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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LocalBroadcastReceiver localReceiver = new LocalBroadcastReceiver(); LocalBroadcastManager.getInstance(context).registerReceiver(localReceiver, new IntentFilter(ACTION_LOCAL_SEND)); |
(3) 发送广播
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LocalBroadcastManager.getInstance(context).sendBroadcast(new Intent(ACTION_LOCAL_SEND)); |
(4) 取消注册
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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。
感觉遗漏了一些点没讲，比如 线程安全。多线程并发的话，executePendingBroadcasts 的处理会不会有问题等。（ mHandler = new Handler(context.getMainLooper())… ） (｡>﹏<｡)
回头再看看
这是好东西，顶起来回头再问问题。