LocalBroadcastManager是Android Support包提供了一个工具类，用于在同一个应用内的不同组件间发送广播，广播数据在本应用内传播，相比在系统内发送全局广播，安全性高，效率也高，广播接收者接收消息是在主线程，所以可以在广播接收者里面更新UI，但不能做耗时操作，以免阻塞UI。
LocalBroadcastManager基本用法
注册广播接收者
LocalBroadcastManager.getInstance(this).registerReceiver(receiver, intentFilter);
发送广播
LocalBroadcastManager.getInstance(this).sendBroadcast(intent);
取消注册广播接收者
LocalBroadcastManager.getInstance(this).unregisterReceiver(receiver);
LocalBroadcastManager.getInstance(this)
使用单例模式创建LocalBroadcastManager对象，在LocalBroadcastManager的构造函数中创建了一个Handler，且在主线程中处理消息。
public static LocalBroadcastManager getInstance(Context context) {
//单例模式
synchronized (mLock) {
if (mInstance == null) {
//创建LocalBroadcastManager对象
//把外部传进来的Context，转化成了ApplicationContext，有效的避免了当前Context的内存泄漏的问题
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}
private LocalBroadcastManager(Context context) {
mAppContext = context;
//创建Handler，由context.getMainLooper()可知, 这个Handler是在主线程中创建的
mHandler = new Handler(context.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
//收到MSG_EXEC_PENDING_BROADCASTS类型消息，执行executePendingBroadcasts方法处理广播
case MSG_EXEC_PENDING_BROADCASTS:
executePendingBroadcasts();
break;
default:
super.handleMessage(msg);
}
}
};
}
注册广播接收者registerReceiver
mReceivers的类型是HashMap<BroadcastReceiver, ArrayList<IntentFilter>>，key为注册的广播接收者，value为该广播接收者对应的IntentFilter的集合。mActions的类型是HashMap<String, ArrayList<ReceiverRecord>>，key为action，value为该action对应的ReceiverRecord的集合。注册广播就是以要注册的广播接收者BroadcastReceiver为key，把该广播接收者对应的IntentFilter的集合ArrayList<IntentFilter>>放入mReceivers，以IntentFilter匹配的action为key，把该action对应的ReceiverRecord的集合ArrayList<ReceiverRecord>存入mActions。
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
//ReceiverRecord封装了IntentFilter和BroadcastReceiver
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
//一种广播接收者BroadcastReceiver可以对应多个IntentFilter
ArrayList<IntentFilter> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<IntentFilter>(1);
mReceivers.put(receiver, filters);
}
//将filter存入filters集合，也即存入对应receiver的mReceivers
filters.add(filter);
//一个Intent Filter中可声明多个action
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);
}
//将entry存入entries集合，也即存入对应action的mActions
entries.add(entry);
}
}
}
取消注册广播接收者unregisterReceiver
与注册广播接收者相对应，取消注册广播接收者就是从mReceivers中移除该广播接收者对应的IntentFilter的集合ArrayList<IntentFilter>>，从mActions中移除该广播接收者对应的所有IntentFilter匹配的action和相关ReceiverRecord。
public void unregisterReceiver(BroadcastReceiver receiver) {
synchronized (mReceivers) {
//从mReceivers中移除receiver对应的IntentFilter的集合，并返回该集合
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);
//获取action对应的ReceiverRecord的集合
ArrayList<ReceiverRecord> receivers = mActions.get(action);
if (receivers != null) {
for (int k=0; k<receivers.size(); k++) {
if (receivers.get(k).receiver == receiver) {
//从receivers中当前移除receiver对应的ReceiverRecord
receivers.remove(k);
k--;
}
}
if (receivers.size() <= 0) {
//action对应的集合receivers无数据时，从mActions移除
mActions.remove(action);
}
}
}
}
}
}
发送广播sendBroadcast
public boolean sendBroadcast(Intent intent) {
synchronized (mReceivers) {
//获取Intent携带的信息
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();
final boolean debug = DEBUG ||
((intent.getFlags() & Intent.FLAG_DEBUG_LOG_RESOLUTION) != 0);
if (debug) Log.v(
TAG, "Resolving type " + type + " scheme " + scheme
+ " of intent " + intent);
//获取intent的action对应的ReceiverRecord的集合
ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());
if (entries != null) {
if (debug) Log.v(TAG, "Action list: " + entries);
//保存匹配的ReceiverRecord
ArrayList<ReceiverRecord> receivers = null;
for (int i=0; i<entries.size(); i++) {
ReceiverRecord receiver = entries.get(i);
if (debug) Log.v(TAG, "Matching against filter " + receiver.filter);
//如果已经匹配过并加入receivers里了，就执行下一次循环
if (receiver.broadcasting) {
if (debug) {
Log.v(TAG, " Filter's target already added");
}
continue;
}
//将filter与intent的信息进行匹配
int match = receiver.filter.match(action, type, scheme, data,
categories, "LocalBroadcastManager");
//如果匹配成功就把receiver加入receivers
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 (debug) {
String reason;
switch (match) {
case IntentFilter.NO_MATCH_ACTION: reason = "action"; break;
case IntentFilter.NO_MATCH_CATEGORY: reason = "category"; break;
case IntentFilter.NO_MATCH_DATA: reason = "data"; break;
case IntentFilter.NO_MATCH_TYPE: reason = "type"; break;
default: reason = "unknown reason"; break;
}
Log.v(TAG, " Filter did not match: " + reason);
}
}
}
if (receivers != null) {
//将broadcasting置为false，这样下一个Intent到来时，ReceiverRecord才有机会重新加入到receivers中
for (int i=0; i<receivers.size(); i++) {
receivers.get(i).broadcasting = false;
}
//创建BroadcastRecord对象，并加入到mPendingBroadcasts中
//BroadcastRecord包含intent以及与其匹配的ReceiverRecord的集合
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
//如果主线程消息队列中没有MSG_EXEC_PENDING_BROADCASTS类型的消息，则发送该消息
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
}
}
return false;
}
从上面可以看出，sendBroadcast方法的参数是一个intent，先从获取与其匹配的ReceiverRecord的集合，把intent与该集合封装成BroadcastRecord对象，把它加入mPendingBroadcasts集合，然后通过Handler发送MSG_EXEC_PENDING_BROADCASTS消息，调用executePendingBroadcasts()方法。
executePendingBroadcasts方法
executePendingBroadcasts方法从mPendingBroadcasts集合中取出BroadcastRecord，通过br.receivers.get(j).receiver获取BroadcastReceiver，并调用BroadcastReceiver的onReceive方法。
private void executePendingBroadcasts() {
//不停的执行，直到return
while (true) {
BroadcastRecord[] brs = null;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
//保存要处理的BroadcastRecord
brs = new BroadcastRecord[N];
//将集合mPendingBroadcasts的数据都放到数组brs里
mPendingBroadcasts.toArray(brs);
//清空mPendingBroadcasts，若下次while循环时没有新数据加入，就会退出循环
mPendingBroadcasts.clear();
}
for (int i=0; i<brs.length; i++) {
BroadcastRecord br = brs[i];
for (int j=0; j<br.receivers.size(); j++) {
//调用BroadcastReceiver的onReceive方法
br.receivers.get(j).receiver.onReceive(mAppContext, br.intent);
}
}
}
}
最后，我们可以看出LocalBroadcastManager本质是通过Handler机制发送消息到主线程，然后调用BroadcastReceiver的onReceive方法的。
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