Android LocalBroadcastManager解析

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