Handler系列文章:

• Handler源码剖析
• HandlerThread与IntentService源码剖析
• LocalBroadcastManager源码剖析

BroadcastReceiver作为android四大组件之一常常被人使用。但无论是普通广播还是有序广播，都是系统全局广播。即发送的广播可以被任何应用程序接收到，但同时我们也能接收来自任何应用程序的广播。

为了解决广播安全性的问题，Google引入了一套本地广播机制。使用本地广播机制发送的广播只能在应用中进行传递，而广播接收器也只能接收来自应用内的广播。

LocalBroadcast简单使用

本地广播的使用和普通广播基本没区别，主要是使用LocalBroadcastManager进行广播发送、注册广播接收器和注销广播接收器。

#daqiActivity.java
//本地广播管理器
private LocalBroadcastManager mBroadcastManager;
//广播接收器
private daqiBroadcastReceiver mBroadcastReceiver;@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {super.onCreate(savedInstanceState);//.....//获取广播管理类实例mBroadcastManager = LocalBroadcastManager.getInstance(daqiActivity.this);//初始化intent拦截器IntentFilter mIntentFilter = new IntentFilter();mIntentFilter.addAction("com.daqi.demo.LOCAL_BROADCAST");//初始化广播接收器mBroadcastReceiver = new daqiBroadcastReceiver();//注册广播接收器mBroadcastManager.registerReceiver(mBroadcastReceiver,mIntentFilter);mButton.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View v) {Intent intent = new Intent("com.daqi.demo.LOCAL_BROADCAST");//发送本地广播mBroadcastManager.sendBroadcast(intent);}});
}@Override
protected void onDestroy() {super.onDestroy();//注销广播接收器mBroadcastManager.unregisterReceiver(mBroadcastReceiver);
}private class daqiBroadcastReceiver extends BroadcastReceiver{@Overridepublic void onReceive(Context context, Intent intent) {Toast.makeText(daqiActivity.this,"本地广播",Toast.LENGTH_SHORT).show();}
}
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LocalBroadcastManager源码分析

从LocalBroadcastManager.getInstance(this)获取实例可以看出，LocalBroadcastManager是一个单例管理类。

查看LocalBroadcastManager#getInstance()的源码并无异样，继续查看构造方法：

#LocalBroadcastManager.java
@NonNull
public static LocalBroadcastManager getInstance(@NonNull Context context) {synchronized (mLock) {if (mInstance == null) {//获取ApplicationContext的contextmInstance = new LocalBroadcastManager(context.getApplicationContext());}return mInstance;}
}private LocalBroadcastManager(Context context) {mAppContext = context;mHandler = new Handler(context.getMainLooper()) {@Overridepublic void handleMessage(Message msg) {switch (msg.what) {case MSG_EXEC_PENDING_BROADCASTS:executePendingBroadcasts();break;default:super.handleMessage(msg);}}};
}
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LocalBroadcastManager的构造函数只做了两件事:

1、存储ApplicationContext的Context。

2、初始化一个Handler，并将其绑定在主线程。

LocalBroadcastManager居然初始化了一个Handler，莫非本地广播也是通过Handler实现的？

LocalBroadcastManager内部类

在查看注册广播监听器、注销广播监听器和发送本地广播的源码之前，需要先了解LocalBroadcastManager得两个内部类ReceiverRecord 和 BroadcastRecord。

#LocalBroadcastManager.java
//接收器记录
private static final class ReceiverRecord {final IntentFilter filter;final BroadcastReceiver receiver;//标记是否正在处于广播状态boolean broadcasting;//标记该接收器记录是否有效//false为有效，true为无效。boolean dead;ReceiverRecord(IntentFilter _filter, BroadcastReceiver _receiver) {filter = _filter;receiver = _receiver;}//....
}//广播器记录
private static final class BroadcastRecord {//发送广播时的Intent对象final Intent intent;//接收器记录列表final ArrayList<ReceiverRecord> receivers;BroadcastRecord(Intent _intent, ArrayList<ReceiverRecord> _receivers) {intent = _intent;receivers = _receivers;}
}
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从名字上ReceiverRecord-接收器记录，BroadcastRecord-广播器记录，可以很好的理解各类中变量的意义。

ReceiverRecord主要用于存储注册广播接收器时，存储注册广播时传递的参数：IntentFilter 和 BroadcastReceiver。

BroadcastRecord主要存储广播的intent和存储IntentFilter符合该intent.action的ReceiverRecord对象。

LocalBroadcastManager内部集合

#LocalBroadcastManager.java//存储广播接收器和对应接收器记录列表（ArrayList<LocalBroadcastManager.ReceiverRecord>）
//在之前的版本这里value存储的是ArrayList<IntentFilter>
//LocalBroadcastManager.ReceiverRecord中存有IntentFilter，所以存储ArrayList<LocalBroadcastManager.ReceiverRecord>和ArrayList<IntentFilter>道理都一样
private final HashMap<BroadcastReceiver, ArrayList<LocalBroadcastManager.ReceiverRecord>> mReceivers = new HashMap();//String存储action,表示一个action对应多个接收该action的接收器记录（ReceiverRecord）
//（ReceiverRecord内部存储广播接收器和过滤规则）
private final HashMap<String, ArrayList<LocalBroadcastManager.ReceiverRecord>> mActions = new HashMap();//与发送的广播的Action匹配的ReceiverRecord集合
//即存储了广播接收器、广播过滤规则 和 发送广播时的Intent
private final ArrayList<LocalBroadcastManager.BroadcastRecord> mPendingBroadcasts = new ArrayList();
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LocalBroadcastManager定义了3个集合，简单的说就是：

• mReceivers    存储广播接收器（BroadcastReceiver）对象 和 其IntentFilter信息。
• mActions    存储单个action 和 监听该action的接收器记录（ReceiverRecord）列表
• mPendingBroadcasts    存储发送广播时携带的intent对象 和 符合该intent.action的IntentFilter。

如果看不懂，暂时放下，配合下面的源码分析食用。

分别从广播机制的注册广播监听器、发送广播、注销广播监听器入手查看源码。 先从注册广播监听器入手：

registerReceiver()

#LocalBroadcastManager.javapublic void registerReceiver(@NonNull BroadcastReceiver receiver,@NonNull IntentFilter filter) {//加锁synchronized (mReceivers) {//传递进来的receiver广播接收器与filter广播拦截器会被封装在ReceiverRecord对象中。ReceiverRecord entry = new ReceiverRecord(filter, receiver);//从mReceivers中获取有关该receiver广播接收器的接收器记录（ReceiverRecord）对象ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);//如果没有获取到对应的记录（正常是null）if (filters == null) {//创建一个size为1的ArrayList，并将信息存储进去。filters = new ArrayList<>(1);mReceivers.put(receiver, filters);}//将存储注册广播信息的内容存储到filters中filters.add(entry);//遍历注册时传递进来的IntentFilter的action列表for (int i=0; i<filter.countActions(); i++) {//获取actionString action = filter.getAction(i);//获取特定action对应的接收器记录ArrayList<ReceiverRecord> entries = mActions.get(action);//如果原本没有这个条action的记录if (entries == null) {//则创建一个存进去entries = new ArrayList<ReceiverRecord>(1);mActions.put(action, entries);}//将这次注册广播监听器生成的接收器记录对象存储到该action对应的接收器记录（ReceiverRecord）器列表中。entries.add(entry);}}
}
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总的来说，LocalBroadcastManager#registerReceiver()做了三件事：

• 将注册广播监听器的信息：IntentFilter 和 BroadcastReceiver。封装在ReceiverRecord中。
• 为广播接收器添加IntentFilter，存储在mReceivers中。
• 为注册时IntentFilter中的action添加接收器记录（ReceiverRecord），存储在mActions中。

unregisterReceiver()

再查看注销广播接收器的源码：

#LocalBroadcastManager.javapublic void unregisterReceiver(@NonNull BroadcastReceiver receiver) {//加锁synchronized (mReceivers) {//移除该广播接收器的IntentFilter规则final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);if (filters == null) {return;}//遍历其存储的所有IntentFilterfor (int i=filters.size()-1; i>=0; i--) {//取出其中一个ReceiverRecord对象，其实是获取其对应的IntentFilterfinal ReceiverRecord filter = filters.get(i);//将该接收器记录标志为失效（死亡）filter.dead = true;//遍历这个IntentFilter过滤规则的actionfor (int j=0; j<filter.filter.countActions(); j++) {//获取其中一个actionfinal String action = filter.filter.getAction(j);//依据该action获取对应的ReceiverRecord列表，即获取其对应的广播接收器（BroadcastReceiver）列表final ArrayList<ReceiverRecord> receivers = mActions.get(action);//如果该action的接收器记录（ReceiverRecord）列表不为空if (receivers != null) {for (int k=receivers.size()-1; k>=0; k--) {//倒序或许更快final ReceiverRecord rec = receivers.get(k);if (rec.receiver == receiver) {//找到则将其移除。rec.dead = true;receivers.remove(k);}}//如果接收器记录（ReceiverRecord）列表移除后为空，即该action无对应的广播接收器，则将该action移除。if (receivers.size() <= 0) {mActions.remove(action);}}}}}
}
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注销广播接收器和注册广播接收器做了相反的事情：

• 在mReceivers中，移除广播接收器和其IntentFilter。
• 在mActions中，移除IntentFilter#action对应的接收器记录（ReceiverRecord）,若action无对应的接收器记录（ReceiverRecord）,则将action移除。

sendBroadcast()

#LocalBroadcastManager.javapublic boolean sendBroadcast(@NonNull Intent intent) {synchronized (mReceivers) {//获取一堆intent的参数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();//根据intent对象的action获取其对应的接收器记录（ReceiverRecord）列表ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());//如果该action的接收器记录（ReceiverRecord）列表不为空if (entries != null) {//..//这个列表用来记录实际用来接收该广播的广播接收器（BroadcastReceiver）ArrayList<ReceiverRecord> receivers = null;for (int i=0; i<entries.size(); i++) {//获取单个接收器记录，即获取单个广播接收器ReceiverRecord receiver = entries.get(i);//..//如果当前广播接收器正在广播，则先跳过，防止重复注册if (receiver.broadcasting) {//..continue;}//测试此过滤器是否与给定的意图数据匹配。 只有当Intent中的操作和类别与过滤器匹配时，匹配才会成功int match = receiver.filter.match(action, type, scheme, data,categories, "LocalBroadcastManager");//匹配成功if (match >= 0) {//..//如果存储实际用来接收该广播的广播接收器列表没初始化，则初始化。if (receivers == null) {receivers = new ArrayList<ReceiverRecord>();}//将该广播接收器存储到列表中，并设置为正在广播的状态。receivers.add(receiver);receiver.broadcasting = true;} else {//..}}//最后接受广播的广播接收器列表不为空，则将intent对象传递进去。if (receivers != null) {for (int i=0; i<receivers.size(); i++) {//恢复不是正在广播的状态receivers.get(i).broadcasting = false;}//将该广播的intent对象和广播接收器列表封装成接收器记录，存储在mPendingBroadcasts中。mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {//发送特定what的Message，交由Handler处理。 mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);}return true;}}}return false;
}
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sendBroadcast()做了4个步骤：

1、获取广播的intent信息，用于后续的匹对。

2、通过广播的action在mActions中获取其对应的接收器记录（ReceiverRecord）列表。

3、遍历接收器记录（ReceiverRecord）列表，将列表内元素的IntentFilter对象与广播的intent对象进行匹对，如果匹对成功，则存储在新的接收器记录（ReceiverRecord）列表receivers中。

4、将新的接收器记录（ReceiverRecord）列表receivers 和 广播的intent对象存储在广播记录(BroadcastRecord)对象中，并将该广播记录(BroadcastRecord)对象存储在mPendingBroadcasts中。

5、发送what = MSG_EXEC_PENDING_BROADCASTS的Message,让Handler传递广播intent对象到对应的接收器中。

注：对列表receivers存储接收器记录过程中，将接收器记录中的broadcasting标记为true，可以防止重复注册广播接收器造成的多次广播。

重归Handler的handleMessage方法中：

#LocalBroadcastManager.java
@Override
public void handleMessage(Message msg) {switch (msg.what) {case MSG_EXEC_PENDING_BROADCASTS:executePendingBroadcasts();break;default:super.handleMessage(msg);}
}
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what = MSG_EXEC_PENDING_BROADCASTS的Message,会调用executePendingBroadcasts进行处理。

#LocalBroadcastManager.java
void executePendingBroadcasts() {while (true) {final BroadcastRecord[] brs;synchronized (mReceivers) {//获取待广播的广播记录对象。final int N = mPendingBroadcasts.size();if (N <= 0) {return;}mPendingBroadcasts//将广播记录BroadcastRecord对象存储在数组中，并清空mPendingBroadcasts对象。brs = new BroadcastRecord[N];mPendingBroadcasts.toArray(brs);mPendingBroadcasts.clear();}//解锁//遍历广播记录数组for (int i=0; i<brs.length; i++) {final BroadcastRecord br = brs[i];//遍历接收器记录列表，即广播接收器列表final int nbr = br.receivers.size();for (int j=0; j<nbr; j++) {final ReceiverRecord rec = br.receivers.get(j);//广播接收器列表没有被标记失效，则进行onReceive()回调进行广播。if (!rec.dead) {rec.receiver.onReceive(mAppContext, br.intent);}}}}
}
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executePendingBroadcasts()将mPendingBroadcasts转换为数组，遍历广播记录数组。对还生效的接收器对象进行onReceive()回调，进行广播。

注：当广播接收器移除时，ReceiverRecord#dead会被标记为true，则标记为无效。

再回看3个列表对象：

• mReceivers    存储广播接收器（BroadcastReceiver）对象 和 其IntentFilter信息。
  作用：注销广播接收器时，通过广播接收器对象，寻找到其IntentFilter列表信息，方便根据IntentFilter列表中的action信息在mActions中进行移除。
• mActions    存储单个action 和 监听该action的接收器记录（ReceiverRecord）列表。
  作用:发送广播时，根据广播中intent对象的action，可以快速寻找到其对应接收的接收记录对象，再根据接收器对象的IntentFilter与广播中intent对象做匹配，寻找符合该广播intent对象条件的广播接收器对象。
• mPendingBroadcasts    存储发送广播时携带的intent对象 和 符合该intent.action的IntentFilter。
  作用:短暂存储需要传递的广播intent对象和接收的该intent对象广播接收器。

总结:

本地广播其实也是使用handler机制，将广播接收器与action、IntentFilter相关联。因为Handler的原因，本地广播也就只能在该应用内接收。