（五十六）Android O WifiManager和WifiSerivceImpl间的AsyncChannel双向通道建立流程

前言：看了这篇https://blog.csdn.net/u010961631/article/details/48179305#t13 写的AsyncChannel写的很好，就着这篇博客梳理下WifiManager和WifiSerivceImpl间的双向通道建立流程。

1.WifiManager

WifiManager相当于客户端，主动建立单向通道

    private synchronized AsyncChannel getChannel() {
        if (mAsyncChannel == null) {
            Messenger messenger = getWifiServiceMessenger();
            if (messenger == null) {
                throw new IllegalStateException(
                        "getWifiServiceMessenger() returned null!  This is invalid.");
            }

            mAsyncChannel = new AsyncChannel();
            mConnected = new CountDownLatch(1);

            Handler handler = new ServiceHandler(mLooper);
            mAsyncChannel.connect(mContext, handler, messenger);
            try {
                mConnected.await();
            } catch (InterruptedException e) {
                Log.e(TAG, "interrupted wait at init");
            }
        }
        return mAsyncChannel;
    }

连接成功收到消息AsyncChannel.CMD_CHANNEL_HALF_CONNECTED，发送AsyncChannel.CMD_CHANNEL_FULL_CONNECTION建立双向通道

    // Ensure that multiple ServiceHandler threads do not interleave message dispatch.
    private static final Object sServiceHandlerDispatchLock = new Object();

    private class ServiceHandler extends Handler {
        ServiceHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message message) {
            synchronized (sServiceHandlerDispatchLock) {
                dispatchMessageToListeners(message);
            }
        }

        private void dispatchMessageToListeners(Message message) {
            Object listener = removeListener(message.arg2);
            switch (message.what) {
                case AsyncChannel.CMD_CHANNEL_HALF_CONNECTED:
                    if (message.arg1 == AsyncChannel.STATUS_SUCCESSFUL) {
                        mAsyncChannel.sendMessage(AsyncChannel.CMD_CHANNEL_FULL_CONNECTION);
                    } else {
                        Log.e(TAG, "Failed to set up channel connection");
                        // This will cause all further async API calls on the WifiManager
                        // to fail and throw an exception
                        mAsyncChannel = null;
                    }
                    mConnected.countDown();
                    break;

这边写一下AsynChannel connect原理

    /**
     * Connect handler and messenger.
     *
     * Sends a CMD_CHANNEL_HALF_CONNECTED message to srcHandler when complete.
     *      msg.arg1 = status
     *      msg.obj = the AsyncChannel
     *
     * @param srcContext
     * @param srcHandler
     * @param dstMessenger
     */
    public void connect(Context srcContext, Handler srcHandler, Messenger dstMessenger) {
        if (DBG) log("connect srcHandler to the dstMessenger  E");

        // We are connected
        connected(srcContext, srcHandler, dstMessenger);

        // Tell source we are half connected
        replyHalfConnected(STATUS_SUCCESSFUL);

        if (DBG) log("connect srcHandler to the dstMessenger X");
    }

    /**
     * Connect handler to messenger. This method is typically called
     * when a server receives a CMD_CHANNEL_FULL_CONNECTION request
     * and initializes the internal instance variables to allow communication
     * with the dstMessenger.
     *
     * @param srcContext
     * @param srcHandler
     * @param dstMessenger
     */
    public void connected(Context srcContext, Handler srcHandler, Messenger dstMessenger) {
        if (DBG) log("connected srcHandler to the dstMessenger  E");

        // Initialize source fields
        mSrcContext = srcContext;
        mSrcHandler = srcHandler;
        mSrcMessenger = new Messenger(mSrcHandler);

        // Initialize destination fields
        mDstMessenger = dstMessenger;
        if (DBG) log("connected srcHandler to the dstMessenger X");
    }

    /**
     * Reply to the src handler that we're half connected.
     * see: CMD_CHANNEL_HALF_CONNECTED for message contents
     *
     * @param status to be stored in msg.arg1
     */
    private void replyHalfConnected(int status) {
        Message msg = mSrcHandler.obtainMessage(CMD_CHANNEL_HALF_CONNECTED);
        msg.arg1 = status;
        msg.obj = this;
        msg.replyTo = mDstMessenger;
        if (!linkToDeathMonitor()) {
            // Override status to indicate failure
            msg.arg1 = STATUS_BINDING_UNSUCCESSFUL;
        }

        mSrcHandler.sendMessage(msg);
    }

这边表示connect内部逻辑是会发出一个CMD_CHANNEL_HALF_CONNECTED消息给自己的handler。

2.WifiServiceImpl

WifiServiceImpl作为客户端，接收到CMD_CHANNEL_HALF_CONNECTED，建立起剩下的通道。

    private class ClientHandler extends WifiHandler {

        ClientHandler(String tag, Looper looper) {
            super(tag, looper);
        }

        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            switch (msg.what) {
                case AsyncChannel.CMD_CHANNEL_HALF_CONNECTED: {
                    if (msg.arg1 == AsyncChannel.STATUS_SUCCESSFUL) {
                        if (DBG) Slog.d(TAG, "New client listening to asynchronous messages");
                        // We track the clients by the Messenger
                        // since it is expected to be always available
                        mTrafficPoller.addClient(msg.replyTo);
                    } else {
                        Slog.e(TAG, "Client connection failure, error=" + msg.arg1);
                    }
                    break;
                }
                case AsyncChannel.CMD_CHANNEL_DISCONNECTED: {
                    if (msg.arg1 == AsyncChannel.STATUS_SEND_UNSUCCESSFUL) {
                        if (DBG) Slog.d(TAG, "Send failed, client connection lost");
                    } else {
                        if (DBG) Slog.d(TAG, "Client connection lost with reason: " + msg.arg1);
                    }
                    mTrafficPoller.removeClient(msg.replyTo);
                    break;
                }
                case AsyncChannel.CMD_CHANNEL_FULL_CONNECTION: {
                    AsyncChannel ac = mFrameworkFacade.makeWifiAsyncChannel(TAG);
                    ac.connect(mContext, this, msg.replyTo);
                    break;
                }

双向通道建立完成。