LocalBroadcastManager 的实现原理，Handler还是 Binder？

原文: http://www.trinea.cn/android/localbroadcastmanager-impl/

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

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

(3) 发送广播

Java

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

(4) 取消注册

Java

1	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。

(3) 因为是 Handler 实现的通信, 只能在同一个进程的不同组件间进行通信, 不能进行夸进程通信.