Xamarin 表单：如何为 StackLayout 添加 SwipeGestureRecognizer？

Question

我的主页上有 15 个选项。最初，我将在 UI 中显示 9 个选项。要查看剩余的 6 个图标，用户可以向右滑动，然后再向左滑动以查看上一个。我尝试实现如下所示的滑动功能，但它不起作用。

XAML

<StackLayout x:Name="firstLlayout">
    <Grid>
        //3 icons in horizontal
    </Grid>

    <Grid>
        //3 icons in horizontal
    </Grid>

    <Grid>
        //3 icons in horizontal
    </Grid>
    <StackLayout.GestureRecognizers>
        <SwipeGestureRecognizer Direction="Right" Swiped="RightSwipe"/>
    </StackLayout.GestureRecognizers>
</StackLayout>

<StackLayout IsVisible="False" x:Name="secondLayout">
     <Grid>
        //3 icons in horizontal
    </Grid>

    <Grid>
        //3 icons in horizontal
    </Grid>
    <StackLayout.GestureRecognizers>
        <SwipeGestureRecognizer Direction="Left" Swiped="LeftSwipe"/>
    </StackLayout.GestureRecognizers>
</StackLayout>

XAML.CS

public void RightSwipe(object sender, EventArgs e)
{
    firstLlayout.IsVisible = false;
    secondLayout.IsVisible = true;
}

public void LeftSwipe(object sender, EventArgs e)
{
    secondLayout.IsVisible = false;
    firstLlayout.IsVisible = true;
}

当尝试左右滑动时，UI 中没有发生任何事情，并且代码执行不会进入事件函数。我在这里缺少什么？

Answer 1

原因1：

如果您将堆栈布局放在 ScrollView 中，滑动操作将与滚动操作发生冲突。

解决方案：

从 Root StackLayout 中移除 ScrollView，然后滑动就可以了。

原因2：需要在StackLayout中添加子控件（如Image或Label），否则永远不会调用滑动动作。

解决方案：如果你想让StackLayout的内容默认不显示，可以查看以下代码。

在后面的代码中

using System;
using Xamarin.Forms;

namespace xxx
{
    public class GestureScrollView : ScrollView
    {
        public event EventHandler SwipeLeft;
        public event EventHandler SwipeRight;

        public void OnSwipeLeft() =>
            SwipeLeft?.Invoke(this, null);

        public void OnSwipeRight() =>
            SwipeRight?.Invoke(this, null);
    }
}

在 Android 项目中

using System;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using xxx;
using xxx.Droid;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ExportRenderer(typeof(GestureScrollView), typeof(GestureScrollViewRenderer))]
namespace xxx.Droid
{
    public class GestureScrollViewRenderer : ScrollViewRenderer
    {
        readonly CustomGestureListener _listener;
        readonly GestureDetector _detector;

        public GestureScrollViewRenderer(Context context) : base(context)
        {
            _listener = new CustomGestureListener();
            _detector = new GestureDetector(context, _listener);
        }

        public override bool DispatchTouchEvent(MotionEvent e)
        {
            if (_detector != null)
            {
                _detector.OnTouchEvent(e);
                base.DispatchTouchEvent(e);
                return true;
            }

            return base.DispatchTouchEvent(e);
        }

        public override bool OnTouchEvent(MotionEvent ev)
        {
            base.OnTouchEvent(ev);

            if (_detector != null)
                return _detector.OnTouchEvent(ev);

            return false;
        }

        protected override void OnElementChanged(VisualElementChangedEventArgs e)
        {
            base.OnElementChanged(e);

            if (e.NewElement == null)
            {
                _listener.OnSwipeLeft -= HandleOnSwipeLeft;
                _listener.OnSwipeRight -= HandleOnSwipeRight;
            }

            if (e.OldElement == null)
            {
                _listener.OnSwipeLeft += HandleOnSwipeLeft;
                _listener.OnSwipeRight += HandleOnSwipeRight;
            }
        }

        void HandleOnSwipeLeft(object sender, EventArgs e) =>
            ((GestureScrollView)Element).OnSwipeLeft();

        void HandleOnSwipeRight(object sender, EventArgs e) =>
            ((GestureScrollView)Element).OnSwipeRight();
    }

    public class CustomGestureListener : GestureDetector.SimpleOnGestureListener
    {
        static readonly int SWIPE_THRESHOLD = 100;
        static readonly int SWIPE_VELOCITY_THRESHOLD = 100;

        MotionEvent mLastOnDownEvent;

        public event EventHandler OnSwipeLeft;
        public event EventHandler OnSwipeRight;

        public override bool OnDown(MotionEvent e)
        {
            mLastOnDownEvent = e;

            return true;
        }

        public override bool OnFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY)
        {
            if (e1 == null)
                e1 = mLastOnDownEvent;

            float diffY = e2.GetY() - e1.GetY();
            float diffX = e2.GetX() - e1.GetX();

            if (Math.Abs(diffX) > Math.Abs(diffY))
            {
                if (Math.Abs(diffX) > SWIPE_THRESHOLD && Math.Abs(velocityX) > SWIPE_VELOCITY_THRESHOLD)
                {
                    if (diffX > 0)
                        OnSwipeRight?.Invoke(this, null);
                    else
                        OnSwipeLeft?.Invoke(this, null);
                }
            }

            return base.OnFling(e1, e2, velocityX, velocityY);
        }
    }
}

在 Xaml 中

将StackLayout放到ScrollView中

<local:GestureScrollView SwipeRight="RightSwipe">
     <StackLayout x:Name="firstLlayout" >

        //...        
     </StackLayout>

          
</local:GestureScrollView>

Answer 2

我喜欢并使用 Lucas Zhang 对这个问题的回答 answer-59190549 但是，可以更改 GestureScrollView 以使用 iOS 将使用的相同 GestureRecognizer，如下所示：

public class GestureScrollView : ScrollView
{
    private bool isInitialized = false;
    private List<SwipeGestureRecognizer> LeftSwipeRecognizers { get; } = new();
    private List<SwipeGestureRecognizer> RightSwipeRecognizers { get; } = new();

    public GestureScrollView() : base()
    {
    }

    protected override void LayoutChildren(double x, double y, double width, double height)
    {
        base.LayoutChildren(x, y, width, height);

        //Not sure if this is the best place, but the ctor wasn't getting called.
        if (!isInitialized)
        {
            isInitialized = true;
            foreach (SwipeGestureRecognizer swipeGestureRecognizer in GestureRecognizers.Where(x => x is SwipeGestureRecognizer))
            {
                if (swipeGestureRecognizer.Direction.HasFlag(SwipeDirection.Left))
                {
                    LeftSwipeRecognizers.Add(swipeGestureRecognizer);
                }

                if (swipeGestureRecognizer.Direction.HasFlag(SwipeDirection.Right))
                {
                    RightSwipeRecognizers.Add(swipeGestureRecognizer);
                }
            }
        }
    }

    private void ExecuteGestureCommands(List<SwipeGestureRecognizer> swipeRecognizers, SwipedEventArgs e)
    {
        foreach (var gesture in swipeRecognizers)
        {
            gesture.SendSwiped(this, e.Direction);
        }
    }

    public void OnSwipeLeft(object sender, SwipedEventArgs e)
    {
        ExecuteGestureCommands(LeftSwipeRecognizers, e);
    }

    public void OnSwipeRight(object sender, SwipedEventArgs e)
    {
        ExecuteGestureCommands(RightSwipeRecognizers, e);
    }
}

渲染器需要与这些类似的更改（基本上将所有“EventArgs”引用更改为“SwipedEventArgs”并在事件调用中传入一个新的 SwipedEventArgs）：

... 
void HandleOnSwipeLeft(object sender, SwipedEventArgs e) => ((GestureScrollView)Element).OnSwipeLeft(sender, e);
void HandleOnSwipeRight(object sender, SwipedEventArgs e) => ((GestureScrollView)Element).OnSwipeRight(sender, e);
...
public event EventHandler<SwipedEventArgs> OnSwipeLeft;
public event EventHandler<SwipedEventArgs> OnSwipeRight;
...
if (diffX > 0)
{
    OnSwipeRight?.Invoke(this, new SwipedEventArgs(null, SwipeDirection.Right));
}
else
{
    OnSwipeLeft?.Invoke(this, new SwipedEventArgs(null, SwipeDirection.Left));
}
...

它并不完美，因为它忽略了 Android 中的向上/向下方向，但其他实现也是如此。