揭秘LocalBroadcastManager实现原理

LocalBroadcastManager经常用的App内通信组件，也是官方推荐的App内广播发送组件
           

起初，用这个组件的时候，见名知意，因为LocalBroadcast，所以以为也是用Binder实现的底层，结果不是这样的。

1、平时我们都是这样用LocalBroadcastManager     a、注册并接收

LocalBroadcastManager broadcastManager = LocalBroadcastManager.getInstance(context);
IntentFilter intentFilter = new IntentFilter();
intentFilter.addAction(ACTION);
mReceiver = new BroadcastReceiver() {
	@Override
	public void onReceive(Context context, Intent intent) {
		String Key = intent.getExtras().getString("key");
		if("Key".equals(Key)){
		Log.i("tag","接收到广播");		
		}
	}
};
broadcastManager.registerReceiver(mReceiver, intentFilter);
           

b、发送广播

LocalBroadcastManager.getInstance(context).sendBroadcast(new Intent(ACTION));
           

c、取消注册

LocalBroadcastManager.getInstance(context).unregisterReceiver(mReceiver);
           

2、怎么实现的？ a、异步到源码：LocalBroadcastManager.java  看看里面到底是怎么实现的，标准的单例实现：

static final int MSG_EXEC_PENDING_BROADCASTS = 1;
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 = new Handler(context.getMainLooper()) {
       @Override
       public void handleMessage(Message msg) {
           switch (msg.what) {
           case MSG_EXEC_PENDING_BROADCASTS:
               executePendingBroadcasts();
           break;
           default:
               super.handleMessage(msg);
           }
        }
     };
}
           

基于主线程的Looper（context.getMainLooper()） 新实例化一个Handler，handleMessage方法中调用接收器对广播消息进行处理。

b、注册接收

public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
        synchronized (mReceivers) {
            ReceiverRecord entry = new ReceiverRecord(filter, receiver);
            ArrayList<IntentFilter> filters = mReceivers.get(receiver);
            if (filters == null) {
                filters = new ArrayList<IntentFilter>(1);
                mReceivers.put(receiver, filters);
            }
            filters.add(filter);
            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);
            }
        }
    }
           

注册存储广播和过滤信息，以BroadcastReceiver为Key，IntentFilter为Value，存储到ArrayList。

这里mReceivers中，为什么保存的IntentFilter是ArrayList形式，IntentFilter中是可以保存多个action的，这也就为什么他初始化成长度为1的List。这里的ArrayList能够传入多个IntentFilter，虽然如此，但是action如果一样的话，也是会按照一个来出来。action是以key来存储的。

c、取消注册

public void unregisterReceiver(BroadcastReceiver receiver) {
        synchronized (mReceivers) {
            ArrayList<IntentFilter> filters = mReceivers.remove(receiver);
            if (filters == null) {
                return;
            }
            for (int i=0; i<filters.size(); i++) {
                IntentFilter filter = filters.get(i);
                for (int j=0; j<filter.countActions(); j++) {
                    String action = filter.getAction(j);
                    ArrayList<ReceiverRecord> receivers = mActions.get(action);
                    if (receivers != null) {
                        for (int k=0; k<receivers.size(); k++) {
                            if (receivers.get(k).receiver == receiver) {
                                receivers.remove(k);
                                k--;
                            }
                        }
                        if (receivers.size() <= 0) {
                            mActions.remove(action);
                        }
                    }
                }
            }
        }
    }
           

只是从mReceivers和mActions中remove掉相应的元素。      

d、此时应该看看executePendingBroadcasts()方法源码：

private void executePendingBroadcasts() {
        while (true) {
            BroadcastRecord[] brs = null;
            synchronized (mReceivers) {
             final int N = mPendingBroadcasts.size();
                if (N <= 0) {
                    return;
                }
                brs = new BroadcastRecord[N];
                mPendingBroadcasts.toArray(brs);
                mPendingBroadcasts.clear();
            }
            for (int i=0; i<brs.length; i++) {
                BroadcastRecord br = brs[i];
                for (int j=0; j<br.receivers.size(); j++) {
                    br.receivers.get(j).receiver.onReceive(mAppContext, br.intent);
                }
            }
        }
    }
           

这个函数，mPendingBroadcasts转为BroadcastRecord[]，再循环遍历每一个receiver，调用Broadcast的onReceive函数，完成广播逻辑。消息处理完成。

e、那怎么发出去的，发送广播

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<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);

            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++) {
                    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;
                    }

                    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>();
                        }
                        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<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;
    }
           

根据Action从mActions中取出ReceiverRecord列表，循环每一个ReceiverRecord判断filter中的action\type\scheme\data\categoried是否match，匹配的话就保存到receivers中，然后发送空消息，what=MSG_EXEC_PENDING_BROADCASTS，这个时候Handler就去处理啦。

总结： a、注册和反注册保持一致，onStart ，onStop b、一个广播，最好将action都放到一个IntentFilter中。 c、LocalBroadcastManager只对注册到的起作用，产生互动。 d、LocalBroadcastManager的核心就是Handler，利用了IntentFIlter的match功能。 e、Handler实现的应用内代码间的通信，他持有BroadcastReceiver的对象，直接调用onReceive方法，安全效率高。