Question

I’m making a drawing app, and randomly when I press down, the app crashes. I can’t figure out how to fix it, and have no Idea where to start. I don’t know whether it is the List or the timing, or something else. Here’s my code:

package com.orangewhales.paint.views;

import java.util.ArrayList;
import java.util.List;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.Paint.Style;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.SurfaceHolder;
import android.view.SurfaceHolder.Callback;
import android.view.SurfaceView;

public class PaintView extends SurfaceView implements Callback {

boolean run = true;
Thread tUpdate;
public List<PaintPath> paths = new ArrayList<PaintPath>();

public Paint stroke = new Paint();

@Override
public void draw(Canvas canvas) {
    // TODO Auto-generated method stub
    super.draw(canvas);

    canvas.drawRGB(255, 255, 255);

    for (PaintPath path : paths) {
        canvas.drawPath(path.path, path.paint);
    }
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    // TODO Auto-generated method stub
    switch(event.getAction()) {
    case MotionEvent.ACTION_DOWN:
        Path current = new Path();
        current.moveTo(event.getX(), event.getY());
        paths.add(new PaintPath(current, stroke));
        break;

    case MotionEvent.ACTION_MOVE:
        PaintPath path = paths.get(paths.size() - 1);
        path.path.lineTo(event.getX(), event.getY());
        break;
    }


    return true;
}

public PaintView(Context context, AttributeSet attrs) {
    super(context, attrs);

    stroke.setStyle(Style.STROKE);
    stroke.setColor(Color.BLACK);
    stroke.setStrokeWidth(10);

    tUpdate = new Thread() {

        public void run() {
            while (run) {
                Canvas c = getHolder().lockCanvas();
                draw(c);
                getHolder().unlockCanvasAndPost(c);

                update();

                try {
                    sleep(10);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }

    };

    getHolder().addCallback(this);
}

public void update() {
    // TODO Auto-generated method stub

}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width,
        int height) {
    // TODO Auto-generated method stub

}

@Override
public void surfaceCreated(SurfaceHolder holder) {
    // TODO Auto-generated method stub
    tUpdate.start();
}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {
    // TODO Auto-generated method stub
    run = false;

    try {
        tUpdate.join();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

}

Answer 1

I have not touched my Android SDK in a while. So I’ll start with a question.

You said the app crashes at random times when the GUI is pressed down. At function onTouchEvent, which action occurs first, ACTION_DOWN or ACTION_MOVE?
I hope it’s well defined but we can not assume that.

I find code in the function “PaintPath path = paths.get(paths.size() – 1)” to be suspicious because paths.size can be 0, in the beginning anyway.

I think it’s a good idea to monitor the function onTouchEvent closely, can use debugger. Log messages if you have nothing else.

Fixing random intermittent issues is part of the job. Learn from it and have fun.