How to draw lines without deleting the old ones every time?

Question

I'm working on a code, which should make lines. You can touch the screen on two diffrent places, the program gets the x/y values and draws a line. So that's working fine! I want the program now to save the lines. So you can touch the screen multiple times and the program makes a line from the first to the second touch, from the third to the fourth and so on, and displays them all. I tried to do this with an arraylist, but it still didn't worked. Here is the code, which draws the lines, but doesn't sve them.

So the code works now! I made it with Paths and it's working fine! Thank for your help!

package com.example.linienmachen;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

public class Touchclass extends View {
private Paint paint = new Paint();
private Path path = new Path();
int count = 1;

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

paint.setAntiAlias(true);
paint.setStrokeWidth(6f);
paint.setColor(Color.BLACK);
paint.setStyle(Paint.Style.STROKE);
paint.setStrokeJoin(Paint.Join.ROUND);

}



@Override
protected void onDraw(Canvas canvas) {

if ((count%2 != 0)){
canvas.drawPath(path, paint);}
}

 @Override
public boolean onTouchEvent(MotionEvent event) {
float eventX = event.getX();
float eventY = event.getY();

switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:

    count++;

    if(count%2 != 1){
    path.moveTo(eventX, eventY);
  return true;
    }
    if(count%2 != 0){
    path.lineTo(eventX, eventY);
    break;
    }
case MotionEvent.ACTION_MOVE:

  break;
case MotionEvent.ACTION_UP:
  // nothing to do
  break;
default:
  return false;
}

// Schedules a repaint.
invalidate();
return true;
}
} 

Answer 1

There's two ways to do this.

1)Store the location of each line, and redraw each line in every onDraw call.

2)Use a bitmap. Every time a new line is created, draw the line directly to the bitmap. Then your onDraw function just draws this bitmap to the screen.

Method 2 uses more memory but has better performance. Method 1 doesn't scale to hundreds of lines, but it takes a lot less memory.