iOS UIView Free Hand Drawing无法用一根手指

Question

我有一个基于this article的自由绘画课程。

public partial class HandDrawingView : ReactiveView, IViewFor<ColorPickerViewModel>
{
    UIBezierPath path;
    UIColor strokeColor = UIColor.Black;
    UIImage incrementalImage;
    CGPoint[] points = new CGPoint[5]; // we now need to keep track of the four points of a Bezier segment and the first control point of the next segment
    int counter; // a counter variable to keep track of the point index
    nfloat strokeWidth = 4;

    public HandDrawingView(IntPtr handle) : base(handle)
    {
    }

    public override void AwakeFromNib()
    {
        base.AwakeFromNib();

        this.MultipleTouchEnabled = false;
        this.BackgroundColor = UIColor.White;

        path = new UIBezierPath();
    }

    public override void Draw(CGRect rect)
    {
        base.Draw(rect);

        if (incrementalImage != null)
        {
            incrementalImage.DrawAsPatternInRect(rect);
        }

        if (this.strokeColor != null)
        {
            this.strokeColor.SetStroke();
            path.Stroke();
        }
    }

    public override void TouchesBegan(NSSet touches, UIEvent evt)
    {
        base.TouchesBegan(touches, evt);
        Debug.WriteLine("began");
        counter = 0;

        path.LineWidth = strokeWidth;
        path.LineCapStyle = CGLineCap.Round;

        var touch = touches.AnyObject as UITouch;
        points[0] = touch.LocationInView(this);
    }

    public override void TouchesMoved(NSSet touches, UIEvent evt)
    {
        base.TouchesMoved(touches, evt);
        Debug.WriteLine("move");
        var touch = touches.AnyObject as UITouch;
        var p = touch.LocationInView(this);

        counter++;
        points[counter] = p;
        if (counter == 4)
        {
            // move the endpoint to the middle of the line joining the second control point of the first Bezier segment and the first control point of the second Bezier segment 
            points[3] = new CGPoint((points[2].X + points[4].X) / 2, (points[2].Y + points[4].Y) / 2);

            path.MoveTo(points[0]);
            path.AddCurveToPoint(points[3], points[1], points[2]);
            this.SetNeedsDisplay();
            points[0] = points[3];
            points[1] = points[4];
            counter = 1;
        }
    }

    public override void TouchesEnded(NSSet touches, UIEvent evt)
    {
        base.TouchesEnded(touches, evt);
        Debug.WriteLine("end");
        this.DrawBitmap();
        this.SetNeedsDisplay();
        path.RemoveAllPoints();
        counter = 0;
    }

    public override void TouchesCancelled(NSSet touches, UIEvent evt)
    {
        base.TouchesCancelled(touches, evt);

        Debug.WriteLine("cancel");
        this.TouchesEnded(touches, evt);
    }

    void DrawBitmap()
    {
        UIGraphics.BeginImageContextWithOptions(this.Bounds.Size, false, 0f);

        if (incrementalImage != null)
        {
            incrementalImage.Draw(CGPoint.Empty);
        }
        this.strokeColor.SetStroke();
        path.Stroke();
        incrementalImage = UIGraphics.GetImageFromCurrentImageContext();
        UIGraphics.EndImageContext();
    }

    public void ClearAll()
    {
        counter = 0;
        points = new CGPoint[5];
        path.RemoveAllPoints();
        SetNeedsDisplay();
    }

    public ColorPickerViewModel _viewModel;

    public ColorPickerViewModel ViewModel
    {
        get { return _viewModel; }
        set { this.RaiseAndSetIfChanged(ref _viewModel, value); }
    }

    object IViewFor.ViewModel
    {
        get { return ((IViewFor<ColorPickerViewModel>)this).ViewModel; }
        set { ((IViewFor<ColorPickerViewModel>)this).ViewModel = (ColorPickerViewModel)value; }
    }
}

此课程以前在iOS 10中有效。自从我更新到iOS 11以来，它已不再适用了。如果我只使用一根手指，则会按以下顺序调用触摸事件：开始 - 移动 - 取消 - 结束，即使我的手指仍在移动且视图中没有绘制任何内容。

但如果我用两根手指（或更多），那么我可以用第一根手指触摸视图来绘制视图。触摸序列将变为这样：开始 - 移动 - 移动 - 移动 - {很多移动} - 移动 - 取消 - 结束

更新

我尝试将segue从Show（push）更改为以模态方式呈现，现在它可以正常工作。

为什么不使用segue Show（推送）？