LocalBroadcastManager原理分析及应用

Posted 2022-12-01 aspook

引言

android页面或模块之间通信的方法有很多，如Intent传值、startActivityForResult、EventBus（RxBus）等，大家追求的无非是解耦以及高灵活性；我们自己的应用中使用了基于Android消息机制封装的一套通信体系，且不谈这些，今天的主角是本地广播。

本地广播是系统提供的一种应用内通信方式，它跟全局广播相比，其运行机制是不同的。全局广播是系统级别的，需要跨进程调用，而且作用域是整个设备，而本地广播的作用域只在当前应用之内，因此无需IPC的过程，本应用发送的本地广播，其他应用接收不到，其他应用发送的本地广播，本应用也接收不到。

本地广播的优点

1. 本地广播作用于App内部，广播数据不会泄露，本应用也不会接收到其他应用的广播，因此安全性较高
2. 本地广播是在应用内部执行，无需跨进程逻辑，与普通广播相比效率更高
3. 使用简单，无需静态注册

原理分析

本地广播的原理也很简单，大概流程如下

1. 首先需要接收广播的页面创建一个BroadcastReceiver（广播声明跟全局广播是一样的）并注册到一个应用内的本地广播接收器列表（注册方式跟全局广播不同，下文会详细说明）
2. 某个页面利用Intent发送广播（发送方式跟全局广播不同，下文会详细说明）
3. 本地广播接收器列表根据IntentFilter匹配得到可以接收该广播的BroadcastReceiver，然后匹配的广播接收器会响应广播（注意这里区分广播的异步与同步发送）
4. 在适当时候反注册BroadcastReceiver，比如Activity的onDestroy()回调中

为了方便大家的使用，Android提供了LocalBroadcastManager类（姑且称之为本地广播管理器），该类帮我们封装了注册、反注册、广播发送等过程，接下来分析一下源码实现。

LocalBroadcastManager定义

首先LocalBroadcastManager定义为一个单例，方便App内全局使用。

private static LocalBroadcastManager mInstance;

public static LocalBroadcastManager getInstance(Context context) 
    synchronized (mLock) 
        if (mInstance == null) 
            mInstance = new LocalBroadcastManager(context.getApplicationContext());
        
        return mInstance;
    


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);
            
        
    ;

注意到LocalBroadcastManager的构造函数中创建了一个Handler实例，并且是运行在主线程的，它响应一种what为MSG_EXEC_PENDING_BROADCASTS的消息，这是用来处理异步广播的，具体如何处理稍后说明。

两个内部类

对于广播接收器的注册过程，并非简单地把BroadcastReceiver进行注册，而是进行了简单的包装，这里涉及到LocalBroadcastManager的两个内部类，如下：

ReceiverRecord

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();
    

顾名思义，ReceiverRecord简单封装了BroadcastReceiver和IntentFilter。

BroadcastRecord

private static class BroadcastRecord 
    final Intent intent;
    final ArrayList<ReceiverRecord> receivers;

    BroadcastRecord(Intent _intent, ArrayList<ReceiverRecord> _receivers) 
        intent = _intent;
        receivers = _receivers;
    

BroadcastRecord也很简单，封装了广播Intent以及能够匹配该Intent的接收器列表（这里并非直接是BroadcastReceiver，而是包装类ReceiverRecord）。

基于上述两个包装类，LocalBroadcastManager提供了如下两个HashMap用来存储注册的接收器

private final HashMap<BroadcastReceiver, ArrayList<IntentFilter>> mReceivers
        = new HashMap<BroadcastReceiver, ArrayList<IntentFilter>>();
private final HashMap<String, ArrayList<ReceiverRecord>> mActions
        = new HashMap<String, ArrayList<ReceiverRecord>>();

此外还有一个叫mPendingBroadcasts的容器如下

private final ArrayList<BroadcastRecord> mPendingBroadcasts
        = new ArrayList<BroadcastRecord>();

它是用来存储待处理广播的列表，其元素为BroadcastRecord。

注册过程

注册过程源码如下，具体流程分析请看注释

/**
 * Register a receive for any local broadcasts that match the given IntentFilter.
 *
 * @param receiver The BroadcastReceiver to handle the broadcast.
 * @param filter Selects the Intent broadcasts to be received.
 *
 * @see #unregisterReceiver
 */
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) 
    // 加锁同步
    synchronized (mReceivers) 
        // 将BroadcastReceiver和IntentFilter封装成ReceiverRecord
        ReceiverRecord entry = new ReceiverRecord(filter, receiver);
        // BroadcastReceiver作为HashMap的key，注意一般来说注册时一个BroadcastReceiver对应一个IntentFilter。但有一种情况下同一个页面内使用同一个BroadcastReceiver，配合不同的IntentFilter注册多次；更极端情况是应用内只有一个BroadcastReceiver，使用不同的IntentFilter进行多次注册
        ArrayList<IntentFilter> filters = mReceivers.get(receiver);
        if (filters == null) 
            filters = new ArrayList<IntentFilter>(1);
            mReceivers.put(receiver, filters);
        
        filters.add(filter);
        // 一个IntentFilter可以对应多个action
        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);
        
    

反注册过程

对应的反注册过程如下，代码逻辑请看注释

/**
 * Unregister a previously registered BroadcastReceiver.  <em>All</em>
 * filters that have been registered for this BroadcastReceiver will be
 * removed.
 *
 * @param receiver The BroadcastReceiver to unregister.
 *
 * @see #registerReceiver
 */
public void unregisterReceiver(BroadcastReceiver receiver) 
    // 加锁同步
    synchronized (mReceivers) 
        // 同一个BroadcastReceiver可能对应多个IntentFilter，移除以BroadcastReceiver为key的键值对
        ArrayList<IntentFilter> filters = mReceivers.remove(receiver);
        if (filters == null) 
            return;
        
        for (int i=0; i<filters.size(); i++) 
            IntentFilter filter = filters.get(i);
            // 每个IntentFilter可对应多个action
            for (int j=0; j<filter.countActions(); j++) 
                String action = filter.getAction(j);
                // 以action作为key
                ArrayList<ReceiverRecord> receivers = mActions.get(action);
                if (receivers != null) 
                    for (int k=0; k<receivers.size(); k++) 
                        // 移除包含目标BroadcastReceiver的ReceiverRecord
                        if (receivers.get(k).receiver == receiver) 
                            receivers.remove(k);
                            k--;
                        
                    
                    if (receivers.size() <= 0) 
                        mActions.remove(action);
                    
                
            
        
    

异步发送广播

异步发送广播利用了Android消息机制，发送完成后即可返回，广播会异步响应，通常情况下我们都是使用异步方式，否则会被阻塞。

/**
 * Broadcast the given intent to all interested BroadcastReceivers.  This
 * call is asynchronous; it returns immediately, and you will continue
 * executing while the receivers are run.
 *
 * @param intent The Intent to broadcast; all receivers matching this
 *     Intent will receive the broadcast.
 *
 * @see #registerReceiver
 */
public boolean sendBroadcast(Intent intent) 
    synchronized (mReceivers) 
        // 提取出Intent中的action、type、data、scheme、categories等，稍后用于匹配
        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();
        // debug仅用于打印log，可忽略
        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);

        // 首先根据广播Intent的action进行匹配，匹配不上则直接返回false
        ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());
        if (entries != null) 
            if (debug) Log.v(TAG, "Action list: " + entries);

            // receivers用来存储匹配上的接收器包装类ReceiverRecord
            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;
                

                // InterFilter的匹配，若匹配成功，返回值match>=0
                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>();
                    
                    // 匹配成功的接收器加入列表，并设置一个标志
                    receivers.add(receiver);
                    receiver.broadcasting = true;
                 else 
                    // 匹配失败在debug为true的情况下仅输出log
                    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);
                    
                
            

            // 匹配成功的接收器
            if (receivers != null) 
                // 将上述匹配成功时设置的标志重新置为false
                for (int i=0; i<receivers.size(); i++) 
                    receivers.get(i).broadcasting = false;
                
                // 将匹配成功的接收器封装为BroadcastRecord，加入待处理广播列表
                mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
                // 利用Handler消息机制，发送消息，异步处理广播
                if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) 
                    mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
                
                return true;
            
        
    
    return false;

可以看到异步广播最终利用Android消息机制切换到主线程执行广播接收过程，具体实现逻辑则是在LocalBroadcastManager的构造器中：

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);
            
        
    ;

最终调用方法为executePendingBroadcasts()，稍后详解。

同步发送广播

同步发送广播比较简单，实现如下

/**
 * Like @link #sendBroadcast(Intent), but if there are any receivers for
 * the Intent this function will block and immediately dispatch them before
 * returning.
 */
public void sendBroadcastSync(Intent intent) 
    if (sendBroadcast(intent)) 
        executePendingBroadcasts();
    

具体的广播执行也是在executePendingBroadcasts()中。

执行广播

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);
            
        
    

上述代码逻辑比较简单，从mPendingBroadcasts中遍历执行每个BroadcastReceiver的onReceive方法，触发BroadcastReceiver声明页面的回调。在回调中，我们就可以实现自己的广播接收逻辑了。

本地广播的响应逻辑为

@Override
public void onReceive(Context context, Intent intent) 
    // 处理逻辑

因此如果要传值的话则要依赖于Intent的能力。

注意事项

1. 发送广播可以在主线程也可以在子线程
2. 接收广播在哪个线程要看是异步广播还是同步广播。如果是异步广播，无论发送广播是在主线程还是在子线程，内部都会通过Handler消息机制切换到主线程执行。如果是同步广播，发送广播的线程和接收广播的线程为同一线程，此种方式会导致线程阻塞，使用时需谨慎。
3. 如果广播接收在主线程的话，注意不要进行耗时操作
4. 通过广播传值要依赖于Intent的传值方式及要求

下表是一个简单总结

发送本地广播的线程	同步 or 异步	是否阻塞	接收本地广播的线程
主线程	异步	否	主线程
主线程	同步	是	主线程
子线程	异步	否	主线程
子线程	同步	是	子线程

应用场景

1. 单对单通信

   场景描述：一个页面向另一个页面发送消息，可以是相邻页面也可以不相邻

   实现方式：使用特定的action注册BroadcastReceiver，并使用具有该action的Intent发送本地广播

2. 单对多通信

   场景描述：一个页面发送广播，多个页面可以接收

   实现方式：每个页面都使用相同的action注册BroadcastReceiver即可

3. 多对单通信

   场景描述：一个页面可以接收到多个不同广播

   实现方式：该页面使用多个不同的action（一个IntentFilter可以有多个action）注册BroadcastReceiver，这样就能接收到具有不同action的广播了

应用示例

以下示例有3个页面，分别为ActivityA、ActivityB、ActivityC，其中ActivityA和ActivityB分别注册了广播接收器，ActivityC发送广播，ActivityA和ActivityB都可以接收到广播，接收到广播之后取出传值显示在UI上。

代码中注释了子线程发送广播、同步发送广播等逻辑，同时还有Log输出，可以查看同步、异步的执行顺序以及发送广播和接收广播所在的线程信息。源代码很简单，如下。

1. OnReceiveBrodadcastListener接口

   public interface OnReceiveBroadcastListener 
      void onReceive(Context context, Intent intent);
   

2. LocalBroadcastReceiver类，继承自BroadcastReceiver

   public class LocalBroadcastReceiver extends BroadcastReceiver 
      private OnReceiveBroadcastListener mListener;
   
      @Override
      public void onReceive(Context context, Intent intent) 
          if (mListener != null) 
              mListener.onReceive(context, intent);
          
      
   
      public LocalBroadcastReceiver(OnReceiveBroadcastListener listener) 
          mListener = listener;
      
   

3. ActivityA的实现

   public class ActivityA extends AppCompatActivity 
      private static final String TAG = "LocalBroadcastManager";
      private TextView tv_a;
      private Button btn_a;
      private LocalBroadcastReceiver mReceiver;
   
      @Override
      protected void onCreate(Bundle savedInstanceState) 
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_a);
   
          Log.d(TAG, "main thread id:" + Thread.currentThread().getId() + ",name:" + Thread.currentThread().getName());
   
          tv_a = (TextView) findViewById(R.id.tv_a);
          btn_a = (Button) findViewById(R.id.btn_a);
   
          btn_a.setOnClickListener(new View.OnClickListener() 
              @Override
              public void onClick(View v) 
                  Intent intent = new Intent(ActivityA.this, ActivityB.class);
                  startActivity(intent);
              
          );
   
          mReceiver = new LocalBroadcastReceiver(new OnReceiveBroadcastListener() 
              @Override
              public void onReceive(Context context, Intent intent) 
                  // 注意：异步消息是在主线程处理的，务必避免耗时操作
                  // 如果接收多个广播，可以根据action分别处理
                  String action = intent.getAction();
   
                  Log.d(TAG, "A onReceive=== thread id:" + Thread.currentThread().getId() + ",name:" + Thread.currentThread().getName());
                  Log.d(TAG, "A received broadcast,action=" + action);
   
                  // 取出广播传值
                  String name = intent.getStringExtra("NAME");
                  tv_a.setText(name);
              
          );
   
          IntentFilter filter = new IntentFilter();
          // 一个IntentFilter可以配置多个action
          filter.addAction("com.aspook.localbroadcast_A");
          filter.addAction("com.aspook.localbroadcast_AA");
          LocalBroadcastManager.getInstance(this).registerReceiver(mReceiver, filter);
   
          // 同一个BroadcastReceiver可以注册多次，极端情况下，全局可以只有一个
   //        IntentFilter filter2 = new IntentFilter();
   //        filter2.addAction("com.aspook.localbroadcast_AAA");
   //        LocalBroadcastManager.getInstance(this).registerReceiver(mReceiver, filter2);
      
   
      @Override
      protected void onDestroy() 
          super.onDestroy();
          LocalBroadcastManager.getInstance(this).unregisterReceiver(mReceiver);
      
   

4. ActivityB的实现

   public class ActivityB extends AppCompatActivity 
      private static final String TAG = "LocalBroadcastManager";
      private TextView tv_b;
      private Button btn_b;
      private LocalBroadcastReceiver mReceiver;
   
      @Override
      protected void onCreate(Bundle savedInstanceState) 
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_b);
   
          tv_b = (TextView) findViewById(R.id.tv_b);
          btn_b = (Button) findViewById(R.id.btn_b);
   
          btn_b.setOnClickListener(new View.OnClickListener() 
              @Override
              public void onClick(View v) 
                  Intent intent = new Intent(ActivityB.this, ActivityC.class);
                  startActivity(intent);
              
          );
   
          mReceiver = new LocalBroadcastReceiver(new OnReceiveBroadcastListener() 
              @Override
              public void onReceive(Context context, Intent intent) 
                  Log.d(TAG, "B onReceive=== thread id:" + Thread.currentThread().getId() + ",name:" + Thread.currentThread().getName());
                  Log.d(TAG, "B received broadcast，action=" + intent.getAction());
   
                  Bundle bundle = intent.getExtras();
                  int age = bundle.getInt("AGE");
                  tv_b.setText(age + "");
              
          );
   
          IntentFilter filter = new IntentFilter();
          // 允许定义一个action数组，以接收不同的消息
          filter.addAction("com.aspook.localbroadcast_B");
          filter.addAction("com.aspook.localbroadcast_BB");
          filter.addAction("com.aspook.localbroadcast_AA");
          LocalBroadcastManager.getInstance(this).registerReceiver(mReceiver, filter);
      
   
      @Override
      protected void onDestroy() 
          super.onDestroy();
          LocalBroadcastManager.getInstance(this).unregisterReceiver(mReceiver);
      
   

5. ActivityC的实现

   public class ActivityC extends AppCompatActivity 
      private static final String TAG = "LocalBroadcastManager";
      private TextView tv_c;
      private Button btn_c;
   
      @Override
      protected void onCreate(Bundle savedInstanceState) 
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_c);
   
          tv_c = (TextView) findViewById(R.id.tv_c);
          btn_c = (Button) findViewById(R.id.btn_c);
   
          btn_c.setOnClickListener(new View.OnClickListener() 
              @Override
              public void onClick(View v) 
                  // 在主线程发送本地广播
                  Log.d(TAG, "send broadcast=== thread id:" + Thread.currentThread().getId() + ",name:" + Thread.currentThread().getName());
   
                  Intent intent = new Intent();
                  // Intent传值示例
                  intent.putExtra("NAME", "kevin");
                  Bundle bundle = new Bundle();
                  bundle.putInt("AGE", 22);
                  intent.putExtras(bundle);
   
                  intent.setAction("com.aspook.localbroadcast_AA");
                  // 异步执行
                  LocalBroadcastManager.getInstance(getApplicationContext()).sendBroadcast(intent);
                  // 同步执行
                  //LocalBroadcastManager.getInstance(getApplicationContext()).sendBroadcastSync(intent);
                  Log.d(TAG, "do other things after localbroadcast!!!");
   
                  // 在子线程发送本地广播
   //                new Thread(new Runnable() 
   //                    @Override
   //                    public void run() 
   //                        Log.d(TAG, "send broadcast=== thread id:" + Thread.currentThread().getId() + ",name:" + Thread.currentThread().getName());
   //
   //                        Intent intent = new Intent();
   //                        intent.putExtra("NAME", "kevin");
   //                        Bundle bundle = new Bundle();
   //                        bundle.putInt("AGE", 22);
   //                        intent.putExtras(bundle);
   //                        // action should defined as message constant
   //                        intent.setAction("com.aspook.localbroadcast_AAA");
   //                        // 异步执行
   //                        LocalBroadcastManager.getInstance(getApplicationContext()).sendBroadcast(intent);
   //                        // 同步执行
   //                        //LocalBroadcastManager.getInstance(getApplicationContext()).sendBroadcastSync(intent);
   //                        Log.d(TAG, "do other things after localbroadcast!!!");
   //                    
   //                ).start();
              
          );
      
   
      @Override
      protected void onDestroy() 
          super.onDestroy();
      
   

上面代码只是一个本地广播的应用演示，具体使用时可以再进一步抽象封装，可用作应用内的通信机制。