Android四大组件：BroadcastReceiver史上最全面解析

概述https://www.jianshu.com/p/ca3d87a4cdf3LocalBroadcastManager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

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

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