本地广播介绍

  LocalBroadcastManager是Android Support包提供了一个工具，是用来在同一个应用内的不同组件间发送Broadcast的。

使用LocalBroadcastManager有如下好处：

1. 发送的广播只会在自己App内传播，不会泄露给其他App，确保隐私数据不会泄露
2. 其他App也无法向你的App发送该广播，不用担心其他App会来搞破坏比系统全局广播更加高效

使用方式

先通过

LocalBroadcastManager lbm = LocalBroadcastManager.getInstance(this);

获取实例,然后通过函数 registerReceiver来注册监听器

lbm.registerReceiver(new BroadcastReceiver() {  
        @Override  
        public void onReceive(Context context, Intent intent) {  
        // TODO Handle the received local broadcast  
        }  
    }, new IntentFilter(LOCAL_ACTION));  

通过 sendBroadcast 函数来发送广播

lbm.sendBroadcast(new Intent(LOCAL_ACTION));  

以下我们简单来做个Demo,实际体会一下。例子很简单，一个布局文件加一个主Activity。

布局文件

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">


    <Button
        android:id="@+id/sendLocal"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="发送本地广播" />
</RelativeLayout>

主Activity

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private final String ACTION = "org.bridge.localreceiver.LOCAL_BROADCAST";
    private LocalBroadcastManager manager;

    private LocalReceiver localReceiver;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        manager = LocalBroadcastManager.getInstance(this);
        //新建intentFilter
        IntentFilter filter = new IntentFilter();
        filter.addAction(ACTION);
        localReceiver = new LocalReceiver();
        manager.registerReceiver(localReceiver, filter);

        findViewById(R.id.sendLocal).setOnClickListener(this);

    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.sendLocal:
                Intent intent = new Intent(ACTION);
                manager.sendBroadcast(intent);
//                manager.sendBroadcastSync();
                break;
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        manager.unregisterReceiver(localReceiver);
    }
}

运行结果

点击发送按钮，我们的应用成功收到了广播，接下来我们分析一下，本地广播是如何实现只在当前应用内发送和接受的。

原理分析

以下是LocalBroadcastManager的源代码，整个类代码量不多，还是较为简单的。

public final class LocalBroadcastManager {
//静态内部类，ReceiverRecord  封装了BroadcastReceiver和IntentFilter
    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();
        }
    }
//静态内部类BroadcastRecord ，封装了Intent 和对应的ReceiverRecord
    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;

//BroadReceiver和对应IntentFilter映射
    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;

//单例模式获取实例
    public static LocalBroadcastManager getInstance(Context context) {
        synchronized (mLock) {
            if (mInstance == null) {
                mInstance = new LocalBroadcastManager(context.getApplicationContext());
            }
            return mInstance;
        }
    }
//私有构造方法，构造了一个Handler
    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);
                }
            }
        };
    }

   //注册广播
    public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
        synchronized (mReceivers) {
            ReceiverRecord entry = new ReceiverRecord(filter, receiver);
            //从映射表取得receiver对应的IntentFilter
            ArrayList<IntentFilter> filters = mReceivers.get(receiver);
            //如果没有找到，就添加这个映射
            if (filters == null) {
                filters = new ArrayList<IntentFilter>(1);
                mReceivers.put(receiver, filters);
            }
            filters.add(filter);
            //遍历filter,保存action到mActions中
            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);
            }
        }
    }

  //注销广播
    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);
                        }
                    }
                }
            }
        }
    }

    //发送广播
    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;
    }

   //发送同步广播
    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++) {
                    br.receivers.get(j).receiver.onReceive(mAppContext, br.intent);
                }
            }
        }
    }
}

主要成员变量含义

mReceivers存储广播和过滤器信息，以BroadcastReceiver作为key,IntentFilter链表作为value.mReceivers是接收器和IntentFilter的对应表，主要作用是方便在unregisterReceiver()取消注册，同时作为对象锁限制注册接收器，发送广播，取消接收器注册等几个过程的并发访问。

mActions 以Action为key,注册这个Action的BroadCastReceiver链表为value.mActions的主要作用是方便在广播发送后快速得到可以接受它的BroadCastReceiver.

发送广播的过程

先根据Action从mActions 中取出ReceiverRecord列表，循环每个ReceiverRecord 判断filter和intent 中action，type,scheme,data,categoried 是否match,是的话则保存到receivers列表中，发送what为 MSG_EXEC_PENDING_BROADCASTS的消息，通过Handler处理。

消息处理过程

mPendingBroadcasts转换为数组BroadcastRecord,循环每个receiver,调用其onReceive函数，这样便完成了广播的核心逻辑。

取消注册过程

从mReceivers及mActions中移除相应元素。

总结

LocalBroadcastManager 的核心实现实际上还是Handler，只是利用到了IntentFilter的match功能，至于BroadcastReceiver换成其他接口也无所谓，顺便利用了现成的类和概念而已。

因为采用了Handler实现了应用内通信，自然安全性更好，效率更高。

最后

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