0) {
                    continue;
                }
                // If intent has scheme "content", it will need to acccess
                // provider that needs to lock mProviderMap in ActivityThread
                // and also it may need to wait application response, so we
                // cannot lock ActivityManagerService here.
                if (filter.match(resolver, intent, true, TAG) >= 0) {
                    if (allSticky == null) {
                        allSticky = new ArrayList<Intent>();
                    }
                    allSticky.add(intent);
                }
            }
        }

        // 直接把最近的一个匹配到的sticky广播返回
        Intent sticky = allSticky != null ? allSticky.get(0) : null;
        // 广播注册的时候receiver是可以为null的，这种情况下这里直接return
        if (receiver == null) {
            return sticky;
        }

        synchronized (this) {
            // 校验caller进程是否正常
            if (callerApp != null && (callerApp.thread == null
                    || callerApp.thread.asBinder() != caller.asBinder())) {
                // Original caller already died
                return null;
            }
            // mRegisteredReceivers中存放了所有的已注册的receiver
            // 每个BroadcastReceiver对应一个InnerReceiver，即Binder对象
            // binder对象做key，value是ReceiverList
            // ReceiverList是一个ArrayList
            ReceiverList rl = mRegisteredReceivers.get(receiver.asBinder());
            if (rl == null) {
                rl = new ReceiverList(this, callerApp, callingPid, callingUid,
                        userId, receiver);
                if (rl.app != null) {
                    rl.app.receivers.add(rl);
                } else {
                    try {
                        // 如果是新创建的receiver，还需要linkToDeath
                        receiver.asBinder().linkToDeath(rl, 0);
                    } catch (RemoteException e) {
                        return sticky;
                    }
                    rl.linkedToDeath = true;
                }
                // 放入mRegisteredReceivers
                mRegisteredReceivers.put(receiver.asBinder(), rl);
            } else if (rl.uid != callingUid) {
                throw new IllegalArgumentException(
                        "Receiver requested to register for uid " + callingUid
                        + " was previously registered for uid " + rl.uid
                        + " callerPackage is " + callerPackage);
            } else if (rl.pid != callingPid) {
                throw new IllegalArgumentException(
                        "Receiver requested to register for pid " + callingPid
                        + " was previously registered for pid " + rl.pid
                        + " callerPackage is " + callerPackage);
            } else if (rl.userId != userId) {
                throw new IllegalArgumentException(
                        "Receiver requested to register for user " + userId
                        + " was previously registered for user " + rl.userId
                        + " callerPackage is " + callerPackage);
            }
            // 每一个IntentFilter对应一个BroadcastFilter
            BroadcastFilter bf = new BroadcastFilter(filter, rl, callerPackage,
                    permission, callingUid, userId, instantApp, visibleToInstantApps);
            // receiverList中存放了通过这个receiver注册的所有的filter
            // 每调用一次register就会add一次
            rl.add(bf);
            // mReceiverResolver中存放所有的BroadcastFilter
            mReceiverResolver.addFilter(bf);

            // 有匹配的sticky广播，则直接开始调度派发
            if (allSticky != null) {
                ArrayList receivers = new ArrayList();
                receivers.add(bf);

                // 对于每一个sticky广播，创建BroadcastRecord并入队（并行）
                final int stickyCount = allSticky.size();
                for (int i = 0; i < stickyCount; i++) {
                    Intent intent = allSticky.get(i);
                    // 根据flag是否有FLAG_RECEIVER_FOREGROUND判断入队是前台还是后台队列
                    BroadcastQueue queue = broadcastQueueForIntent(intent);
                    BroadcastRecord r = new BroadcastRecord(queue, intent, null,
                            null, -1, -1, false, null, null, AppOpsManager.OP_NONE, null, receivers,
                            null, 0, null, null, false, true, true, -1);
                    // 入队，并行队列
                    queue.enqueueParallelBroadcastLocked(r);
                    // 启动广播的调度，也就是开始派发广播
                    queue.scheduleBroadcastsLocked();
                }
            }
            return sticky;
        }
    }

上面主要做了几件事情：

1. 对caller的判断
2. 遍历action，查询是否有匹配的sticky广播
3. 将本次注册的广播放到mRegisteredReceivers中记录
4. 如果是sticky广播，开始派发

5. 总结

本文从App的一次广播注册发起开始，到systemserver的注册流程，整体上流程还是比较简单的，顾名思义，注册，正是把要注册的广播在systemserver中进行登记，等到这个事件真正来临的时候，从登记中取出需要被通知的receiver，这也就是后面广播的派发了。
从设计模式的角度讲，这正是经典的观察者模式。