TouchMove事件在xamarin ios中结束得如此之快

我创建了一个画布视图，如下所示。当我把它应用到一个新项目中时，它是正确的，但当我把它们应用到我的真实项目中时时，我无法画出一条实线。它总是被打破（如下图所示）。第一次，我认为这是一个问题，因为我推了太多的屏幕，直到这个绘图。但每次我推送一个新视图时，我都试图禁用上一个视图的UserInteraction，但它不起作用。请帮忙！

public class CanvasView : UIView
{
    public CanvasView (CGRect frame) : base(frame)
    {
        this.drawPath = new CGPath();
        BackgroundColor = UIColor.White;
    }

    private CGPoint touchLocation;
    private CGPoint previousTouchLocation;
    private CGPath drawPath;
    private bool fingerDraw;
    public override void LayoutSubviews ()
    {
        base.LayoutSubviews ();
        Layer.BorderWidth = 0.5f;
        Layer.BorderColor = UIColor.FromRGB (146, 146, 146).CGColor;
    }
    public override void TouchesBegan (NSSet touches, UIEvent evt)
    {
        base.TouchesBegan (touches, evt);
        UITouch touch = touches.AnyObject as UITouch;
        this.fingerDraw = true;
        this.touchLocation = touch.LocationInView(this);
        this.previousTouchLocation = touch.PreviousLocationInView(this);
        this.SetNeedsDisplay();
    }
    public override void TouchesMoved (NSSet touches, UIEvent evt)
    {
        base.TouchesMoved (touches, evt);
        UITouch touch = touches.AnyObject as UITouch;
        this.touchLocation = touch.LocationInView(this);
        this.previousTouchLocation = touch.PreviousLocationInView(this);
        this.SetNeedsDisplay();
    }
    public override void TouchesEnded (NSSet touches, UIEvent evt)
    {
        base.TouchesEnded (touches, evt);
        var alert = new UIAlertView () {
            Message = "Xui"
        };
        alert.AddButton("OK");
        alert.Show();
    }
    public override void Draw (CGRect rect)
    {
        base.Draw (rect);
        if (this.fingerDraw)
        {
            using (CGContext context = UIGraphics.GetCurrentContext())
            {
                context.SetStrokeColor(UIColor.Black.CGColor);
                context.SetLineWidth(1.5f);
                context.SetLineJoin(CGLineJoin.Round);
                context.SetLineCap(CGLineCap.Round);
                this.drawPath.MoveToPoint(this.previousTouchLocation);
                this.drawPath.AddLineToPoint(this.touchLocation);
                context.AddPath(this.drawPath);
                context.DrawPath(CGPathDrawingMode.Stroke);
            }
        }
    }
}

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