Android Question How to draw on a bitmap, save and can undo?

    'Make a new copy of the existing bitmap and save it
    Dim bt As Bitmap
    bt.InitializeMutable(b.Width,b.Height)
    Dim cv As Canvas
    cv.Initialize2(bt)
    'Dim rect1 As Rect
    'rect1.Initialize(0,0,b.Width,b.Height)
    'cv.DrawBitmap(b,Null,rect1)

    'c.DrawCircle(X,Y,5dip,Colors.Red,True,1dip)
    'ImageView1.Invalidate
    undo.Add(bt)

    Select action
        Case Activity.ACTION_DOWN
            x1 = X
            y1 = Y
            c.DrawCircle(x1, y1, Radius, LineColor, True, 1dip)
            rect1.Left = x1 - Radius
            rect1.Top = y1 - Radius
            rect1.Right = rect1.Left + LineWidth
            rect1.Bottom = rect1.Top + LineWidth
            ImageView1.Invalidate2(rect1)
        Case Activity.ACTION_MOVE
            x2 = X
            y2 = Y
            c.DrawCircle(x1, y1, Radius, LineColor, True, 1)
            c.DrawLine(x1, y1, x2, y2, LineColor, LineWidth)
            rect1.Left = Min(x1, x2) - Radius
            rect1.Top = Min(y1, y2) - Radius
            rect1.Right = Max(x1, x2) + LineWidth
            rect1.Bottom = Max(y1, y2) + LineWidth
            ImageView1.Invalidate2(rect1)
            x1 = x2
            y1 = y2
    End Select