源码阅读

LocalBroadcastManager源码分析

Posted by alonealice on 2016-11-21

当我们在发送广播时，有时会遇到这样的问题，就是我不想让该广播被其他应用接收，或者我的广播接收器不想接收其他的应用的广播，这该怎么做呢？解决的方案有很多中，比如可以自定义广播的权限；比如可以给intent设置package等。但是这些方法，一旦应用被反编译，就完全无法保障其他应用再来发冒充我们的广播了。在Android系统中，有一个LocalBroadcastManager类，就是专门来解决广播只在应用内部流通的问题。
LocalBroadcastManager是在android.support.v4包中，它能使发送的广播只在本应用中传递，同时自己的接收器也不会接受到其他应用的广播。那它内部是怎样实现的呢？
LocalBroadcastManager使用了单例模式，其构造方法如下：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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);
                }
            }
        };
    }

这里的mLock对象是一个空Object对象，只是单纯的在这里用来锁一下。同时，在构造方法找那个，实现了一个在主线程中的Handler，这个hander具体怎么样，后面再说。
LocalBroadcastManager主要有三个方法，registerReceiver、unregisterReceiver和sendBroadcast。从名字就可以知道，一个用来注册接收器，一个用来注销接收器，还有一个用来发送广播，那我们一次来看看是怎么实现的。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
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);
            }
        }
    }


    private static class ReceiverRecord {
           final IntentFilter filter;
           final BroadcastReceiver receiver;
           boolean broadcasting;

           ReceiverRecord(IntentFilter _filter, BroadcastReceiver _receiver) {
               filter = _filter;
               receiver = _receiver;
           }

          ...
       }

首先是registerReceiver，首先先将BroadcastReceiver和IntentFilter封装成一个ReceiverRecord，再判断是否存有相同的BroadcastReceiver，如果没有，则以BroadcastReceiver为key，ArrayList为值，添加到列表，同时在ArrayList添加传入的IntentFilter；同样，又以IntentFilter的action为key，将ReceiverRecord存入到列表。
发送广播：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
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;
    }

简单分析一下：首先是根据传入intent的action，遍历所有该action的ReceiverRecord，同时通过IntentFilter的match方法匹配传入intent的各项数据是否符合ReceiverRecord中的IntentFilter，如果匹配，则保存该ReceiverRecord，同时将该ReceiverRecord的状态设置为发送中。最后将这些ReceiverRecord的状态通信置为为发送，将这些ReceiverRecord和intent保存成BroadcastRecord,添加到BroadcastRecord列表，再使用handler在主线程中发送广播。

1
2
3
4
5
6
7
8
9
private static class BroadcastRecord {
      final Intent intent;
      final ArrayList<ReceiverRecord> receivers;

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

具体发送代码：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
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);
               }
           }
       }
   }

将BroadcastRecord列表转成数组后，循环遍历每个intent 的ReceiverRecord，直接调用里面的BroadcastReceiver的onReceive方法，完成广播的发送。
最后再来看看注销广播：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
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);
                       }
                   }
               }
           }
       }
   }

注销广播无非也就是根据receive，将ReceiverRecord列表和actions列表中的数据移除。
这里我们看到了，避免外部广播的方式其实就是广播的发送不通过系统的进程间通信，而是直接在内部直接调用的方式，从而完成广播的传递。




FEATURED TAGS
源码阅读