LocalBroadcastManager源码分析

1、LocalBroadcastManager是什么

LocalBroadcastManager是Android Support包提供了一个工具，用于在同一个应用内的不一样组件间发送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，适合局部通讯，能够用来代替Handler更新UI。

2、LocalBroadcastManager的优势

• 因广播数据在本应用范围内传播，你不用担忧隐私数据泄露的问题。因此LocalBroadcastManager比BroadcastReceiver更安全。
• 不用担忧别的应用伪造广播，形成安全隐患。
• 相比在系统内发送全局广播，它更高效。

3、LocalBroadcastManager使用

• 获取LocalBroadcastManager实例

LocalBroadcastManager localBroadcastManager = LocalBroadcastManager.getInstance(ctx);

• 注册，须要传入BroadcastReceiver和IntentFilter对象

localBroadcastManager.registerReceiver(receiver, filter);

• 发送，需传入一个Intenet对象

localBroadcastManager.sendBroadcast(intent);

• 注销，须要经过建立的BroadcastReceiver对象来实现注销

localBroadcastManager.unregisterReceiver(receiver);

4、LocalBroadcastManager使用小例子

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

    <TextView
        android:id="@+id/local_textview"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="LocalBroadcastReciver"
        android:textColor="@android:color/holo_orange_light"
        android:textSize="20sp" />
</LinearLayout>

public class LocalBroadcastReciverAct extends AppCompatActivity {

    private LocalBroadcastManager localBroadcastManager = null;
    private BroadcastReceiver broadcastReceiver = null;
    private IntentFilter intentFilter = null;

    private static final String ACTION = "LOCAL_BROADCAST_RECIVER_ACTION";
    private TextView localTextview = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_local_broadcast_reciver);
        localTextview = findViewById(R.id.local_textview);
        localTextview.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                localBroadcastManager.sendBroadcast(new Intent(ACTION));
            }
        });
        localBroadcastManager = LocalBroadcastManager.getInstance(this);
    }

    @Override
    protected void onStart() {
        super.onStart();
        initReciverFilter();
        localBroadcastManager.registerReceiver(broadcastReceiver, intentFilter);
    }

    private void initReciverFilter() {
        broadcastReceiver = new MyBroadcastReciver();
        intentFilter = new IntentFilter(ACTION);
    }

    @Override
    protected void onStop() {
        super.onStop();
        localBroadcastManager.unregisterReceiver(broadcastReceiver);
    }

    private class MyBroadcastReciver extends BroadcastReceiver {

        @Override
        public void onReceive(Context context, Intent intent) {
            localTextview.setText("已接受到LocalBroadcastReciver");
        }
    }

}

效果图

5、源码分析

LocalBroadcastManager localBroadcastManager = LocalBroadcastManager.getInstance(ctx);

private final Handler mHandler;

    private static final Object mLock = new Object();
    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);
                }
            }
        };
    }

从构造方法私有化，很明显这是一个单利模式，而且进行了加锁操做因此是一个线程安全的单利模式；而且在LocalBroadcastManager的构造方法中咱们看见了mHandler = new Handler(context.getMainLooper())，因此能够初步判定LocalBroadcastManager是经过handler+message实现的消息广播，context.getMainLooper()是主线程的Looper，因此这个handler是运行在主线程的，不能进行耗时操做！
 

localBroadcastManager.registerReceiver(broadcastReceiver, intentFilter);

// 广播接收器做为key，接收器记录集合做为value
private final HashMap<BroadcastReceiver, ArrayList<ReceiverRecord>> mReceivers = new HashMap<>();
// 筛选动做(类型)做为key，接收器记录集合做为value
private final HashMap<String, ArrayList<ReceiverRecord>> mActions = new HashMap<>();

public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
        synchronized (mReceivers) {
            ReceiverRecord entry = new ReceiverRecord(filter, receiver);
            ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);
            if (filters == null) {
                filters = new ArrayList<>(1);
                mReceivers.put(receiver, filters);
            }
            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);
                }
                entries.add(entry);
            }
        }
    }

registerReceiver只是作了一些数据的分类保存操做。ReceiverRecord是一个内部类，主要是存储广播接收器BroadcastReceiver，筛选器IntentFilter，及广播运行状态。

private static final class ReceiverRecord {
        final IntentFilter filter; // 筛选器
        final BroadcastReceiver receiver; // 广播接收器
        boolean broadcasting; // 是否处于激活状态
        boolean dead; // 广播是否已经销毁
        ............
    }

 

private static final String ACTION = "LOCAL_BROADCAST_RECIVER_ACTION";
localBroadcastManager.sendBroadcast(new Intent(ACTION));

public boolean sendBroadcast(Intent intent) {
        // 保证同一时间，只有一个线程在操做
        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);
            
            // 得到对象动做(类型)的广播接收器记录集合
            ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());
            if (entries != null) {
                if (debug) Log.v(TAG, "Action list: " + entries);
            
                // 真正符合条件的接收器记录集
                ArrayList<ReceiverRecord> receivers = null;
                // 遍历集合，找到符合条件的ReceiverRecord
                for (int i=0; i<entries.size(); i++) {
                    ReceiverRecord receiver = entries.get(i); 
                    if (debug) Log.v(TAG, "Matching against filter " + receiver.filter);
                    
                    // 已经处于激活状态的ReceiverRecord过滤掉（主要避免重复加入）
                    if (receiver.broadcasting) {
                        if (debug) {
                            Log.v(TAG, "  Filter's target already added");
                        }
                        continue;
                    }

                    // 判断本次循环取出的ReceiverRecord，在动做、数据，分类是否匹配
                    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>();
                        }
                        receivers.add(receiver); // 符合条件的加入到receivers中
                        receiver.broadcasting = true; // 而且标记广播状态为已激活
                    } 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;  // 筛选完后，将状态重置
                    }
                    // 从新构造一个BroadcastRecord对象，并添加到待发送状态下的广播列表
                    mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
                    // 查看handler在排队执行的消息中是否有MSG_EXEC_PENDING_BROADCASTS类型，
                    // 没有，添加一个MSG_EXEC_PENDING_BROADCASTS类型的消息
                    if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
                        mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
                    }
                    return true;
                }
            }
        }
        return false;
    }

sendBroadcast()中主要进行的是广播接受器的筛选操做，而后通知handler准备调用筛选出来的广播接收器。

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);
                }
            }
        };
    }

private void executePendingBroadcasts() {
        while (true) {
            final BroadcastRecord[] brs; // 使用数组存放，广播接受记录
            synchronized (mReceivers) {
                final int N = mPendingBroadcasts.size();
                if (N <= 0) {
                    return;
                }
                brs = new BroadcastRecord[N];
                // 转换成功数组格式并清空集合
                mPendingBroadcasts.toArray(brs); 
                mPendingBroadcasts.clear();
            }
            for (int i=0; i<brs.length; i++) {
                final BroadcastRecord br = brs[i];
                final int nbr = br.receivers.size();
                // 遍历调用接收器方法
                for (int j=0; j<nbr; j++) {
                    final ReceiverRecord rec = br.receivers.get(j);
                    // 不在销毁状态下，调用接收器的onReceive()方法执行
                    if (!rec.dead) {
                        rec.receiver.onReceive(mAppContext, br.intent);
                    }
                }
            }
        }
    }

executePendingBroadcasts()执行接收器的onReceive()方法。
 

localBroadcastManager.unregisterReceiver(broadcastReceiver);

public void unregisterReceiver(BroadcastReceiver receiver) {
        synchronized (mReceivers) {
            // 从mReceivers中移除，并返回对应的记录集合
            final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);
            if (filters == null) {
                return;
            }
            // 遍历接收器记录集合，并设置dead = true; 避免正在运行的executePendingBroadcasts()执行此接收器的回调
            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);
                            }
                        }
                        if (receivers.size() <= 0) {
                            mActions.remove(action);
                        }
                    }
                }
            }
        }
    }

unregisterReceiver()中主要是资源的释放，并将注册的BroadcastReceiver相关信息，从集合中删除，并设置BroadcastReceiver对应的ReceiverRecord的dead状态标记为true，避免被执行。

6、总结

LocalBroadcastManager的内部是经过handler来实现广播消息的分发，这样的广播只能在本应用内分发，因此没必要担忧安全问题，由于别人根本够不到你~~

做者：Lawrence飝 连接：https://www.jianshu.com/p/8c706a8968f0 来源：简书 简书著做权归做者全部，任何形式的转载都请联系做者得到受权并注明出处。