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Android LocalBroadcastManager解析
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阿里巴巴Android开发手册
【强制】避免使用隐式 Intent 广播敏感信息，信息可能被其他注册了对应BroadcastReceiver 的 App 接收。
说明：
通过 Context#sendBroadcast()发送的隐式广播会被所有感兴趣的 receiver 接收，恶意应用注册监听该广播的 receiver 可能会获取到 Intent 中传递的敏感信息，并进行其他危险操作。如果发送的广播为使用 Context#sendOrderedBroadcast()方法发送的有序广播，优先级较高的恶意 receiver 可能直接丢弃该广播，造成服务不可用，或者向广播结果塞入恶意数据。
如果广播仅限于应用内，则可以使用 LocalBroadcastManager#sendBroadcast()实现，避免敏感信息外泄和 Intent 拦截的风险
【推荐】对于只用于应用内的广播，优先使用 LocalBroadcastManager 来进行注册和发送，LocalBroadcastManager 安全性更好，同时拥有更高的运行效率。
说明：
对于使用 Context#sendBroadcast()等方法发送全局广播的代码进行提示。如果该广播仅用于应用内，则可以使用 LocalBroadcastManager 来避免广播泄漏以及广播被拦截等安全问题，同时相对全局广播本地广播的更高效
基础概念
1.BroadcastReceiver安全问题
BroadcastReceiver设计的初衷是从全局考虑可以方便应用程序和系统、应用程序之间、应用程序内的通信，所以对单个应用程序而言BroadcastReceiver是存在安全性问题的(恶意程序脚本不断的去发送你所接收的广播)。为了解决这个问题LocalBroadcastManager就应运而生了。
2.LocalBroadcastManager 是V4包中的一个类，主要负责程序内部广播的注册与发送。也就是说，它只是适用代码中注册发送广播，对于在AndroidManifest中注册的广播接收，则不适用。
3.LocalBroadcastManager是Android Support包提供了一个工具，用于在同一个应用内的不同组件间发送Broadcast。LocalBroadcastManager也称为局部通知管理器，这种通知的好处是安全性高，效率也高，适合局部通信，可以用来代替Handler更新UI
4.LocalBroadcastManager注册广播只能通过代码注册的方式。传统的广播可以通过代码和xml两种方式注册。
5.LocalBroadcastManager注册广播后，一定要记得取消监听。这一步可以有效的解决内存泄漏的问题。
6.LocalBroadcastManager所发送的广播action，只能与注册到LocalBroadcastManager中BroadcastReceiver产生互动
LocalBroadcastManager用法
1.LocalBroadcastManager对象的创建
LocalBroadcastManager localBroadcastManager = LocalBroadcastManager.getInstance( this ) ;
2.注册广播接收器
LocalBroadcastManager.registerReceiver( broadcastReceiver , intentFilter );
3.发送广播
LocalBroadcastManager.sendBroadcast( intent ) ;
4.取消注册广播接收器
LocalBroadcastManager.unregisterReceiver( broadcastReceiver );
源码分析
源码路径:
/frameworks/support/core-utils/java/android/support/v4/content/LocalBroadcastManager.java
public final class LocalBroadcastManager {
//内部类ReceiverRecord，保存IntentFilter和BroadcastReceiver
private static class ReceiverRecord {
final IntentFilter filter;
final BroadcastReceiver receiver;
boolean broadcasting;
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);
builder.append("}");
return builder.toString();
}
}
private static class BroadcastRecord {
final Intent intent;
final ArrayList<ReceiverRecord> receivers;
BroadcastRecord(Intent _intent, ArrayList<ReceiverRecord> _receivers) {
intent = _intent;
receivers = _receivers;
}
}
private static final String TAG = "LocalBroadcastManager";
private static final boolean DEBUG = false;
private final Context mAppContext;
private final HashMap<BroadcastReceiver, ArrayList<IntentFilter>> mReceivers
= new HashMap<BroadcastReceiver, ArrayList<IntentFilter>>();
private final HashMap<String, ArrayList<ReceiverRecord>> mActions
= new HashMap<String, ArrayList<ReceiverRecord>>();
private final ArrayList<BroadcastRecord> mPendingBroadcasts
= new ArrayList<BroadcastRecord>();
static final int MSG_EXEC_PENDING_BROADCASTS = 1;
private final Handler mHandler;
private static final Object mLock = new Object();
private static LocalBroadcastManager mInstance;
//单例模式的做法，并且把外部传进来的Context 转化成了ApplicationContext，有效的避免了当前Context的内存泄漏的问题
public static LocalBroadcastManager getInstance(Context context) {
synchronized (mLock) {
if (mInstance == null) {
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}
private LocalBroadcastManager(Context context) {
mAppContext = context;
//构造函数中创建了Handler，使用的getMainLooper是主线程的消息队列，所以避免耗时操作
mHandler = new Handler(context.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
//收到MSG_EXEC_PENDING_BROADCASTS，调用函数处理广播
case MSG_EXEC_PENDING_BROADCASTS:
executePendingBroadcasts();
break;
default:
super.handleMessage(msg);
}
}
};
}
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);
}
}
}
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);
}
}
}
}
}
}
//根据Intent的信息，匹配出对应的BroadcastReceiver，然后用BroadcastReceiver构造出BroadcastRecord对象，并发送消息，触发主线程进行处理
public boolean sendBroadcast(Intent intent) {
synchronized (mReceivers) {
//首先解析出Intent中携带的信息
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);
//根据Action取出所有初步匹配的ReceiverRecord，其中包含IntentFilter和BroadcastReceiver
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;
}
//利用IntentFilter的接口进行完整的匹配
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>();
}
//匹配成功后，将receiver加入到receivers中，并将broadcasting标志置为true
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;
}
//最后，构造BroadcastRecord，并加入到mPendingBroadcasts中
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
//若主线程队列中没有MSG_EXEC_PENDING_BROADCASTS，则发送该消息
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
}
}
return false;
}
public void sendBroadcastSync(Intent intent) {
if (sendBroadcast(intent)) {
executePendingBroadcasts();
}
}
private void executePendingBroadcasts() {
while (true) {
BroadcastRecord[] brs = null;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord[N];
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++) {
//回调BroadcastReceiver的onReceive函数
//从这里可以看出，无论在什么线程利用LocalBroadcastManager注册BroadcastReceiver
//BroadcastReceiver的onReceive函数均在主线程被回调
//这与普通广播的处理相似
br.receivers.get(j).receiver.onReceive(mAppContext, br.intent);
}
}
}
}
}
源码分析总结:
1.在获取LocalBroadcastManager对象实例的时候，这里用了单例模式。并且把外部传进来的Context 转化成了ApplicationContext，有效的避免了当前Context的内存泄漏的问题。这一点我们在设计单例模式框架的时候是值得学习的，看源码可以学习到很多东西。
2.在LocalBroadcastManager构造函数中创建了一个Handler.可见 LocalBroadcastManager 的本质上是通过Handler机制发送和接收消息的。
3.在创建Handler的时候，用了 context.getMainLooper() , 说明这个Handler是在Android 主线程中创建的，广播接收器的接收消息的时候会在Android 主线程，所以我们决不能在广播接收器里面做耗时操作，以免阻塞UI。
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参考:
Android LocalBroadcastManager 的使用总结
http://www.cnblogs.com/zhaoyanjun/p/6048369.html
Android编程之LocalBroadcastManager源码详解
http://blog.csdn.net/xyz_fly/article/details/18970569
Android中LocalBroadcastManager的基本用法及源码分析
http://blog.csdn.net/gaugamela/article/details/56671312
Android接收广播阻塞解决和优先级的问题
http://www.cnblogs.com/lijunamneg/p/5859662.html
