如何在Xamarin Forms中触发触摸开始事件（而非手指松开时）

TapGestureRecognizer的Tapped事件仅在手指抬起时触发，若要实现触摸按下瞬间就响应，可采用以下两种方案：

方案1：使用TouchGestureRecognizer（MAUI 7+推荐） #

MAUI 7及以上版本新增了TouchGestureRecognizer，专门用于捕获触摸的按下、移动、抬起等事件，能直接满足需求：

XAML代码 #

<Image>
    <Image.GestureRecognizers>
        <TouchGestureRecognizer 
            TouchDown="OnImageTouchDown"
            TouchUp="OnImageTouchUp" /> <!-- 可选，触摸抬起时触发 -->
    </Image.GestureRecognizers>
</Image>

后台逻辑 #

private void OnImageTouchDown(object sender, TouchEventArgs e)
{
    // 在这里编写触摸开始时需要执行的代码
}

private void OnImageTouchUp(object sender, TouchEventArgs e)
{
    // 可选：触摸抬起时的处理逻辑
}

方案2：自定义触摸效果（兼容Xamarin.Forms/低版本MAUI） #

如果项目基于Xamarin.Forms或低版本MAUI，可通过自定义路由效果监听原生触摸事件：

1. 创建共享项目的效果类 #

using Xamarin.Forms;

namespace YourAppNamespace
{
    public class TouchDownEffect : RoutingEffect
    {
        public event EventHandler TouchDown;

        public TouchDownEffect() : base("YourAppNamespace.TouchDownEffect")
        {
        }

        public void TriggerTouchDown()
        {
            TouchDown?.Invoke(this, EventArgs.Empty);
        }
    }
}

2. Android平台实现 #

using Android.Views;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ResolutionGroupName("YourAppNamespace")]
[assembly: ExportEffect(typeof(YourAppNamespace.Droid.TouchDownEffectDroid), "TouchDownEffect")]
namespace YourAppNamespace.Droid
{
    public class TouchDownEffectDroid : PlatformEffect
    {
        private TouchEventListener _listener;

        protected override void OnAttached()
        {
            _listener = new TouchEventListener(Control ?? Container);
            _listener.TouchDown += OnTouchDown;
            (Control ?? Container).Touch += _listener.OnTouch;
        }

        private void OnTouchDown(object sender, EventArgs e)
        {
            var effect = Element.Effects.FirstOrDefault(eff => eff is TouchDownEffect) as TouchDownEffect;
            effect?.TriggerTouchDown();
        }

        protected override void OnDetached()
        {
            if (_listener != null)
            {
                _listener.TouchDown -= OnTouchDown;
                (Control ?? Container).Touch -= _listener.OnTouch;
            }
        }

        private class TouchEventListener : Java.Lang.Object, View.IOnTouchListener
        {
            public event EventHandler TouchDown;

            public bool OnTouch(View v, MotionEvent e)
            {
                if (e.Action == MotionEventActions.Down)
                {
                    TouchDown?.Invoke(this, EventArgs.Empty);
                }
                return false;
            }
        }
    }
}

3. iOS平台实现 #

using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ResolutionGroupName("YourAppNamespace")]
[assembly: ExportEffect(typeof(YourAppNamespace.iOS.TouchDownEffectiOS), "TouchDownEffect")]
namespace YourAppNamespace.iOS
{
    public class TouchDownEffectiOS : PlatformEffect
    {
        protected override void OnAttached()
        {
            var nativeView = Control ?? Container;
            nativeView.TouchDown += OnTouchDown;
        }

        private void OnTouchDown(object sender, EventArgs e)
        {
            var effect = Element.Effects.FirstOrDefault(eff => eff is TouchDownEffect) as TouchDownEffect;
            effect?.TriggerTouchDown();
        }

        protected override void OnDetached()
        {
            var nativeView = Control ?? Container;
            nativeView.TouchDown -= OnTouchDown;
        }
    }
}

4. XAML中使用效果 #

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:YourAppNamespace"
             x:Class="YourAppNamespace.MainPage">

    <Image>
        <Image.Effects>
            <local:TouchDownEffect TouchDown="OnImageTouchDown" />
        </Image.Effects>
    </Image>

</ContentPage>

后台逻辑 #

private void OnImageTouchDown(object sender, EventArgs e)
{
    // 触摸开始时执行的逻辑
}

内容的提问来源于stack exchange，提问作者Alex Zaitsev