本地广播（ LocalBroadcastManager）源码解析

本地广播（ LocalBroadcastManager）源码解析

一、什么是LocalBroadcastManager

LocalBroadcastManager是注册和发送本地广播的helper。本地广播与全局广播有如下优势：

• 广播携带的数据不会离开app，所以无需担心数据泄露。
• 不允许其他app向你发送广播所以不必担心存在安全问题
• 相对于全局广播，本地广播效率更高

二、LocalBroadcastManager类

先看下其结构：
[image:536CD83A-7465-4287-916E-29490CC0F734-286-000C6069B3001F04/C9A82A22-9A65-4B6A-82E5-1FA647789D64.png]

我们看到其有两个内部类ReceiverRecord和BroadcastRecord.然后提供了几个方法，这几个方法就是让我们进行绑定、注册、注销本地广播用的。

先来看下其中两个内部类ReceiverRecord和BroadcastRecord。这两个内部类的作用就是用来记录广播接收器和注册的本地广播。

private static final class ReceiverRecord {
    final IntentFilter filter;
    final BroadcastReceiver receiver;
    boolean broadcasting;
    boolean dead;

    ReceiverRecord(IntentFilter _filter, BroadcastReceiver _receiver) {
        filter = _filter;
        receiver = _receiver;
    }

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder(128);
        builder.append("Receiver{");
        builder.append(receiver);
        builder.append(" filter=");
        builder.append(filter);
        if (dead) {
            builder.append(" DEAD");
        }
        builder.append("}");
        return builder.toString();
    }
}


private static final class BroadcastRecord {
    final Intent intent;
    final ArrayList<ReceiverRecord> receivers;

    BroadcastRecord(Intent _intent, ArrayList<ReceiverRecord> _receivers) {
        intent = _intent;
        receivers = _receivers;
    }
}

可以看出这两个类并未对Broadcast和Receiver做什么具体的操作。

我们一般使用本地广播都是通过下面代码：

LocalBroadcastManager.getInstance(getContext()).sendBroadcast();
LocalBroadcastManager.getInstance(getContext()).sendBroadcastSync();
LocalBroadcastManager.getInstance(getContext()).registerReceiver();
LocalBroadcastManager.getInstance(getContext()).unregisterReceiver();

可以看到都是通过LocalBroadcastManager.getInstance来获取一个LocalBroadcastManager实例，此处实际是一个单例。

private static LocalBroadcastManager mInstance;

public static LocalBroadcastManager getInstance(Context context) {
    //采用DCL模式的创建单例
    synchronized (mLock) {
        if (mInstance == null) {
            mInstance = new LocalBroadcastManager(context.getApplicationContext());
        }
        return mInstance;
    }
}
//LocalBroadcastManager唯一的构造函数。
private LocalBroadcastManager(Context context) {
    mAppContext = context;
//用传入的context参数构造handler
    mHandler = new Handler(context.getMainLooper()) {

        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_EXEC_PENDING_BROADCASTS:
                    executePendingBroadcasts();
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    };
}

接下来我们来依次看下LocalBroadcastManager提供的几个可调用函数。

registerReceiver

public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
    synchronized (mReceivers) {
        ReceiverRecord entry = new ReceiverRecord(filter, receiver);//构造一个ReceiverRecord实例
        ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);//1、查找当前receiver是否已有对应的filters
        if (filters == null) {
            filters = new ArrayList<>(1);
            mReceivers.put(receiver, filters);
        }
        filters.add(entry);
        //2遍历filter的Action存入list中
        for (int i=0; i<filter.countActions(); i++) {
            String action = filter.getAction(i);
            ArrayList<ReceiverRecord> entries = mActions.get(action);
            if (entries == null) {
                entries = new ArrayList<ReceiverRecord>(1);
                mActions.put(action, entries);
            }
            entries.add(entry);
        }
    }
}

registerReceiver主要做了两件事：把与Receiver对应的filters存到一个list中，把filter包含的action存到一个list中。这其中涉及了两个变量mReceivers和mActions，他俩定义如下

private final HashMap<BroadcastReceiver, ArrayList<ReceiverRecord>> mReceivers
        = new HashMap<>();
private final HashMap<String, ArrayList<ReceiverRecord>> mActions = new HashMap<>();

可以看出mReceivers一个以BroadcastReceiver为key以ReceiverRecord list为value的HashMap。mActions是一个以String为key以ReceiverRecord为value的HashMap。
定义mReceivers的目的是可以根据Receiver找到其所有的filter，定义mActions的目的则是可以根据某个action找到所有的Receiver。

unregisterReceiver

public void unregisterReceiver(BroadcastReceiver receiver) {
    synchronized (mReceivers) {
        final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);//1 在mReceivers移除对应的receiver
        if (filters == null) {
            return;
        }
        for (int i=filters.size()-1; i>=0; i--) {//2 遍历注释1获得的ReceiverRecord list，该list中存放的是要注销的receiver所对应的filter。
            final ReceiverRecord filter = filters.get(i);
            filter.dead = true;//3 置dead为true表示当前ReceiverRecord不在活跃
            for (int j=0; j<filter.filter.countActions(); j++) { // 4 遍历当前ReceiverRecord的filter包含的action
                final String action = filter.filter.getAction(j);
                final ArrayList<ReceiverRecord> receivers = mActions.get(action);// 5 根据action查找对应的ReceiverRecord list
                if (receivers != null) {
                    for (int k=receivers.size()-1; k>=0; k--) {
                        final ReceiverRecord rec = receivers.get(k);
                        if (rec.receiver == receiver) { // 6 注释5中查到的list中如果包含要注销的receiver则把其dead置为true
                            rec.dead = true;
                            receivers.remove(k);
                        }
                    }
                    if (receivers.size() <= 0) {//7 receivers.size()小于等于0表示没有recriver会接收receivers对应的action
                        mActions.remove(action);
                    }
                }
            }
        }
    }
}

总结下LocalBroadcastManager注册和注销过程：
首先LocalBroadcastManager定义了两个hashMap，mReceivers和mActions。mReceivers一个以BroadcastReceiver为key以ReceiverRecord list为value的HashMap。mActions是一个以String为key以ReceiverRecord为value的HashMap。
我们注册时会传入receiver、filter，receiver就是我们要注册的广播接收器而filter则表示我们的接收器要接收哪种类型的广播（实际filter中的action，每个action表示一个广播类型）。LocalBroadcastManager的注册过程是先利用传入的receiver、filter构造一个ReceiverRecord实例这样这个ReceiverRecord实例就建立一个receiver和其filter的记录块。然后就会以receiver为key把之前构造的ReceiverRecord实例存入mReceivers中。同样用类似操作会以注册时传入的filter参数中的action为key把之前构造的ReceiverRecord实例存入mActions中。
在注销时会根据receiver把其对应的ReceiverRecord list从mReceivers中移除。因为在注册时构造的ReceiverRecord存了两遍在mReceivers和mActions都存过一遍，所以此时还需要把mActions中要注销的receiver之前存入的ReceiverRecord移除。

由上面可以看出注册和注销正好是相反的操作，注册是在mReceivers和mActions存入对应的ReceiverRecord，注销则是相反的要在mReceivers和mActions移除对应的ReceiverRecord。

sendBroadcast

public boolean sendBroadcast(Intent intent) {
    synchronized (mReceivers) {
        //1 获取传入的intent参数的相关属性
        final String action = intent.getAction();
        final String type = intent.resolveTypeIfNeeded(
                mAppContext.getContentResolver());
        final Uri data = intent.getData();
        final String scheme = intent.getScheme();
        final Set<String> categories = intent.getCategories();

        final boolean debug = DEBUG ||
                ((intent.getFlags() & Intent.FLAG_DEBUG_LOG_RESOLUTION) != 0);
        if (debug) Log.v(
                TAG, "Resolving type " + type + " scheme " + scheme
                + " of intent " + intent);

        //2 根据intent的action获取对应的ReceiverRecord list
        ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());
        if (entries != null) {
            if (debug) Log.v(TAG, "Action list: " + entries);

            ArrayList<ReceiverRecord> receivers = null;
            for (int i=0; i<entries.size(); i++) {//3 遍历注释2处获取的list
                ReceiverRecord receiver = entries.get(i);
                if (debug) Log.v(TAG, "Matching against filter " + receiver.filter);

                if (receiver.broadcasting) {
                    if (debug) {
                        Log.v(TAG, "  Filter's target already added");
                    }
                    continue;
                }
                 //4 判断当前的ReceiverRecord所记录的filter是否跟传入的intent相匹配
                int match = receiver.filter.match(action, type, scheme, data,
                        categories, "LocalBroadcastManager");
                if (match >= 0) {
                    if (debug) Log.v(TAG, "  Filter matched!  match=0x" +
                            Integer.toHexString(match));
                    if (receivers == null) {
                        receivers = new ArrayList<ReceiverRecord>();
                    }
                    //5 如果匹配则添加到receivers中，receivers表示接受此广播的receiver list
                    receivers.add(receiver);
                    //6 broadcasting置为true表示正在处理广播
                    receiver.broadcasting = true;
                } else {
                    if (debug) {
                        String reason;
                        switch (match) {
                            case IntentFilter.NO_MATCH_ACTION: reason = "action"; break;
                            case IntentFilter.NO_MATCH_CATEGORY: reason = "category"; break;
                            case IntentFilter.NO_MATCH_DATA: reason = "data"; break;
                            case IntentFilter.NO_MATCH_TYPE: reason = "type"; break;
                            default: reason = "unknown reason"; break;
                        }
                        Log.v(TAG, "  Filter did not match: " + reason);
                    }
                }
            }

            if (receivers != null) {//7 如果接收此广播的receiver list不为空则依次添加到mPendingBroadcasts中（mPendingBroadcasts表示需要处理的广播），然后通过发送MSG_EXEC_PENDING_BROADCASTS消息
                for (int i=0; i<receivers.size(); i++) {
                    receivers.get(i).broadcasting = false;
                }
                mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
                if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
                    mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
                }
                return true;
            }
        }
    }
    return false;
}

sendBroadcast是异步发送广播，由上面注释我们可以知道发送的流程是会根据intent中的action找到可以接收此广播的receiver list（实际找到的是ReceiverRecord list但是ReceiverRecord中记录了相关的Receiver和filter信息），然后会把receiver的filter和传入的intent做匹配对比，如果匹配上那么就把当前的信息块（ReceiverRecord）添加到一个名为receivers的list中，此处的receivers表示接可以接收此广播的receiver list，之后会把receivers中的元素添加到mPendingBroadcasts中（mPendingBroadcasts表示需要处理的广播，它是一个BroadcastRecord list，BroadcastRecord则是用来记录广播和其对应的receivers）最后会通过handler发送一个MSG_EXEC_PENDING_BROADCASTS消息。消息的处理在LocalBroadcastManager构造函数中我们已经看过了，当收到MSG_EXEC_PENDING_BROADCASTS消息时会调用executePendingBroadcasts()。

private void executePendingBroadcasts() {
    while (true) {
        final BroadcastRecord[] brs;
        //把mPendingBroadcasts中的元素转换为数组
        synchronized (mReceivers) {
            final int N = mPendingBroadcasts.size();
            if (N <= 0) {
                return;
            }
            brs = new BroadcastRecord[N];
            mPendingBroadcasts.toArray(brs);
            mPendingBroadcasts.clear();
        }
        //遍历上边的brs数组，取出BroadcastRecord中的ReceiverRecord list（此处的ReceiverRecord list可以看做是接收广播的receiver list），如果ReceiverRecord的记录还“有效”那么调用其中存储的Receiver的onReceive，至此广播接收器就收到了要接收的广播
        for (int i=0; i<brs.length; i++) {
            final BroadcastRecord br = brs[i];
            final int nbr = br.receivers.size();
            for (int j=0; j<nbr; j++) {
                final ReceiverRecord rec = br.receivers.get(j);
                if (!rec.dead) {
                    rec.receiver.onReceive(mAppContext, br.intent);
                }
            }
        }
    }
}

executePendingBroadcasts主要完成的还是广播的分发。我们可以看到本地广播的异步分发其实是通过Handler完成的。

sendBroadcastSync

sendBroadcast是异步发送广播，sendBroadcastSync则是同步发送广播。

public void sendBroadcastSync(Intent intent) {
    if (sendBroadcast(intent)) {
        executePendingBroadcasts();
    }
}

可以看到它的内部是调用了sendBroadcast但是跟sendBroadcast不同的是广播发送完后它直接调用了executePendingBroadcasts来对广播进行分发。

总结：
我们通过对LocalBroadcastManager分析可知，本地广播通过Handler来实现发送和分发的异步处理，我们还可以知道本地广播之所以不能跨进程、效率高是因为，其注册、分发等都是通过本地变量完成的，而不像全局广播那样通过binder机制实现。