Thread: paint brush not leaving trail?

Hey, I'm new to android, and I just started fiddling with some simple example code I found online to get some practice. I found some code that has a red dot follow your finger, but I'd like to make the dot leave a trail like a paint brush. It looks like the program erases the previous paint area and puts down a new dot at your finger each frame; I made the 'V' in the upper left corner move to find that. Anyone have any clues?

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnTouchListener;
import android.view.ViewGroup.LayoutParams;
 
public class MainActivity extends Activity implements OnTouchListener  {
    private float x;
    private float y;
    private int moveX;
    Paint paint = new Paint();
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        MyCustomPanel view = new MyCustomPanel(this);
 
        ViewGroup.LayoutParams params = 
                            new ViewGroup.LayoutParams(LayoutParams.FILL_PARENT,
                                                       LayoutParams.FILL_PARENT);
        addContentView(view, params);
        view.setOnTouchListener(this);
 
    }
    private class MyCustomPanel extends View {
 
        public MyCustomPanel(Context context) {
            super(context);
 
        }
        @Override
        public void onDraw(Canvas canvas) {
 
            paint.setColor(Color.GREEN);
            paint.setStrokeWidth(6);
 
            canvas.drawLine(moveX,10,50,50,paint);
            paint.setColor(Color.RED);
 
            canvas.drawLine(50, 50, 90, 10, paint);
            canvas.drawCircle(50, 50, 3, paint);
            moveX++;
            canvas.drawCircle(x,y,3,paint);
        }
    }
    public boolean onTouch(View v, MotionEvent event) {
        x = event.getX();
        y = event.getY();
        v.invalidate();
        return true;
    }
}

After the user starts drawing for a while, won't it need more and more resources to remember all the points, and slow it down?

...By slightly changing the graphic each time a touchEvent is invoked, then displaying it? Wait, do all graphics remember the previous points and then display them like that?

Originally Posted by JavaInProgress

...By slightly changing the graphic each time a touchEvent is invoked, then displaying it?

If you just want to maintain a screenshot (so to speak) of the canvas, that will work, but there would be no way to undo what has been done.

Originally Posted by JavaInProgress

Wait, do all graphics remember the previous points and then display them like that?

You would have to store the points somewhat in this manner to support an undo feature. Otherwise how would the program determine the previous color of any pixel that is overwritten?

Both work, which one suits your needs? (or a combination of the two)

I was planning on a combo of the two. It would remember the last 10 or so modifications, then when a new modification is made, the last in the series is permanently added to the canvas.