draw line- Android

I want to draw a line on screen using touch listener, but when i try to draw line again it erases the previous line. I am using this code:-

I am unable to find a solution to the problem.

public class Drawer extends View
{
    public Drawer(Context context)
    {
        super(context);
    }

    protected void onDraw(Canvas canvas)
    {
        Paint p = new Paint();
        p.setColor(color开发者_如何学运维draw);
        canvas.drawLine(x1, y1, x2, y2, p);
        invalidate();
    }
}

I'm pretty sure that invalidate() wipes the canvas, so you have to keep a collection of lines that you want to draw. Then you need to draw ALL of them EVERY time before calling invalidate().

private class Line {

    public Line(int x1, int y1, int x2, int y2) {
        this.x1 = x1;
        this.y1 = y1;
        this.x2 = x2;
        this.y2 = y2;
    }
    ...    
}

public class Drawer extends View
{  
    ArrayList<Line> lines;
    public Drawer(Context context)
    {
        super(context);
        lines = new ArrayList<Line>();
    }

    public void addLine(int x1, int y1, int x2, int y2) {
        Line newLine = new Line(x1, y1, x2, y2);
        lines.add(newLine);
    }

    protected void onDraw(Canvas canvas)
    {
        Paint p = new Paint();
        p.setColor(colordraw);
        for (Line myLine : lines) {
            canvas.drawLine(myLine.getX1(), myLine.getY1(), myLine.getX2(), myLine.getY2(), p);
        }
        invalidate();
    }
}