https://www.jianshu.com/p/ca3d87a4cdf3

LocalBroadcastManager 实现原理

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。

初始化

LocalBroadcastManager 采用的是单例模式，并且在私有的构造函数中创建了一个mHandler,该Handler关联的是主线程的Looper。所以消息的处理都是在主线程中处理的。

  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);
                }
            }
        };
    }

注册广播registerReceiver

    /**
     * Register a receive for any local broadcasts that match the given IntentFilter.
     *
     * @param receiver The BroadcastReceiver to handle the broadcast.
     * @param filter Selects the Intent broadcasts to be received.
     *
     * @see #unregisterReceiver
     */
    public void registerReceiver(@NonNull BroadcastReceiver receiver,
            @NonNull IntentFilter filter) {
        synchronized (mReceivers) {
            ReceiverRecord entry = new ReceiverRecord(filter, receiver);
            ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);
            if (filters == null) {
                filters = new ArrayList<>(1);
                mReceivers.put(receiver, filters);
            }
            filters.add(entry);
            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);
            }
        }
    }

会将 Receiver 和action 存在下方的数据结构中

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

unregisterReceiver

    /**
     * Unregister a previously registered BroadcastReceiver.  <em>All</em>
     * filters that have been registered for this BroadcastReceiver will be
     * removed.
     *
     * @param receiver The BroadcastReceiver to unregister.
     *
     * @see #registerReceiver
     */
    public void unregisterReceiver(@NonNull BroadcastReceiver receiver) {
        synchronized (mReceivers) {
            final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);
            if (filters == null) {
                return;
            }
            for (int i=filters.size()-1; i>=0; i--) {
                final ReceiverRecord filter = filters.get(i);
                filter.dead = true;
                for (int j=0; j<filter.filter.countActions(); j++) {
                    final String action = filter.filter.getAction(j);
                    final ArrayList<ReceiverRecord> receivers = mActions.get(action);
                    if (receivers != null) {
                        for (int k=receivers.size()-1; k>=0; k--) {
                            final ReceiverRecord rec = receivers.get(k);
                            if (rec.receiver == receiver) {
                                rec.dead = true;
                                receivers.remove(k);
                            }
                        }
                        if (receivers.size() <= 0) {
                            mActions.remove(action);
                        }
                    }
                }
            }
        }
    }

总结

优点：
1：相对于串行广播，更加安全，数据只会在应用内部传输，并且限定在同一进程中，一个APK中其他的进程也是不行的
2：性能较好：相对于串行广播，其不对system_server造成任何的性能问题，并且也没有跨进程调用带来的耗时问题
缺点：
1：不支持跨进程，如果跨进程必须考虑其他实现方式
2：不支持静态xml的方式注册，无法实现监听诸如系统开机广播等