Android应用内广播LocalBroadcastManager机制

1. 简介

通常我们在使用android广播的时候都会直接将广播注册到的ams当中，由于ams任务繁忙，一般可能不会立即能处理到我们发出的广播，如果我们使用广播是在应用内的单个进程中使用，则完全可以采用localbroadcastmanager来处理。localbroadcastmanager采用的是handler的消息机制来处理的广播，而注册到系统中的是通过binder机制实现的，速度是应用内广播要快很多。不过由于handler的消息机制是为了同一个进程的多线程间进行通信的，因而跨进程时无法使用应用内广播。

1.1 使用

在使用上和普通的broadcast类似，主要分5步。具体如下：

//1. 自定义广播接收者
public class localreceiver extends broadcastreceiver {
    public void onreceive(context context, intent intent) {
        ...
    }
}
localreceiver localreceiver = new localreceiver();

//2. 注册广播
localbroadcastmanager.getinstance(context)
             .registerreceiver(localreceiver, new intentfilter(“test”));
//4. 发送广播
localbroadcastmanager.getinstance(context).sendbroadcast(new intent("test"));
//5. 取消注册广播
localbroadcastmanager.getinstance(context).unregisterreceiver(localreceiver);

自定义广播和普通的广播一样，在注册广播的时候将该广播接受者注册到localbroadcatmanager中。当发生时也是调用localbroadcastmanager的sendbroadcast进行发生。同样在不使用时记得取消广播注册。

2. localbroadcastmanager

2.1 初始化

localbroadcastmanager采用的是单例模式，其构造函数是私有的，获取该类实例的方法是getinstance，具体代码如下：

  private final handler mhandler;

    private static final object mlock = new object();
    private static localbroadcastmanager minstance;

    public static localbroadcastmanager getinstance(context context) {
        synchronized (mlock) {
            if (minstance == null) {
                minstance = new localbroadcastmanager(context.getapplicationcontext());
            }
            return minstance;
        }
    }

    private localbroadcastmanager(context context) {
        mappcontext = context;
        //mhandler是主线程的
        mhandler = new handler(context.getmainlooper()) {

            @override
            public void handlemessage(message msg) {
                switch (msg.what) {
                    case msg_exec_pending_broadcasts:
                        executependingbroadcasts();//这里去执行广播分发
                        break;
                    default:
                        super.handlemessage(msg);
                }
            }
        };
    }

在构造函数中创建了一个mhandler，该mhandler关联的是主线程的looper。即消息处理时都在主线程中处理。

2.2 registerreceiver

public void registerreceiver(broadcastreceiver receiver, intentfilter filter) {
        //在注册，取消注册，发送广播的时候都需要先获取mreceivers的锁
        synchronized (mreceivers) {
            //新建一个receiverrecord实体表示该receiver及对应的filter
            receiverrecord entry = new receiverrecord(filter, receiver);
            //获取receiver对应的filters
            arraylist filters = mreceivers.get(receiver);
            if (filters == null) {
                //如果该receiver没有对应的filters则，新建一个。
                filters = new arraylist(1);
                mreceivers.put(receiver, filters);
            }
            //将filter放入该receiver对应的filters中
            filters.add(filter);
            for (int i=0; i entries = mactions.get(action);
                if (entries == null) {
                    entries = new arraylist(1);
                    //将action放入mactions中
                    mactions.put(action, entries);
                }
                entries.add(entry);
            }
        }
    }

注册的时候也就是将receiver自己和对应的filter及action放入到mreceivers和mactions当中。代码比较简单。

2.3 发送广播sendbroadcast

public boolean sendbroadcast(intent intent) {
        synchronized (mreceivers) {
            final string action = intent.getaction();
            final string type = intent.resolvetypeifneeded(
                    mappcontext.getcontentresolver());
            final uri data = intent.getdata();
            final string scheme = intent.getscheme();
            final set 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);

            arraylist entries = mactions.get(intent.getaction());
            if (entries != null) {
                if (debug) log.v(tag, "action list: " + entries);

                arraylist receivers = null;
                for (int i=0; i<entries.size(); i++)="" {="" receiverrecord="" receiver="entries.get(i);" if="" (debug)="" log.v(tag,="" "matching="" against="" filter="" "="" +="" receiver.filter);="" (receiver.broadcasting)="" filter's="" target="" already="" added");="" }="" continue;="" int="" match="receiver.filter.match(action," type,="" scheme,="" data,="" categories,="" "localbroadcastmanager");="" (match="">= 0) {
                        if (debug) log.v(tag, "  filter matched!  match=0x" +
                                integer.tohexstring(match));
                        if (receivers == null) {
                            receivers = new arraylist();
                        }
                        receivers.add(receiver);
                        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) {
                    for (int i=0; i
主要步骤：1.根据intent的action来查询相应的广播接收者列表； 

2.创建相应广播，添加到mpendingbroadcasts队列； 

3.发送msg_exec_pending_broadcasts消息。将消息传给主线程进行处理。 

4.主线程mhandler接受到后就由该类的handlermessage进行处理。在该方法中调用executependingbroadcasts()进行处理
    private void executependingbroadcasts() {
        while (true) {
            broadcastrecord[] brs = null;
            synchronized (mreceivers) {//注意多线程下的同步
                final int n = mpendingbroadcasts.size();
                if (n <= 0) {
                    return;
                }
                brs = new broadcastrecord[n];
                mpendingbroadcasts.toarray(brs);//把待处理的广播转成数组形式
                mpendingbroadcasts.clear();//然后就可以把mpendingbroadcasts清空
            }
            //for循环变量每个接受者，然后调用对应的onreceive
            for (int i=0; i
处理也很简单，查询相应的变量，找到有多少个接受者，然后调用接受者的onreceive，该调用在主线程中，因而不要做耗时操作。在localbroadcastmanager中还提供了同步发送广播处理的方法：
    //使用该方法会立即去让接受者处理广播。
    public void sendbroadcastsync(intent intent) {
        if (sendbroadcast(intent)) {
            executependingbroadcasts();
        }
    }
2.4 广播的注销
 public void unregisterreceiver(broadcastreceiver receiver) {
        synchronized (mreceivers) {
            arraylist filters = mreceivers.remove(receiver);
            if (filters == null) {
                return;
            }
            for (int i=0; i receivers = mactions.get(action);
                    if (receivers != null) {
                        for (int k=0; k
注销广播也很简单，找到注册时候添加到list中的变量，然后remove掉。注意要讲mreceivers,mactions里面保存的都remove了。
3.总结
普通广播采用binder和系统通信，由ams进行管理，而应用内广播是通过handler的消息机制来进行通信，仅在一个进程中使用。和普通广播比，应用内广播安全，速度快。缺点是只能在应用的一个进程中使用，不能跨进程使用。
        
   









   
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