xamarin.forms - 如何在 SKCanvasView 上实现双指缩放和滑动？

Question

我正在使用 xamarin 表单开发图像查看器。我的源图像是无限大的，比我的视口(viewport)大得多。所以我使用 SKCanvasView 控件逐部分绘制，以便我应该在图像中滑动。

我可以通过 SKCanvasView 上可用的 Touch 事件获得 Touch 点。在这里，我能够检测到双击。滑动和平移。

我无法决定何时向上/向下滑动需要向上/向下移动多少图像？

最初，我尝试将 SKCanvasView 放置在 ScrollView 控件之上，并使用我绘制 SKCanvas 的滚动位置值。这里滚动和打印效果完美。但是在这里，双指缩放不起作用，因为双指手势事件没有触发，而是在滚动。

Answer 1

虽然我个人从未使用过 Skia，但我曾经为我的一位 friend 做过类似的 SKCanvas。

无论如何，它都有 Pinch、Pan 和 Tap 缩放事件，您可以根据需要随意进行任何更改:

 public class CustomSKCanvas : SKCanvasView
{
    private const double MIN_SCALE = 1;
    private const double MAX_SCALE = 4;
    private const double OVERSHOOT = 0.15;
    private double StartScale, LastScale;
    private double StartX, StartY;

    public CustomSKCanvas()
    {
        var pinch = new PinchGestureRecognizer();
        pinch.PinchUpdated += OnPinchUpdated;
        GestureRecognizers.Add(pinch);

        var pan = new PanGestureRecognizer();
        pan.PanUpdated += OnPanUpdated;
        GestureRecognizers.Add(pan);

        var tap = new TapGestureRecognizer { NumberOfTapsRequired = 2 };
        tap.Tapped += OnTapped;
        GestureRecognizers.Add(tap);

        Scale = MIN_SCALE;
        TranslationX = TranslationY = 0;
        AnchorX = AnchorY = 0;
    }

    protected override SizeRequest OnMeasure(double widthConstraint, double heightConstraint)
    {
        Scale = MIN_SCALE;
        TranslationX = TranslationY = 0;
        AnchorX = AnchorY = 0;
        return base.OnMeasure(widthConstraint, heightConstraint);
    }

    private void OnTapped(object sender, EventArgs e)
    {
        if (Scale > MIN_SCALE)
        {
            this.ScaleTo(MIN_SCALE, 250, Easing.CubicInOut);
            this.TranslateTo(0, 0, 250, Easing.CubicInOut);
        }
        else
        {
            AnchorX = AnchorY = 0.5; //TODO tapped position
            this.ScaleTo(MAX_SCALE, 250, Easing.CubicInOut);
        }
    }

    private void OnPanUpdated(object sender, PanUpdatedEventArgs e)
    {
        switch (e.StatusType)
        {
            case GestureStatus.Started:
                StartX = (1 - AnchorX) * Width;
                StartY = (1 - AnchorY) * Height;
                break;
            case GestureStatus.Running:
                AnchorX = Clamp(1 - (StartX + e.TotalX) / Width, 0, 1);
                AnchorY = Clamp(1 - (StartY + e.TotalY) / Height, 0, 1);
                break;
        }
    }

    private void OnPinchUpdated(object sender, PinchGestureUpdatedEventArgs e)
    {
        switch (e.Status)
        {
            case GestureStatus.Started:
                LastScale = e.Scale;
                StartScale = Scale;
                AnchorX = e.ScaleOrigin.X;
                AnchorY = e.ScaleOrigin.Y;
                break;
            case GestureStatus.Running:
                if (e.Scale < 0 || Math.Abs(LastScale - e.Scale) > (LastScale * 1.3) - LastScale)
                { return; }
                LastScale = e.Scale;
                var current = Scale + (e.Scale - 1) * StartScale;
                Scale = Clamp(current, MIN_SCALE * (1 - OVERSHOOT), MAX_SCALE * (1 + OVERSHOOT));
                break;
            case GestureStatus.Completed:
                if (Scale > MAX_SCALE)
                    this.ScaleTo(MAX_SCALE, 250, Easing.SpringOut);
                else if (Scale < MIN_SCALE)
                    this.ScaleTo(MIN_SCALE, 250, Easing.SpringOut);
                break;
        }
    }

    private T Clamp<T>(T value, T minimum, T maximum) where T : IComparable
    {
        if (value.CompareTo(minimum) < 0)
            return minimum;
        else if (value.CompareTo(maximum) > 0)
            return maximum;
        else
            return value;
    }
}

祝你好运

如有疑问，请回复。