LocalBroadcastManager 的实现原理，还是 Binder？

更新时间：2021-10-04 06:02:15

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

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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

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1 2	LocalBroadcastReceiver localReceiver = new LocalBroadcastReceiver(); LocalBroadcastManager.getInstance(context).registerReceiver(localReceiver, new IntentFilter(ACTION_LOCAL_SEND));

(3) 发送广播

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

(4) 取消注册

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

2. 实现

LocalBroadcastManager 源代码可见：LocalBroadcastManager.java

(1) 构造函数

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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

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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

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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

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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

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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。

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LocalBroadcastManager 的实现原理，还是 Binder？

1. LocalBroadcastManager 使用

(1) 自定义 BroadcastReceiver 子类

(2) 注册接收器

(3) 发送广播

(4) 取消注册

2. 实现

(1) 构造函数

(2) 注册接收器

(3) 发送广播

(4) 消息处理

(5) 取消注册

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