揭秘LocalBroadcastManager实现原理

LocalBroadcastManager经常用的App内通信组件，也是官方推荐的App内广播发送组件

起初，用这个组件的时候，见名知意，因为LocalBroadcast，所以以为也是用Binder实现的底层，结果不是这样的。

1、平时我们都是这样用LocalBroadcastManager
    a、注册并接收

LocalBroadcastManager broadcastManager = LocalBroadcastManager.getInstance(context);
IntentFilter intentFilter = new IntentFilter();
intentFilter.addAction(ACTION);
mReceiver = new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
String Key = intent.getExtras().getString("key");
if("Key".equals(Key)){
Log.i("tag","接收到广播");
}
}
};
broadcastManager.registerReceiver(mReceiver, intentFilter);

b、发送广播

LocalBroadcastManager.getInstance(context).sendBroadcast(new Intent(ACTION));

c、取消注册

LocalBroadcastManager.getInstance(context).unregisterReceiver(mReceiver);

2、怎么实现的？
a、异步到源码：LocalBroadcastManager.java 
看看里面到底是怎么实现的，标准的单例实现：

static final int MSG_EXEC_PENDING_BROADCASTS = 1;
private final Handler mHandler;
private static final Object mLock = new Object();
private static LocalBroadcastManager mInstance;
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);
}
}
};
}

基于主线程的Looper（context.getMainLooper()） 新实例化一个Handler，handleMessage方法中调用接收器对广播消息进行处理。

b、注册接收

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);
}
}
}

注册存储广播和过滤信息，以BroadcastReceiver为Key，IntentFilter为Value，存储到ArrayList。

这里mReceivers中，为什么保存的IntentFilter是ArrayList形式，IntentFilter中是可以保存多个action的，这也就为什么他初始化成长度为1的List。这里的ArrayList能够传入多个IntentFilter，虽然如此，但是action如果一样的话，也是会按照一个来出来。action是以key来存储的。

c、取消注册

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);
}
}
}
}
}
}

只是从mReceivers和mActions中remove掉相应的元素。

d、此时应该看看executePendingBroadcasts()方法源码：

private void executePendingBroadcasts() {
while (true) {
BroadcastRecord[] brs = null;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord
;
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);
}
}
}
}

这个函数，mPendingBroadcasts转为BroadcastRecord[]，再循环遍历每一个receiver，调用Broadcast的onReceive函数，完成广播逻辑。消息处理完成。

e、那怎么发出去的，发送广播

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();

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);

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;
}

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 (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) {
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;
}

根据Action从mActions中取出ReceiverRecord列表，循环每一个ReceiverRecord判断filter中的action\type\scheme\data\categoried是否match，匹配的话就保存到receivers中，然后发送空消息，what=MSG_EXEC_PENDING_BROADCASTS，这个时候Handler就去处理啦。

总结：

a、注册和反注册保持一致，onStart ，onStop

b、一个广播，最好将action都放到一个IntentFilter中。

c、LocalBroadcastManager只对注册到的起作用，产生互动。

d、LocalBroadcastManager的核心就是Handler，利用了IntentFIlter的match功能。

e、Handler实现的应用内代码间的通信，他持有BroadcastReceiver的对象，直接调用onReceive方法，安全效率高。