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类注释
/**
* Helper to register for and send broadcasts of Intents to local objects
* within your process. This has a number of advantages over sending
* global broadcasts with {@link android.content.Context#sendBroadcast}:
* <ul>
* <li> You know that the data you are broadcasting won't leave your app, so
* don't need to worry about leaking private data.
* <li> It is not possible for other applications to send these broadcasts to
* your app, so you don't need to worry about having security holes they can
* exploit.
* <li> It is more efficient than sending a global broadcast through the
* system.
* </ul>
*/
LocalBroadcastManager本地广播是帮助您在自己的进程内去注册和发送广播，它和全局的广播（cotext.sendBroadcast）相比，有几个优点：
- 本地广播发送的数据不会离开app进程，也就是进程内发送和接收数据，所以不必担心会泄露私有的数据。
- 别的进程也不能向你的app发送本地广播，也不必担心会有漏洞被利用。
- 它相比全局广播会更加地高效。
单例模式
private static final Object mLock = new Object();
private static LocalBroadcastManager mInstance;
public static LocalBroadcastManager getInstance(Context context) {
synchronized (mLock) {
if (mInstance == null) {
//传入的Context会直接取Application的Context，不必担心传入Activity的Context造成泄露
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}
构造函数
在其构造方法我们发现，其new了一个Handler：
private final Handler mHandler;
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);
}
}
};
}
而且，注意看到，Handler绑定的是主线程的Looper，而看到 executePendingBroadcasts() 中的 receiver.onReceive （后面再仔细看这个方法），我们定义的接收者的回调方法，是跑在主线程中，所以，本地广播的 onReceive 中也不适应做耗时的任务。
registerReceiver注册广播
private final HashMap<BroadcastReceiver, ArrayList<IntentFilter>> mReceivers
= new HashMap<BroadcastReceiver, ArrayList<IntentFilter>>();
private final HashMap<String, ArrayList<ReceiverRecord>> mActions
= new HashMap<String, ArrayList<ReceiverRecord>>();
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
//1. 记录receiver映射到filter
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);
//2. 记录action映射到ReceiverRecordList
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 存储着广播接收者和过滤器，以广播接收者为key，过滤器List为value，它是接收器和IntentFilter的对应表，可以看到其主要是方便在 unregisterRecevier() 中取消注册，同时作为一个对象锁限制方法的并发访问。
mActions 是存储 Action 动作意图 和 RecevierRecord的映射关系，其中 RecevierRecord 存储着过滤器和对应的接收者
private static class ReceiverRecord {
final IntentFilter filter;
final BroadcastReceiver receiver;
boolean broadcasting;
}
unregisterReceiver反注册广播
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);
}
}
}
}
}
}
有两个地方存储了receiver，一个是 mReceivers，存储 mReceiver 和过滤器过滤器 filters ，还有一个地方是 mActions 存储action和对应的接收者，先remove掉 mReceiver 中对应的接收者，然后根据接收者取出对应的 filters，遍历 filters，取出 ations，遍历 actions ，取出 ArrayList<ReceiverRecord> receivers = mActions.get(action); 找到对应的receiver，然后 remove。
sendBroadcast发送广播
public boolean sendBroadcast(Intent intent) {
synchronized (mReceivers) {
//1. 取出intent的action，type，data 等信息，后面 用于 match 匹配
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();
//2. 根据 action 获取到对应的 recevierRecord List
ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());
if (entries != null) {
ArrayList<ReceiverRecord> receivers = null;
//3. 遍历所有的 recevierRecord
for (int i=0; i<entries.size(); i++) {
ReceiverRecord receiver = entries.get(i);
if (receiver.broadcasting) {
continue;
}
//4. 匹配，如果匹配成功则放到receviers中
int match = receiver.filter.match(action, type, scheme, data, categories, "LocalBroadcastManager");
if (match >= 0) {
if (receivers == null) {
receivers = new ArrayList<ReceiverRecord>();
}
receivers.add(receiver);
receiver.broadcasting = true;
} else {
//省略日志输出
}
}
if (receivers != null) {
for (int i=0; i<receivers.size(); i++) {
receivers.get(i).broadcasting = false;
}
//5. 把符合action的接收者放到 mPendingBroadcasts 中，然后使用 handler发送处理消息
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
}
}
return false;
}
-
取出intent的action，type，data 等信息，后面 用于 match 匹配
-
先通过 mActions 取出 ReceiverRecord List，
ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());，entries里面就存放着，action 对应的 receivers， -
遍历所有的广播接收者
-
然后进行一个匹配：
int match = receiver.filter.match(action, type, scheme, data, categories, "LocalBroadcastManager");，将匹配成功的 ReceiverRecord 放到一个容器里面 receivers.add(receiver); ， -
把符合action的接收者放到 mPendingBroadcasts 中，然后使用 handler发送处理消息
executePendingBroadcasts
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);
}
}
}
}
执行待发的广播。从 mPendingBroadcasts 待发送广播取出转换成数组 BroadcastRecord[] brs ，遍历 BroadcastRecord ，里面再变量广播接收者，调用他的 onReceive 方法进行回调。
这里为什么要额外地去把待发送广播取出转换成数组 BroadcastRecord[] 呢？我想的是，为了尽快地去释放对象锁。锁作用期间就把对象给转移出来，释放锁后，才去回调onReceive方法。如果直接在锁作用期间去回调onReceive方法，在我们的onReceive有可能会做一些耗时操作，这样就会让锁迟迟得不到释放。
sendBroadcastSync
public void sendBroadcastSync(Intent intent) {
if (sendBroadcast(intent)) {
executePendingBroadcasts();
}
}
同步发送广播，其实就是调用executePendingBroadcasts方法，直接回调onReceive方法，我们可以直接在子线程中使用这个方法，然后我们的onRecevier就在子线程中做一个同步的回调。
总结
(1) 本地广播的核心实现实际还是 Handler，只是直接套用了全局广播的概念和API的使用。
(2) 因为是基于Handler所以其效率更高，安全性更好。
(3) 内部的Handler直接是绑定了主线程，使用 sendBroadcast 发送的广播，相应的接收者会在主线程中回调onReceive方法，也可以用 sendBroadcastSync 发送同步广播，让onRecevie直接回调。
(4) 本地广播和全局广播的机制不同，因而本地广播不能去监听全局广播。
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