Android应用内部广播机制详解
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1. 简介
通常我们在使用Android广播的时候都会直接将广播注册到系统的AMS当中，由于AMS任务繁忙，一般可能不会立即能处理到我们发出的广播，如果我们使用广播是在应用内的单个进程中使用，则完全可以采用LocalBroadcastManager来处理。LocalBroadcastManager采用的是Handler的消息机制来处理的广播，而注册到系统中的是通过Binder机制实现的，速度是应用内广播要慢很多。不过由于Handler的消息机制是为了同一个进程的多线程间进行通信的，因而跨进程时无法使用应用内广播。应用内广播onReceive也是在主线程中被回调执行。
1.1 使用
在使用上和普通的Broadcast类似，主要分5步。具体如下：
//1. 自定义广播接收者
public class LocalReceiver extends BroadcastReceiver {
public void onReceive(Context context, Intent intent) {
...
}
}
LocalReceiver localReceiver = new LocalReceiver();
//2. 注册广播
LocalBroadcastManager.getInstance(context)
.registerReceiver(localReceiver, new IntentFilter(“test”));
//4. 发送广播
LocalBroadcastManager.getInstance(context).sendBroadcast(new Intent("test"));
//5. 取消注册广播
LocalBroadcastManager.getInstance(context).unregisterReceiver(localReceiver);
自定义广播和普通的广播一样，在注册广播的时候将该广播接受者注册到LocalBroadcatManager中。当发生时也是调用LocalBroadcastManager的sendBroadcast进行发生。同样在不使用时记得取消广播注册。
2. LocalBroadcastManager
2.1 初始化
LocalBroadcastManager采用的是单例模式，其构造函数是私有的，获取该类实例的方法是getInstance，具体代码如下：
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是主线程的
mHandler = new Handler(context.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_EXEC_PENDING_BROADCASTS:
executePendingBroadcasts();//这里去执行广播分发
break;
default:
super.handleMessage(msg);
}
}
};
}
在构造函数中创建了一个mHandler，该mHandler关联的是主线程的Looper。即消息处理时都在主线程中处理。
2.2 registerReceiver
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
//在注册，取消注册，发送广播的时候都需要先获取mReceivers的锁
synchronized (mReceivers) {
//新建一个ReceiverRecord实体表示该receiver及对应的filter
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
//获取receiver对应的filters
ArrayList<IntentFilter> filters = mReceivers.get(receiver);
if (filters == null) {
//如果该receiver没有对应的filters则，新建一个。
filters = new ArrayList<IntentFilter>(1);
mReceivers.put(receiver, filters);
}
//将filter放入该receiver对应的filters中
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);
//将action放入mActions中
mActions.put(action, entries);
}
entries.add(entry);
}
}
}
注册的时候也就是将receiver自己和对应的filter及action放入到mReceivers和mActions当中。代码比较简单。
2.3 发送广播sendBroadcast
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;
}
主要步骤：1.根据Intent的action来查询相应的广播接收者列表；
2.创建相应广播，添加到mPendingBroadcasts队列；
3.发送MSG_EXEC_PENDING_BROADCASTS消息。将消息传给主线程进行处理。
4.主线程mHandler接受到后就由该类的handlerMessage进行处理。在该方法中调用executePendingBroadcasts()进行处理
private void executePendingBroadcasts() {
while (true) {
BroadcastRecord[] brs = null;
synchronized (mReceivers) {//注意多线程下的同步
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord[N];
mPendingBroadcasts.toArray(brs);//把待处理的广播转成数组形式
mPendingBroadcasts.clear();//然后就可以把mPendingBroadcasts清空
}
//for循环变量每个接受者，然后调用对应的onReceive
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);
}
}
}
}
处理也很简单，查询相应的变量，找到有多少个接受者，然后调用接受者的onReceive，该调用在主线程中，因而不要做耗时操作。在LocalBroadcastManager中还提供了同步发送广播处理的方法：
//使用该方法会立即去让接受者处理广播。
public void sendBroadcastSync(Intent intent) {
if (sendBroadcast(intent)) {
executePendingBroadcasts();
}
}
2.4 广播的注销
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);
}
}
}
}
}
}
注销广播也很简单，找到注册时候添加到List中的变量，然后remove掉。注意要讲mReceivers,mActions里面保存的都remove了。
3.总结
和普通广播比，应用内广播安全，速度快。缺点是只能在应用的一个进程中使用，不能跨进程使用。