“本地广播”LocalBroadcastManager源码解析
在BroadcastReceiver：广播的使用一文中总结了关于广播的基础知识，关于本地广播和普通广播的区别总结如下：
①BroadcastReceiver一般用于应用间消息；LocalBroadcastManager应用内消息传递。
②BroadcastReceiver是Binder进程间通信；LocalBroadcastManager内部用Handler实现，效率更高。
③BroadcastReceiver需要考虑安全等问题；LocalBroadcastManager应用内部使用，安全性高。
仔细看看本地广播LocalBroadcastManager源码是如何实现的，源码并不多，一共320行。巧妙的使用Handler实现，利用IntentFilter匹配广播，以及和Context中BroadcastReceiver相关方法一样的接口设计。
1、内部成员
先看看LocalBroadcastManager内部有哪些成员：一个单例、一个Handle以及若干保存BroadcastReceiver的集合映射。
1.1、LocalBroadcastManager单例
在应用中通过getInstance(Context )获取LocalBroadcastManager实例：
LocalBroadcastManager localBroadcastManager = LocalBroadcastManager.getInstance(this);源码中这样的单例模式很常见，效率比较高：
private static LocalBroadcastManager mInstance;
@NonNull
public static LocalBroadcastManager getInstance(@NonNull Context context) {
synchronized (mLock) {
if (mInstance == null) {
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}1.2、内部Handler
本地广播接收的关键之一是这个Handler，很多优秀的功能设计都是通过Handler，足以见得Handler在Android开发中的重要地位！
static final int MSG_EXEC_PENDING_BROADCASTS = 1;
private final Handler mHandler;获取单例的时候通过私有构造方法初始化mHandler，通过主线程的Looper初始化，广播消息在主线程回调执行。收到MSG_EXEC_PENDING_BROADCASTS消息时执行executePendingBroadcasts()方法(后面详细看看)。
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);
}
}
};
}1.3、全局集合
LocalBroadcastManager内部有两映射，用来保存注册的广播接收器对应的记录、Action对应的接收器集合。后面注册、反注册以及广播分发的时候会用到。
private final HashMap<BroadcastReceiver, ArrayList<ReceiverRecord>> mReceivers = new HashMap<>();
private final HashMap<String, ArrayList<ReceiverRecord>> mActions = new HashMap<>();
//待处理的广播接收器，在sendBroadcast中添加值，在executePendingBroadcasts()中清空
private final ArrayList<BroadcastRecord> mPendingBroadcasts = new ArrayList<>();BroadcastReceiver和ReceiverRecord是两个静态内部类保存对应关系：
private static final class ReceiverRecord {
final IntentFilter filter;
final BroadcastReceiver receiver;
boolean broadcasting;
boolean dead;
ReceiverRecord(IntentFilter _filter, BroadcastReceiver _receiver) {
filter = _filter;
receiver = _receiver;
}
}
private static final class BroadcastRecord {
final Intent intent;
final ArrayList<ReceiverRecord> receivers;
BroadcastRecord(Intent _intent, ArrayList<ReceiverRecord> _receivers) {
intent = _intent;
receivers = _receivers;
}
}2、注册和反注册本地广播
看看本地广播注册和反注册是如何实现的，这两个方法和Context中注册普通广播的签名类型一样。
2.1、注册
注册本地广播接收器的函数registerReceiver(BroadcastReceiver receiver, IntentFilter filter)很面熟吧，和Context中相关方法的签名一样。
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
//将BroadcastReceiver和IntentFilter包装成ReceiverRecord一条记录。
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
//从全局mReceivers中获取receiver对应的filters过滤器
ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);
if (filters == null) {
//如果新注册的广播没有，那么就新建一个，添加到receiver和过滤器的映射中
filters = new ArrayList<>(1);
mReceivers.put(receiver, filters);
}
//将ReceiverRecord添加到过滤器中
filters.add(entry);
//遍历IntentFilter过滤器
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);
}
}
}2.2、反注册
从本地广播集合mReceivers、mActions中移除注册的广播接收器：
public void unregisterReceiver(@NonNull BroadcastReceiver receiver) {
synchronized (mReceivers) {
final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);
if (filters == null) {
return;
}
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);
}
}
}
}
}
}3、发送本地广播和分发广播
LocalBroadcastManager是如何通过Handler发送本地广播的？
3.1、发送本地广播
开发者通过LocalBroadcastManager的sendBroadcast(Intent intent)方法发送本地广播，函数前面和Context发送广播的一致，虽然实现完全不一样。当注册的广播接收器中有能够处理该条广播的接收器时，向Handler发送一条消息MSG_EXEC_PENDING_BROADCASTS。
/**
* Broadcast the given intent to all interested BroadcastReceivers. This
* call is asynchronous; it returns immediately, and you will continue
* executing while the receivers are run.
*/
public boolean sendBroadcast(@NonNull Intent intent) {
synchronized (mReceivers) {
...
if (receivers != null) {
for (int i=0; i<receivers.size(); i++) {
receivers.get(i).broadcasting = false;
}
//添加能够处理这条广播的接收器当待处理广播集合mPendingBroadcasts中
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
}
}
return false;
}还有另外一个方法sendBroadcastSync(Intent intent)，发送同步广播，发送后立即执行
public void sendBroadcastSync(@NonNull Intent intent) {
if (sendBroadcast(intent)) {
executePendingBroadcasts();
}
}3.2、分发本地广播
每次发送一个广播，都会挨个遍历注册的广播接收器，看看有没有能够处理该广播的广播接收器。如果有就回调广播接收器重写的onReceive()方法。
@SuppressWarnings("WeakerAccess") /* synthetic access */
void executePendingBroadcasts() {
while (true) {
final BroadcastRecord[] brs;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord[N];
mPendingBroadcasts.toArray(brs);
//处理完毕后将待处理的广播集合mPendingBroadcasts清空
mPendingBroadcasts.clear();
}
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);
}
}
}
}
}
