LocalBroadcastManager是什么？
LocalBroadcastManager是android中本地广播的核心类。那什么是本地广播呢？它是一种在会在app内部传播的信息通信方式。因为其只在app内部传播，所以它安全；又因为其并不想系统级别的广播那样要有IPC，所以它高效。
但本地广播也有它的缺点，接收信息的只能是Receiver及其继承类的对象，而不能像总线类通信方式那样，只要是注册的类对象，都可以接收数据。
LocalBroadcastManager原理是怎样的？
简单地讲，其原理就是：将广播接收者receiver注册到LocalBroadcastReceiver对象中的一个数据结构里(HashMap)，发送广播时匹配到能够接收该广播的Receiver，因为发送广播有可能是在子线程中，为了保证接收广播的逻辑在主线程中执行，匹配到receiver之后通过持有MainLooper的Handler切换线程到主线程中去执行receiver的onReceiver方法。
1. registerReceiver和unregisterReceiver
在LocalBroadcastManager类中比较重要的方法有两个：registerReceiver(BroadcastReceiver receiver, IntentFilter filter) 和sendBroadcast(Intent intent)。首先说一下registerReceiver方法：
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
//将ReceiverRecord对象存储到mReceivers中
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<>(1);
mReceivers.put(receiver, filters);
}
filters.add(entry);
//将ReceiverRecord对象存储到mActions中，ReceiverRecord对象可能
//会出现在mActions中的多个地方
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);
}
}
}
通过这个方法的名字可以知道，该方法就是一个注册广播接收者的方法。具体都做了哪些事情呢？
1. 将receiver和intentFilter封装成ReceiverRecord对象；
2. 将ReceiverRecord对象存储到一个以receiver为key、以ReceiverRecord列表为value的map中，此map名为mReceivers，是一个成员字段；
3. 将ReceiverRecord对象存储到一个以action为key，以ReceiverRecord列表为value的map中，此map名为mActions，也是一个成员字段；
对于mReceivers，纵观整个LocalBroadcastManager类，感觉它并没有参与到核心流程receiver.onReceive方法的回调中，它的作用仅限于作为锁对象来同步以及在unregisterReceiver(BroadcastReceiver receiver)方法的调用中通过receiver拿到receiver对象的intentFilter。
unregisterReceiver(BroadcastReceiver receiver)方法中都有哪些操作呢？首先，就是拿到IntentFilter对象，拿到此对象后就可以其包含的所有action；其次，遍历所有的action，删除掉mActions这个map中receiver为当前要反注册的receiver的ReceiverRecord对象。
public void unregisterReceiver(BroadcastReceiver receiver) {
synchronized (mReceivers) {
//从mReceivers中删除掉receiver对应的ReceiverRecord对象
final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);
if (filters == null) {
return;
}
//上面根据receiver对象拿到了IntentFilter对象，
//下面遍历所有IntentFilter对象包含的所有action，
//找到这些action所对应的ReceiverRecord对象，
//如果这些ReceiverRecord对象中的Receiver对象与receiver相同，
//则要移除这个ReceiverRecord对象
for (int i=filters.size()-1; i>=0; i--) {
final ReceiverRecord filter = filters.get(i);
filter.dead = true;
for (int j=0; j<filter.filter.countActions(); j++) {
final String action = filter.filter.getAction(j);
final ArrayList<ReceiverRecord> receivers = mActions.get(action);
if (receivers != null) {
for (int k=receivers.size()-1; k>=0; k--) {
final ReceiverRecord rec = receivers.get(k);
if (rec.receiver == receiver) {
rec.dead = true;
receivers.remove(k);
}
}
if (receivers.size() <= 0) {
mActions.remove(action);
}
}
}
}
}
}
对于mActions，这个map就是LocalBroadcastManager中的核心数据结构了。在registerReceiver中，以intentFilter中声明的action为key，存储ReceiverRecord对象。因为intentFilter中本身就可能有多个action，所以有可能在mActions中总共会有多个该ReceiverRecord对象。在发送广播时，intent应该是只能设定一个action，通过该action匹配mActions中的ReceiverRecord，匹配到就执行。当然匹配时，不仅匹配action，还会匹配intentFilter中的其它值，比如category、mimeType等；
2. sendBroadcast
上面讲述了LocalBroadcastManager的registerReceiver和unregisterReceiver方法，下面开始说LocalBroadcastManager类中另一个比较重要的方法：sendBroadcast(Intent intent)
public boolean sendBroadcast(Intent intent) {
synchronized (mReceivers) {
//获取到广播intent中的所有信息
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);
//从mActions中拿到其对应的所有ReceiverRecord对象，
//筛选出与广播intent中信息匹配的ReceiverRecord对象并
//存储到receivers这个列表数据结构中
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 (debug) Log.v(TAG, "Matching against filter " + receiver.filter);
if (receiver.broadcasting) {
if (debug) {
Log.v(TAG, " Filter's target already added");
}
continue;
}
//从当前遍历到的ReceiverRecord对象中拿到intentFilter与
//广播intent进行匹配校验
int match = receiver.filter.match(action, type, scheme, data,
categories, "LocalBroadcastManager");
//匹配了就加入到receivers中
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);
}
}
}
//将可接收此广播的ReceiverRecord对象集合封装到BroadcastRecord对象中，
//加入到mPendingBroadcasts里面
if (receivers != null) {
for (int i=0; i<receivers.size(); i++) {
receivers.get(i).broadcasting = false;
}
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
//利用Handler切换到线程
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
}
}
return false;
}
如注释所示，sendBroadcast方法中主要做的事情是：
1. 根据广播intent中的action，从mActions这个map中拿到所有可接收此广播的ReceiverRecord对象；
2. 将这些ReceiverRecord对象封装在BroadcastRecord对象中；
3. 将BroadcastRecord对象存储在mPendingBroadcasts集合中，之后通过Handler切换线程。
3. 回调广播接收者的onReceive方法
通过Handler切换线程之后，下面就是对onReceive方法的回调了
private void executePendingBroadcasts() {
while (true) {
final BroadcastRecord[] brs;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
//转换成数组
brs = new BroadcastRecord[N];
mPendingBroadcasts.toArray(brs);
mPendingBroadcasts.clear();
}
//逐一遍历BroadcastRecord对象，
//从BroadcastRecord对象中拿到所有的可处理广播的接收者，
//检测一下状态，逐一回调receiver对象的onReceive方法。
for (int i=0; i<brs.length; i++) {
final BroadcastRecord br = brs[i];
final int nbr = br.receivers.size();
for (int j=0; j<nbr; j++) {
final ReceiverRecord rec = br.receivers.get(j);
if (!rec.dead) {
rec.receiver.onReceive(mAppContext, br.intent);
}
}
}
}
}
切换到主线程的操作也很清晰，遍历所有的mPendingBroadcasts，取出所有BroadcastRecord中的Receiver对象，经过sendBroadcast方法中的过滤，这里的Receiver对象都是可接收此广播的Receiver对象，直接调用这些Receiver对象的onReceive方法即可。
小结
整个LocalBroadcastManager类其实代码量很少，只有300多行。其核心数据结构是一个map，注册的广播接收者与intentFilter封装成一个Record对象，存储在该map中，该map的key为intentFilter的action；在发送广播时，根据此广播的action从map中匹配到可接收此广播的广播接收者，然后回调这些广播接收者的onReceive方法。仅此而已。
核心思想其实就只有上面这些，只不过还要处理并发、反注册、切换线程等，增加了其它一些代码。
