Android very basic game development

Question

Can anyone help me with this simple code? Why doesn't the circle move smoothly? What's wrong with it?

package chaseme;

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Point;
import android.os.Bundle;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.Window;

public class ChaseMe extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    setContentView(new SampleView(this));
}

private class SampleView extends SurfaceView implements SurfaceHolder.Callback, Runnable {
    private Point point;
    private Thread thread;
    private SurfaceHolder holder;
    private Paint _rect;
    private Paint _circle;
    private boolean running;
    private int WIDTH;
    private int HEIGHT;
    private float radius = 20;

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

        point = new Point(20, 20);
        _rect = new Paint();
        _rect.setColor(Color.BLACK);

        _circle = new Paint();
        _circle.setColor(Color.BLUE);

        holder = this.getHolder();
        holder.addCallback(this);
        thread = new Thread(this);

    }

    private void updateModel() {
        if(point.x > WIDTH - radius) {
            point.x = 20;
        }
        if(point.y > HEIGHT - radius) {
            point.y = 20;
        }
        point.x++;
        point.y++;
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {}

    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        WIDTH = getWidth();
        HEIGHT = getHeight();
        this.setRunning(true);
        thread.start();
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        boolean retry = true;

        this.setRunning(false);
        while (retry) {
            try {
                thread.join();
                retry = false;
            } 
            catch (InterruptedException e) {}
        }
    }

    private void setRunning(boolean b) {
        running = b;
    }

    @Override
    public void run() {
        while(running) {
            Canvas c = null;
            updateModel();

            try {
                c = holder.lockCanvas(null);
                // synchronized (holder) {                      
                    render(c);
                // }
            }
            catch(Exception e) {
                Log.e("main", e.getMessage());
            }
            finally {
                if(c!=null) {
                    holder.unlockCanvasAndPost(c);
                }                   
            }               
        }
    }

    private void render(Canvas c) {         
        c.drawRect(0, 0, WIDTH, HEIGHT, _rect);     
        c.drawCircle(point.x, point.y, radius , _circle);

        //c.save();
        //c.restore();
    }
}
}

if you print some stuff in the console in render, is the frame rate good ? I wonder if your app is not running too fast as you never ask your thread to sleep between animation frames. — Snicolas, Jul 02 '11 at 09:29
Well, I know what is the problem, it is the emulator runs very slow, I ran a test to see the fps, and it drops from about 20 to 10. — Yotam, Jul 02 '11 at 09:32
A device is only real world test on android. It's really the fastest way to debug an app also. I use the emulator only to test what my devices can't test (resolution, versions of sdk...). But usually I test my code on a device. — Snicolas, Jul 03 '11 at 01:20

score 1 · Answer 1 · answered Jul 02 '11 at 10:25

You can't just call point.x++ as you currently do. You need to calculate movement relative to elapsed time and screen size.

Step 1: At every frame, calculate how much time has passed since last frame by doing

long now = System.currentTimeMillis();
elapsed = (now - mLastTime);
totalTimeElapsed += elapsed;

and then at the end of your main loop you do

mLastTime = now;

Step 2. Get screen ratio:

screenWidth = MyClass.this.getWidth();
screenHeight = MyClass.this.getHeight();
float a = screenWidth;
float b = screenHeight;
screenRatioX = a/WIDTH_OF_YOUR_PHONE;
screenRatioY = b/HEIGTH_OF_YOUR_PHONE;

Step 3. Now you can start doing animations, for instance, if you want to move your circle from right to left:

spriteX1 = (int) ((spriteX1 + (VELOCITY*screenRatioX*elapsed))+0.5);
spriteX2 = spriteX1 + spriteWidth;

Start with a velocity of 2.0 or something and tweak from there.

Good luck!

score 1 · Answer 2 · answered Jul 02 '11 at 10:39

I think for smooth moving of object you should use gesture listener functionality of on touch event .

Below link may help you.

1.http://mobile.tutsplus.com/tutorials/android/android-gesture/ 2.http://stackoverflow.com/questions/937313/android-basic-gesture-detection

BrainCrash · Answer 3 · 2011-09-17T04:31:28.583

0

Have tested the code on my desire and it runs very smooth.

I suggest to test this kind of projects on real devices, the emulator can be very unpredictable and slow depending also on your cpu.

Not a big deal but you can clear your canvas with

canvas.drawColor(Color.BLACK);

instead of drawing a rectangle.

edited Sep 17 '11 at 04:31

answered Jul 02 '11 at 11:12

BrainCrash

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Android very basic game development

3 Answers3