Question

I want to pass my renderer some values from another class. After the renderer has calculated the values, I have a mutex in a helper class that should tell me that the renderer has finished calculating so I can continue with these new values. I can pass the renderer the values without problems, but I can’t figure out how to get them back. I currently use some static variables, but after they are changed by the renderer, they seem to get lost. They aren’t visible in my other class.
Example:

A class

public class View extends SurfaceView{

  private void doSomething(){

     glSurfaceView.queueEvent(new Runnable() {

                @Override
                public void run() {
                  //..
                  renderer.calculate(stack);    
                }
    });
  }

private void doAnotherThing(){

    //Never happens:
    if(Helper.hasCalculated){
    /...
    }
}

}

In my renderer:

public class MyRenderer implements GLSurfaceView.Renderer{

    private void calculate(Stack stack){         
      Helper.hasCalculated = true
    }
}

My helper class:

public class Helper{

 public static volatile boolean hasCalculated = false;

}

hasCalculated is definitely set to true in the renderer, but my other class always sees it as false. Any idea why? My best guess is that it’s because its in another thread, but how would I solve that? If there is a cleaner and safer approach, I’d be happy to hear him.

Answer 1

You can keep hold of your renderer as a variable in your activity (don’t just do mGLView.setRenderer(new MyRenderer()); as a lot of people do, but rather MyRenderer myRenderer = new MyRenderer(); mGLView.setRenderer(myRenderer);). Then you can communicate with your renderer easily through method calls. The problem then just comes down to cross-thread communication. I’ve put two examples below, one with communication between a non-UI thread, the GL thread and the main UI thread. The second example is just for communication between the GL thread and UI thread

public class Test3D extends Activity{

private MyRenderer renderer; // keep hold of the renderer as a variable in activity
private MyAsyncTask gameLoop;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.main);

    myRenderer = new MyRenderer(); // create the renderer object

    GLSurfaceView mGLView = (GLSurfaceView)findViewById(R.id.glsurfaceview1);
    mGLView.setEGLConfigChooser(true);
    mGLView.setRenderer(myRenderer); // set the surfaceView to use the renderer

    gameLoop = new MyAsyncTask(); 
    gameLoop.execute(); // start a new, non-UI, thread to do something

}

/// non-UI thread (inner class of my Test3D activity)
class MyAsyncTask extends AsyncTask<Void, Void, Void>{

    @Override
    protected Void doInBackground(Void... arg0) {

            myRenderer.startCalc(); // tell renderer to start calculation

            while(!myRenderer.isFinishedCalc()){

                // waiting for calc to finish, but not blocking UI thread

                try {
                    long x = 1000;
                    Thread.sleep(x);
                    // sleep the thread for x amount of time to save cpu cycles
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

            }

            publishProgress(null); 
            // when calculation has finished, we will drop out of the loop
            // and update the UI



   }

    protected void onProgressUpdate(Void... progress) {         
        // update UI
    }


}


}

Then in renderer

public class MyRenderer implements Renderer{

    private boolean startCalc = false;
    private boolean finishCalc = false;

    public void startCalc(){
        finishCalc = false;
        startCalc = true;
    }

    public boolean isFinishedCalc(){
        return finishCalc;
    }

    public void onDraw(GL10 gl){

        if(startCalc){
            // do calculation using GL handle
            // always performed in the GL thread

            finishCalc = true;
            startCalc = false;
        }

        // draw

    }



}

I’ve used flags in the renderer example above, but it would be fairly simple to turn that into a queue, if, say, you wanted to tell the renderer “load this array of models”. Since you have to load the models (or at least textures) in the GL thread using the GL handle, you can have other classes and threads do your logic and have just the GL stuff done in the GL thread

---

Alternatively, if you just want to update the UI thread after your calculation is done, rather than interact with any other threads:

public class MyRenderer implements Renderer{

    private Handler handler = null;
    public static final int CALC_FINISHED = 1;

    public void startCalc(Handler handler){
        this.handler = handler;
    }

    public void onDraw(GL10 gl){

        if(handler!=null){
            // do calculation using GL handle
            int flag = MyRenderer.CALC_FINISHED;
            handler.dispatchMessage(Message.obtain(handler, flag));
            // adds a message to the UI thread's message queue

            handler = null;

        }

        // draw

    }

}

and then from anywhere:

myRenderer.startCalc(new Handler(){

    public void handleMessage (Message msg){

        if(msg.what==MyRenderer.CALC_FINISHED){
            // Update UI
            // this code will always be executed in the UI thread

        }

    }

});