Xamarin UIScrollView GestureRecognizerShouldBegin 在滚动视图移动时报告速度为 0

Question

我正在使用 Xamarin 开发 iOS 应用程序，并且我想继承 UIScrollView 以便根据其速度处理滚动视图中的平移手势。因此，我覆盖了GestureRecognizerShouldBegin，并检查了平移手势的VelocityInView。这适用于第一个手势，但随后在滚动视图运动（减速）时触发的平移手势总是报告 (0, 0) 的速度：

public class MyScroll : UIScrollView
{
    public override bool GestureRecognizerShouldBegin(UIGestureRecognizer gestureRecognizer)
    {
        UIPanGestureRecognizer panGesture = gestureRecognizer as UIPanGestureRecognizer;
        if (panGesture != null)
        {
            CGPoint velocity = panGesture.VelocityInView(this);
            Console.WriteLine("Pan gesture velocity: " + velocity);
        }
        return true;
    }
}

在滚动移动时平移一次然后第二次后的输出：

Pan gesture velocity: {X=37.92359, Y=-872.2426}
Pan gesture velocity: {X=0, Y=0}

这是一个错误还是预期的行为？

编辑：在 Xamarin 论坛上交叉发布：https://forums.xamarin.com/discussion/54478/uiscrollview-pan-gesture-velocity-reporting-0-if-it-is-already-moving#latest

编辑澄清：

澄清我最终要做什么：我在水平分页视图中有一个垂直滚动视图。我想检查平移的速度，以便如果平移是“水平的”（即 X 速度 > Y 速度），我可以告诉滚动视图不处理该手势。默认行为是，一旦滚动视图处于运动状态，另一个手势仍会滚动，但这使得用户很难水平滚动（跨页面），直到垂直滚动完全稳定为止。

Answer 1

我终于明白了。感谢@RobertN 的帮助:)

关键是滚动视图使用的默认平移手势识别器将始终报告 0 速度，如果它已经在运动中（例如，前一个手势的惯性仍然有效）。添加新的UIPanGestureRecognizer 是记录后续手势的“实际”速度的好方法，但到那时已经太晚了，无法影响原始平移手势的GestureRecognizerShouldBegin。所以我所要做的就是将一个ShouldBegin 委托添加到我的新UIPanGestureRecognizer 并使用 that 返回false，以防我希望手势“通过”给父级寻呼机。

        public MyScroll() : base()
        {
            UIPanGestureRecognizer panGesture = new UIPanGestureRecognizer();

            panGesture.ShouldBegin = delegate(UIGestureRecognizer recognizer)
            {
                CGPoint v = panGesture.VelocityInView(this);

                if (v.X != 0 || v.Y != 0)
                {
                    if (Math.Abs(v.X) > Math.Abs(v.Y))
                    {
                        return false;
                    }
                }

                return true;
            };

            this.AddGestureRecognizer(panGesture);
        }

这样，我只是让默认的滚动视图平移手势识别器完成它的工作，而我的新 UIPanGestureRecognizer 识别用户何时做出新的水平手势，并传递那个手势以便寻呼机可以分页。这使得父寻呼机和垂直页面滚动视图的组合可以很好地一起运行（想象一下垂直滚动页面并能够翻阅页面，即使垂直页面处于运动状态）。请注意，您还需要实现以下方法以允许两个手势识别器同时操作：

        [Export("gestureRecognizer:shouldRecognizeSimultaneouslyWithGestureRecognizer:")]
        public bool ShouldRecognizeSimultaneously(UIGestureRecognizer gestureRecognizer, UIGestureRecognizer otherGestureRecognizer)
        {
            return true;
        }

Answer 2

这是一个错误还是预期的行为？

就通过 GestureRecognizerShouldBegin 上的 VelocityInView 获取 p/s 而言，在平移运动开始后获得 0,0，但不会停止/重置，这是预期的，至少根据我的经验。 Obj-C/Swift 会返回同样的东西，不要问我为什么，必须让一个真正的 iOS 开发者来问那个原因。

如果您真的需要在 GestureRecognizerShouldBegin 内从任何其他平移识别器（我在下面的示例中执行此操作）在您的 UIScrollView 子类中分配一个私有 CGPoint，那么在“任何地方”获取速度，您应该是黄金级的。 .

示例输出：

2015-10-26 12:07:06.676 iOSVelocity[68486:2309184] Touch-enabed Pan gesture velocity: {X=-608.4813, Y=0}
2015-10-26 12:07:06.703 iOSVelocity[68486:2309184] Touch-enabed Pan gesture velocity: {X=-1213.629, Y=0}
2015-10-26 12:07:06.726 iOSVelocity[68486:2309184] Touch-enabed Pan gesture velocity: {X=-935.5507, Y=0}
2015-10-26 12:07:06.771 iOSVelocity[68486:2309184] Touch-enabed Pan gesture velocity: {X=-1191.385, Y=-8.564461}
2015-10-26 12:07:06.772 iOSVelocity[68486:2309184] otherGestureRecognizer velocity: {X=-1191.385, Y=-8.564461}
2015-10-26 12:07:06.772 iOSVelocity[68486:2309184] otherGestureRecognizer velocity: {X=-1191.385, Y=-8.564461}
2015-10-26 12:07:08.882 iOSVelocity[68486:2309184] !!!! ShouldBegin velocity not reset !!!!
2015-10-26 12:07:08.885 iOSVelocity[68486:2309184] GestureRecognizerShouldBegin velocity: {X=-1191.385, Y=-8.564461}
2015-10-26 12:07:08.887 iOSVelocity[68486:2309184] otherGestureRecognizer velocity: {X=-1191.385, Y=-8.564461}
2015-10-26 12:07:08.889 iOSVelocity[68486:2309184] gestureRecognizer velocity: {X=0, Y=0}
2015-10-26 12:07:08.890 iOSVelocity[68486:2309184] otherGestureRecognizer velocity: {X=0, Y=0}
2015-10-26 12:07:08.891 iOSVelocity[68486:2309184] gestureRecognizer velocity: {X=0, Y=0}
2015-10-26 12:07:08.937 iOSVelocity[68486:2309184] otherGestureRecognizer velocity: {X=0, Y=0}
2015-10-26 12:07:08.938 iOSVelocity[68486:2309184] otherGestureRecognizer velocity: {X=0, Y=0}
2015-10-26 12:07:08.939 iOSVelocity[68486:2309184] gestureRecognizer velocity: {X=-336.9197, Y=0}
2015-10-26 12:07:08.940 iOSVelocity[68486:2309184] Touch-enabled Pan gesture velocity: {X=-336.9197, Y=0}
2015-10-26 12:07:08.954 iOSVelocity[68486:2309184] Touch-enabled Pan gesture velocity: {X=-650.7258, Y=0}
2015-10-26 12:07:08.961 iOSVelocity[68486:2309184] Touch-enabled Pan gesture velocity: {X=-650.7258, Y=0}
2015-10-26 12:07:08.993 iOSVelocity[68486:2309184] Touch-enabled Pan gesture velocity: {X=-914.0547, Y=0}
2015-10-26 12:07:09.027 iOSVelocity[68486:2309184] Touch-enabled Pan gesture velocity: {X=-734.1516, Y=0}
2015-10-26 12:07:09.032 iOSVelocity[68486:2309184] otherGestureRecognizer velocity: {X=-734.1516, Y=0}
2015-10-26 12:07:09.033 iOSVelocity[68486:2309184] otherGestureRecognizer velocity: {X=-734.1516, Y=0}
2015-10-26 12:07:09.060 iOSVelocity[68486:2309184] Touch-enabled Pan gesture velocity: {X=-1086.368, Y=0}

UIScrollView 子类示例：

注意：这使用了shouldRecognizeSimultaneouslyWithGestureRecognizer，以便在用户解除触摸后继续自动平移

注意 2：不确定我是否捕获了所有手势状态排列，因此请根据需要进行调整

using System;
using UIKit;
using CoreGraphics;
using CoreFoundation;
using CoreData;
using Foundation;
using CoreMotion;

namespace iOSVelocity
{
    public class MyScroll : UIScrollView
    {
        UIPanGestureRecognizer panGesture;
        CGPoint velocity;

        public MyScroll (CGRect cGRect) : base (cGRect)
        {
            panGesture = new UIPanGestureRecognizer (() => {
                if ((panGesture.State == UIGestureRecognizerState.Began || panGesture.State == UIGestureRecognizerState.Changed) && (panGesture.NumberOfTouches == 1)) {
                    velocity = panGesture.VelocityInView (this);
                    Console.WriteLine ("Touch-enabled Pan gesture velocity: " + velocity);
                } else if (panGesture.State == UIGestureRecognizerState.Ended) {
                    // Gesture ended, but auto-panning could still be going... 
                }
            });
            AddGestureRecognizer (panGesture);
        }

        [Export ("gestureRecognizer:shouldRecognizeSimultaneouslyWithGestureRecognizer:")]
        public bool ShouldRecognizeSimultaneously (UIGestureRecognizer gestureRecognizer, UIGestureRecognizer otherGestureRecognizer)
        {
            if (gestureRecognizer is UIPanGestureRecognizer) {
                var panRecognizer = (UIPanGestureRecognizer)gestureRecognizer;
                velocity = panRecognizer.VelocityInView (this);
                Console.WriteLine ("gestureRecognizer velocity: " + velocity);
            } else if (otherGestureRecognizer is UIPanGestureRecognizer) {
                var panRecognizer2 = (UIPanGestureRecognizer)otherGestureRecognizer;
                CGPoint beginvelocity = panRecognizer2.VelocityInView(this);
                if (beginvelocity.X != 0 && beginvelocity.Y != 0)
                    velocity = panRecognizer2.VelocityInView (this);
                Console.WriteLine ("otherGestureRecognizer velocity: " + velocity);
            } else {
                // What should we do here?
            }
            return true;
        }

        public override bool GestureRecognizerShouldBegin (UIGestureRecognizer gestureRecognizer)
        {
            UIPanGestureRecognizer panGesture = gestureRecognizer as UIPanGestureRecognizer;
            if (panGesture != null) {
                CGPoint beginvelocity = panGesture.VelocityInView(this);
                if (beginvelocity.X == 0 && beginvelocity.Y == 0) {
                    Console.WriteLine ("!!!! ShouldBegin velocity not reset !!!!");
                } else {
                    velocity = beginvelocity;
                }
                Console.WriteLine ("GestureRecognizerShouldBegin velocity: " + velocity);
            }
            return true;
        }
    }
}