Android-与onSensorChanged同步线程以发送udp数据包

Question

I am trying to make a multiplayer game on android in which players use the accelerometer to move an object around. Initially I had used TCP to send packets which contain the X & Y coordinates of the player as a JSON string, inside the onSensorChanged() attached to the accelerometer(with SENSOR_DELAY_GAME). But due to latency issues, I decided to try out UDP.

Now earlier, I created a seperate thread which created a new socket and I used the following code:

public void onSensorChanged(SensorEvent event) {
// Other Stuff
    PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())),true);
                    out.println(coords.toString());
}

And I had:

socket = new Socket(serverAddr, Port);

in another thread.

Since UDP does not provide any such methods to send the data from the main thread due to NetworkOnMainThreadException , I figured I have to do the packet transmission in another thread. But the problem is that I'm calculating the data to be sent inside the onSensorChanged() , and I want to send a packet each time the onSensorChanged() is invoked.

I'm relatively new to Java and multithreading and tried reading about synchronization, but cannot seem to properly understand and implement it here. So I'd greatly appreciate if someone can point out an approach to solve this. I'd also like to know if sending the coordinates at each SensorChange is a good idea and whether there is a way to optimize the data so that I can still use TCP without latency issues. And I was also wondering if there was any possible way to make onSensorChanged() run as a separate thread...?

Answer 1

For simpliest applications, it can be solved easily:

DatagramSocket socket;

public void onSensorChanged() {
    //calculate and create DatagramPacket here
    new Thread(new Runnable() {
        @Override
        public void run() {
            socket.send(packet);    
        }
    });
}

It's not pretty, but it works (any calculations in onSensorChanged() aren't pretty to be fair...). Question is, whether the sending thread has to be synchronized with onSensorChanged() method. I figured out, that when using SENSOR_DELAY_GAME, sampling frequency on my SGS3 is around 20 ms, so it is long enough not to be synchronized. Of course it would work correctly only if Your calculations of sensor readings are simple and not time-consuming.

But, all in all, calculating any data in onSensorChanged() isn't a good practise. From developer.android.com:

Don't block the onSensorChanged() method

Sensor data can change at a high rate, which means the system may call the onSensorChanged(SensorEvent) method quite often. As a best practice, you should do as little as possible within the onSensorChanged(SensorEvent) method so you don't block it. If your application requires you to do any data filtering or reduction of sensor data, you should perform that work outside of the onSensorChanged(SensorEvent) method.

Therefore what I would suggest is put sensor data in some data container (ie List or sth...) and access this data from another thread, calculate it and send.

And for the last question: even if there was a way to run onSensorChanged() in separate thread, it would be fatal to most applications, due to possible thousands of threads at one time (as far as I'm concerned, Java is quite restraining environment and there are significant error-preventing mechanisms already on API-level).