我尝试在NSImageView上绘图，如果我想画一个椭圆，它可以工作。如果我尝试画一条线，它会失败。为什么？我的对象声明:classMyImgView:NSImageView{varcolor=NSColor.greenColor()varpoint1=NSPoint()varpoint2=NSPoint()init(frame:NSRect){super.init(frame:frame)self.setNeedsDisplay()self.point1=NSPoint(x:0,y:0)self.point2=NSPoint(x:bounds.width,y:bounds.heigh