LocalBroadcastManager实现原理
1：概述
我们都知道广播可以用来实现跨进程通信，它的原理就是binder,通过system_server来实现的，那么当我们只是在应用内部来实现通信的时候，其实我们可以考虑使用LocalBroadcastManager来实现，它有两个好处
1.1：性能较一般的broadcast要好，因为它的实现不需要经过binder调用，它采用的是handler通信来实现的
1.2：安全性较好，不必担心数据泄露或是被截取
LocalBroadcastManager是单利模式，并且只能通过代码的方式注册，不能通过xml的方式注册，下面是谷歌对其的解释
/**
- Helper to register for and send broadcasts of Intents to local objects
- within your process.
- This has a number of advantages over sending
- global broadcasts with {@link android.content.Context#sendBroadcast}:
- You know that the data you are broadcasting won't leave your app, so
- don't need to worry about leaking private data.
- It is not possible for other applications to send these broadcasts to
- your app, so you don't need to worry about having security holes they can
- exploit.
- It is more efficient than sending a global broadcast through the
- system.
*/
2：实现原理
2.1：单例模式
@NonNull
public static LocalBroadcastManager getInstance(@NonNull Context context) {
synchronized (mLock) { //锁
if (mInstance == null) {//创建对象
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}
private LocalBroadcastManager(Context context) {
mAppContext = context;
//getMainLooper是主线程的对应的handler对象
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);
}
}
};
}
2.2注册广播registerReceiver
public void registerReceiver(@NonNull BroadcastReceiver receiver,
@NonNull IntentFilter filter) {
synchronized (mReceivers) {
//创建一个ReceiverRecord对象
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
//先从缓存的HashMap中查询，key为receiver
ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<>(1);
mReceivers.put(receiver, filters);//缓存到hashmap列表中去
}
filters.add(entry);
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);//缓存到action的Hashmap列表中
}
entries.add(entry);
}
}
}
这个过程主要是将数据缓存到两个列表中去
1： private final HashMap<BroadcastReceiver, ArrayList<ReceiverRecord>> mReceivers
= new HashMap<>(); key为receiver,value为ReceiverRecord列表
2： private final HashMap<String, ArrayList<ReceiverRecord>> mActions = new HashMap<>();
将action缓存到Hashmap中去，记录的是action与广播接收者之间的关系
2.3：取消注册unregisterReceiver
public void unregisterReceiver(@NonNull BroadcastReceiver receiver) {
synchronized (mReceivers) {
//从ArratList中移除广播接收者，remove方法返回的是value值
final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);
if (filters == null) {//如果为null的话不做后续处理
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);//从mActions移除接收者为空的action
}
}
if (receivers.size() <= 0) {
mActions.remove(action);
}
}
}
}
}
}
2.4发送广播sendBroadcast
public boolean sendBroadcast(@NonNull 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);
//intent读取action，根据action获取相应的广播接收者列表
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;
}
//根据intent和接收者信息创建广播对象，添加到mPendingBroadcasts
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
} //发送到主线程对应的消息队列中去
return true;
}
}
}
return false;
}
2.5：主线程消息队列：消息的处理
mHandler = new Handler(context.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_EXEC_PENDING_BROADCASTS:
executePendingBroadcasts();//广播的处理
break;
default:
super.handleMessage(msg);
}
}
};
private void executePendingBroadcasts() {
while (true) {
final BroadcastRecord[] brs;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord[N];//缓存到数组中，广播接收者可能有多个
mPendingBroadcasts.toArray(brs);
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);//回调广播接收者的onReceive方法
}
}
}
}
}
3：概括
优点：
1：相对于串行广播，更加安全，数据只会在应用内部传输，并且限定在同一进程中，一个APK中其他的进程也是不行的
2：性能较好：相对于串行广播，其不对system_server造成任何的性能问题，并且也没有跨进程调用带来的耗时问题
缺点：
1：不支持跨进程，如果跨进程必须考虑其他实现方式
2：不支持静态xml的方式注册，无法实现监听诸如系统开机广播等
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