Android之LocalBroadCastManager实现原

LocalBroadcastManager实现原理

1：概述

我们都知道广播可以用来实现跨进程通信，它的原理就是binder,通过system_server来实现的，那么当我们只是在应用内部来实现通信的时候，其实我们可以考虑使用LocalBroadcastManager来实现，它有两个好处
1.1：性能较一般的broadcast要好，因为它的实现不需要经过binder调用，它采用的是handler通信来实现的
1.2：安全性较好，不必担心数据泄露或是被截取
LocalBroadcastManager是单利模式，并且只能通过代码的方式注册，不能通过xml的方式注册，下面是谷歌对其的解释
/**

• Helper to register for and send broadcasts of Intents to local objects
• within your process.
• This has a number of advantages over sending
• global broadcasts with {@link android.content.Context#sendBroadcast}:
• You know that the data you are broadcasting won't leave your app, so
• don't need to worry about leaking private data.
• It is not possible for other applications to send these broadcasts to
• your app, so you don't need to worry about having security holes they can
• exploit.
• It is more efficient than sending a global broadcast through the
• system.
  */

2：实现原理

2.1：单例模式

@NonNull
public static LocalBroadcastManager getInstance(@NonNull Context context) {
    synchronized (mLock) { //锁
        if (mInstance == null) {//创建对象
            mInstance = new LocalBroadcastManager(context.getApplicationContext());
        }
        return mInstance;
    }
}

private LocalBroadcastManager(Context context) {
    mAppContext = context;
    //getMainLooper是主线程的对应的handler对象
    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);
            }
        }
    };
}

2.2注册广播registerReceiver

public void registerReceiver(@NonNull BroadcastReceiver receiver,
        @NonNull IntentFilter filter) {
    synchronized (mReceivers) {
        //创建一个ReceiverRecord对象
        ReceiverRecord entry = new ReceiverRecord(filter, receiver);
        //先从缓存的HashMap中查询，key为receiver
        ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);
        if (filters == null) {
            filters = new ArrayList<>(1);
            mReceivers.put(receiver, filters);//缓存到hashmap列表中去
        }
        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);//缓存到action的Hashmap列表中
            }
            entries.add(entry);
        }
    }
}

这个过程主要是将数据缓存到两个列表中去

1： private final HashMap<BroadcastReceiver, ArrayList<ReceiverRecord>> mReceivers
= new HashMap<>(); key为receiver,value为ReceiverRecord列表

2： private final HashMap<String, ArrayList<ReceiverRecord>> mActions = new HashMap<>();
将action缓存到Hashmap中去，记录的是action与广播接收者之间的关系

2.3：取消注册unregisterReceiver

public void unregisterReceiver(@NonNull BroadcastReceiver receiver) {
    synchronized (mReceivers) {
        //从ArratList中移除广播接收者，remove方法返回的是value值
        final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);
        if (filters == null) {//如果为null的话不做后续处理
            return;
        }
        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);//从mActions移除接收者为空的action
                        }
                    }
                    if (receivers.size() <= 0) {
                        mActions.remove(action);
                    }
                }
            }
        }
    }
}

2.4发送广播sendBroadcast

public boolean sendBroadcast(@NonNull 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);
        //intent读取action，根据action获取相应的广播接收者列表
        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;
                }
                //根据intent和接收者信息创建广播对象，添加到mPendingBroadcasts
                mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
                if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
                    mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
                } //发送到主线程对应的消息队列中去
                return true;
            }
        }
    }
    return false;
}

2.5：主线程消息队列：消息的处理

mHandler = new Handler(context.getMainLooper()) {

        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_EXEC_PENDING_BROADCASTS:
                    executePendingBroadcasts();//广播的处理
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    };

private void executePendingBroadcasts() {
    while (true) {
        final BroadcastRecord[] brs;
        synchronized (mReceivers) {
            final int N = mPendingBroadcasts.size();
            if (N <= 0) {
                return;
            }
            brs = new BroadcastRecord[N];//缓存到数组中，广播接收者可能有多个
            mPendingBroadcasts.toArray(brs);
            mPendingBroadcasts.clear();
        }
        for (int i=0; i<brs.length; i++) {
            final BroadcastRecord br = brs[i];
            final int nbr = br.receivers.size();
            for (int j=0; j<nbr; j++) {
                final ReceiverRecord rec = br.receivers.get(j);
                if (!rec.dead) {
                    rec.receiver.onReceive(mAppContext, br.intent);//回调广播接收者的onReceive方法
                }
            }
        }
    }
}

3：概括

优点：
1：相对于串行广播，更加安全，数据只会在应用内部传输，并且限定在同一进程中，一个APK中其他的进程也是不行的
2：性能较好：相对于串行广播，其不对system_server造成任何的性能问题，并且也没有跨进程调用带来的耗时问题
缺点：
1：不支持跨进程，如果跨进程必须考虑其他实现方式
2：不支持静态xml的方式注册，无法实现监听诸如系统开机广播等