LocalBroadcastManager能够让自己发送的广播不被外部App接收，这样既安全，也又优化了性能，因为传送和接收广播也是跨进程通信。而通过LocalBroadcastManager发送广播和该BroadcastReceiver接收广播都是在自己应用的进程完成的。
接下来我会先简单的说LocalBroadcastManager如何使用，然后在使用的基础上从源码角度讲解这是如何实现的。
1.使用
我的例子很简单，首先是通过单例模式创建LocalBroadcastManager，然后创建BroadcastReceiver并且创建IntentFilter设置过滤规则，最后点击按钮通过LocalBroadcastManager发送广播
public class MainActivity extends AppCompatActivity {
public static final String IntentFilerTest = "test";
LocalBroadcastManager localBroadcastManager;
Button btn;
MyBroadcastReceiver myBroadcastReceiver;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btn = (Button) findViewById(R.id.btn);
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent();
intent.setAction(IntentFilerTest);
localBroadcastManager.sendBroadcast(intent);
}
});
localBroadcastManager = LocalBroadcastManager.getInstance(this);
IntentFilter intentFilter = new IntentFilter();
intentFilter.addAction(IntentFilerTest);
myBroadcastReceiver = new MyBroadcastReceiver();
localBroadcastManager.registerReceiver(myBroadcastReceiver, intentFilter);
}
public void showToast(String s) {
Toast.makeText(this, s , Toast.LENGTH_SHORT).show();
}
class MyBroadcastReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
if (intent != null) {
String action = intent.getAction();
if (action == IntentFilerTest) {
showToast(IntentFilerTest);
}
}
}
}
}2.源码解析
我们按照上面使用LocalBroadcastManager的步骤来讲解源码
首先是LocalBroadcastManager的单例模式，这个单例实现是通过同步锁锁住一个Object类实现线程同步，然后判断mInstance是否为空，为空就初始化
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;
}
}看看构造函数，就保存Context实例和创建一个主线程的Handler，关于executePendingBroadcasts()函数我后面说。
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);
}
}
};
}在我说LocalBroadcastManager如何注册广播之前，我先贴出两个LocalBroadcastManager内部类代码
//记录过滤规则和广播接收器
private static final class ReceiverRecord {
final IntentFilter filter;
final BroadcastReceiver receiver;
//为true判断该广播接收器正在接收广播，目前不接受广播，为false表示处于空闲状态，接收广播
boolean broadcasting;
boolean dead;
ReceiverRecord(IntentFilter _filter, BroadcastReceiver _receiver) {
filter = _filter;
receiver = _receiver;
}
@Override
public String toString() {
StringBuilder builder = new StringBuilder(128);
builder.append("Receiver{");
builder.append(receiver);
builder.append(" filter=");
builder.append(filter);
if (dead) {
builder.append(" DEAD");
}
builder.append("}");
return builder.toString();
}
}
//记录过滤规则中每一个Action和与之对应的ReceiverRecord
private static final class BroadcastRecord {
final Intent intent;
final ArrayList<ReceiverRecord> receivers;
BroadcastRecord(Intent _intent, ArrayList<ReceiverRecord> _receivers) {
intent = _intent;
receivers = _receivers;
}
}注册广播接收器主要依靠以下两个HashMap对象记录信息
//主要记录IntentFilter和与之配对的BroadcastReceiver
private final HashMap<BroadcastReceiver, ArrayList<ReceiverRecord>> mReceivers
= new HashMap<>();
//记录发送的广播里的Action与之配对的BroadcastReceiver
private final HashMap<String, ArrayList<ReceiverRecord>> mActions = new HashMap<>(); public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
//首先是将广播接收器和过滤规则保存在ReceiverRecord对象里
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
//然后在mReceivers这个HashMap里通过BroadCastReceiver实例去取ReceiverRecord对象，如果没有
取到就将receiver和filter保存在HashMap里，这里我感觉是防止
ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<>(1);
mReceivers.put(receiver, filters);
}
filters.add(entry);
//遍历过滤规则，将其中包含的Action一条一条的取出来，如果通过action获取不到ReceiverRecord，就保存在HashMap里
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);
}
}
}虽然两个HashMap里的值都是ArrayList，但是创建的时候都是容量为1，要注意
好，我们现在开始说说LocalBroadcastManager发送广播的代码
public boolean sendBroadcast(Intent intent) {
//先通过mReceivers来个同步锁
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);
//通过Intent设置的Action获取对应的ReceiverRecord对象，
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);
//如果广播接收器处于正在接收广播的状态，不接收广播
if (receiver.broadcasting) {
if (debug) {
Log.v(TAG, " Filter's target already added");
}
continue;
}
//判断这个Intent的参数是否符合广播接收器的过滤规则
int match = receiver.filter.match(action, type, scheme, data,
categories, "LocalBroadcastManager");
//如果符合，receivers就将添加这个receiver，并且设置状态为正在接收广播
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);
}
}
}
//遍历receivers，将状态改为接收完广播
if (receivers != null) {
for (int i=0; i<receivers.size(); i++) {
receivers.get(i).broadcasting = false;
}
//将Intent和recievers保存在mPendingBroadcasts对象，然后如何Handler的消息队列里没有MSG_EXEC_PENDING_BROADCASTS
类型的消息，就发送该类型的消息
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
}
}
return false;
}最后我们再看看executePendingBroadcasts()的代码，看看handler是如何处理消息的
private void executePendingBroadcasts() {
while (true) {
final BroadcastRecord[] brs;
synchronized (mReceivers) {
//判断mPendingBroadcasts有没有东西
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
//如果有就转换为数组
brs = new BroadcastRecord[N];
mPendingBroadcasts.toArray(brs);
mPendingBroadcasts.clear();
}
//接下里就很简单了，遍历数组取出BroadcastReceiver对象，然后遍历receivers，最后很直接的通过广播接收器的实例调用
onReceive函数，传递参数完毕
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);
}
}
}
}
}总结：LocalBroadcastManager是内部是通过Handler实现广播的传递，注册广播时保存BroadcastReceiver和IntentFilter的实例，当发送广播时，保存传递获取过来的Intent，并判断Intent是否符合过滤规则，如果符合再发送消息给Handler，取出BroadcastReceiver调用onReceiver函数，完成广播的接收。
PS:谷歌员工真的很喜欢HashMap保存对象