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LocalBroadcastManager 的实现原理，还是 Binder？

对 LocalBroadcastManager 大家应该都不陌生，相对 BroadcastReceiver，它只能用于应用内通信，安全性更好，同时拥有更高的运行效率。也是需要发送应用内广播时的官方推荐。

大家也都知道 BroadcastReceiver 的通信是走 Binder 机制的，而 LocalBroadcastManager 因为叫 LocalBroadcast ，可能让人产生一种它也是以 Binder 通讯方式为底层实现的错觉，点进源码，我们会发现这个更安全高效的实现原来如此熟悉。

还是先简单提下 LocalBroadcastManager 使用，更多可见： BroadcastReceiver 详细介绍。

1. LocalBroadcastManager 使用

LocalBroadcastManager 的使用跟一般 BroadcastReceiver 差别不大。

(1) 自定义 BroadcastReceiver 子类

Java

public class LocalBroadcastReceiver extends BroadcastReceiver {      @Override     public void onReceive(Context context, Intent intent) {         localMsg.setText(intent.getStringExtra(MSG_KEY));     } }

(2) 注册接收器

Java

LocalBroadcastReceiver localReceiver = new LocalBroadcastReceiver(); LocalBroadcastManager.getInstance(context).registerReceiver(localReceiver, new IntentFilter(ACTION_LOCAL_SEND));

(3) 发送广播

Java

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

(4) 取消注册

Java

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

2. 实现

LocalBroadcastManager 源代码可见： LocalBroadcastManager.java

(1) 构造函数

Java

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);             }         }     }; }

先看构造函数，单例实现因而私有化构造函数。

注意的是基于主线程的 Looper 新建了一个 Handler， handleMessage 中会调用接收器对广播的消息进行处理，也是 LocalBroadcastManager 的核心部分，具体见后面 executePendingBroadcasts() 介绍。

单例函数还可以通过双层条件判断提高效率，双层条件判断的写法可见：单例模式

(2) 注册接收器

Java

HashMap<BroadcastReceiver, ArrayList<IntentFilter>> mReceivers             = new HashMap<BroadcastReceiver, ArrayList<IntentFilter>>(); HashMap<String, ArrayList<ReceiverRecord>> mActions             = new HashMap<String, ArrayList<ReceiverRecord>>();  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);         }     } }

mReceivers 存储广播和过滤器信息，以 BroadcastReceiver 作为 key， IntentFilter 链表作为 value。

mReceivers 是接收器和 IntentFilter 的对应表，主要作用是方便在 unregisterReceiver(…) 取消注册，同时作为对象锁限制注册接收器、发送广播、取消接收器注册等几个过程的并发访问。

mActions 以 Action 为 key，注册这个 Action 的 BroadcastReceiver 链表为 value。mActions 的主要作用是方便在广播发送后快速得到可以接收它的 BroadcastReceiver 。

(3) 发送广播

Java

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();         ……         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 (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 (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; }

先根据 Action 从 mActions 中取出 ReceiverRecord 列表，循环每个 ReceiverRecord 判断 filter 和 intent 中的 action、type、scheme、data、categoried 是否 match，是的话则保存到 receivers 列表中，发送 what 为 MSG_EXEC_PENDING_BROADCASTS 的消息，通过 Handler 去处理。

关于 match 规则可见： Intent Filter介绍。

(4) 消息处理

Java

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);             }         }     } }

以上为消息处理的函数。 mPendingBroadcasts 转换为数组 BroadcastRecord ，循环每个 receiver ，调用其 onReceive 函数，这样便完成了广播的核心逻辑。

(5) 取消注册

Java

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);                     }                 }             }         }     } }

从 mReceivers 及 mActions 中移除相应元素。

到此为止我们便非常清晰了：

(1) LocalBroadcastManager 的核心实现实际还是 Handler，只是利用到了 IntentFilter 的 match 功能，至于 BroadcastReceiver 换成其他接口也无所谓，顺便利用了现成的类和概念而已。

(2) 因为是 Handler 实现的应用内的通信，自然安全性更好，效率更高。

正文到此结束