Threading UI updates in Android

Question

I've just started with android development and updating the UI is really bugging me :/

This is what I've got working so far -


package projects.Move;

import android.os.Bundle;
import android.view.View;
import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Color;

public class Move extends Activity {

    private float y = 0;
    private long now = 0;  
    private float delay = 75;
    private Paint paint = new Paint();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(new SimpleMotion(this));
        paint.setColor(Color.BLACK);
    }
    private class SimpleMotion extends View {

        public SimpleMotion(Context context) {
            super(context);
        }

        @Override protected void onDraw(Canvas canvas) {

            float x = canvas.getWidth() / 2;
            canvas.drawColor(Color.WHITE);
            canvas.drawCircle(x, y, 30, paint);  
            if(System.currentTimeMillis() - now > delay) y++;    
            invalidate();
        }
    }
}

It works fine but everybody says that doing your graphics in the main thread, so I'm trying (and failing) to pass it off to another thread. Trouble is, I have absolutely no idea how since really I've never used Threads.

The examples that Google gives on using Threads doesn't seem to be very clear and I couldn't really follow it for what I want to do. Could I ask somebody out here to give me the most basic example of how I could do what I'm doing here efficiently using Threads?

Thanks in advance :)

Cpt.Ohlund Cpt.Ohlund · Accepted Answer · 2010-09-20T14:27:32

Well, I guess there is some confusion going on here. You HAVE TO do your GUI updates from the main thread (also called the GUI thread) - otherwise you well get something like "Exception, blabla has leaked a view".

I guess what have misunderstood is that expensive operations, such as networking, should be done in a different thread than the main thread. And if you would like to update the GUI from the network thread you would do as ArtWorkAD says (or his links says).

So for what you want to do, you could achieve with something like replacing your SimpleMotion class with the following:

private class SimpleMotion extends View {

        public SimpleMotion(Context context) {
            super(context);

            new Thread(new Runnable() {
                public void run() {
                    while(true){
                        try {
                            Thread.sleep(75);
                            y++;
                            postInvalidate();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
              }).start();
        }

        @Override protected void onDraw(Canvas canvas) {

            float x = canvas.getWidth() / 2;
            canvas.drawColor(Color.WHITE);
            canvas.drawCircle(x, y, 30, paint);  
//            if(System.currentTimeMillis() - now > delay) y++;    
//            invalidate();
        }
    }

With your old code of having invalidate() in onDraw() you would continously be redrawing the gui even while there are no change to it.

The important part of the new code is postInvalidate(). This makes it possible to tell the GUI thread - from another thread - to redraw the GUI.

Threading UI updates in Android

4 Answers