.NET Maui中如何通过C#后端代码触发Picker打开选项列表？

Picker.Focus()在控件隐藏时可能因为平台原生的焦点机制限制而失效，以下是针对MAUI和Xamarin.Forms的可行实现方式：

一、平台特定原生调用（推荐） #

通过自定义Handler（MAUI）或渲染器（Xamarin.Forms）直接调用原生控件的弹出方法，绕过焦点限制。

MAUI 实现 #

Android 端

1. 在MauiProgram.cs中注册自定义Handler：

builder.ConfigureMauiHandlers(handlers =>
{
#if ANDROID
    handlers.AddHandler(typeof(Picker), typeof(CustomPickerHandler));
#endif
});

2. 在Android项目中创建CustomPickerHandler：

using Android.Widget;
using Microsoft.Maui.Handlers;

namespace YourAppName.Droid
{
    public class CustomPickerHandler : PickerHandler
    {
        protected override void ConnectHandler(AppCompatSpinner platformView)
        {
            base.ConnectHandler(platformView);
            PlatformView.Tag = this;
        }

        public void OpenPicker()
        {
            PlatformView?.PerformClick();
        }
    }
}

3. 共享代码中触发弹出：

#if ANDROID
var pickerHandler = MyPicker.Handler as CustomPickerHandler;
pickerHandler?.OpenPicker();
#endif

iOS 端

1. 在MauiProgram.cs中注册自定义Handler：

builder.ConfigureMauiHandlers(handlers =>
{
#if IOS
    handlers.AddHandler(typeof(Picker), typeof(CustomPickerHandler));
#endif
});

2. 在iOS项目中创建CustomPickerHandler：

using Microsoft.Maui.Handlers;
using UIKit;
using System.Reflection;

namespace YourAppName.iOS
{
    public class CustomPickerHandler : PickerHandler
    {
        protected override void ConnectHandler(UIPickerView platformView)
        {
            base.ConnectHandler(platformView);
            PlatformView.Tag = this;
        }

        public void OpenPicker()
        {
            // 调用MAUI内部的弹出逻辑
            var showMethod = GetType().BaseType.GetMethod("ShowPicker", BindingFlags.NonPublic | BindingFlags.Instance);
            showMethod?.Invoke(this, null);
        }
    }
}

3. 共享代码中触发弹出：

#if IOS
var pickerHandler = MyPicker.Handler as CustomPickerHandler;
pickerHandler?.OpenPicker();
#endif

Xamarin.Forms 实现 #

Android 自定义渲染器

1. 在Android项目中创建渲染器：

using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;
using Android.Widget;

[assembly: ExportRenderer(typeof(Picker), typeof(CustomPickerRenderer))]
namespace YourAppName.Droid
{
    public class CustomPickerRenderer : PickerRenderer
    {
        public CustomPickerRenderer(Context context) : base(context)
        {
        }

        protected override void OnElementChanged(ElementChangedEventArgs<Picker> e)
        {
            base.OnElementChanged(e);
            if (Control != null)
                Control.Tag = this;
        }

        public void OpenPicker()
        {
            Control?.PerformClick();
        }
    }
}

2. 共享代码中调用：

var renderer = Platform.GetRenderer(MyPicker) as CustomPickerRenderer;
renderer?.OpenPicker();

iOS 自定义渲染器

1. 在iOS项目中创建渲染器：

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

[assembly: ExportRenderer(typeof(Picker), typeof(CustomPickerRenderer))]
namespace YourAppName.iOS
{
    public class CustomPickerRenderer : PickerRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<Picker> e)
        {
            base.OnElementChanged(e);
            if (Control != null)
                Control.Tag = this;
        }

        public void OpenPicker()
        {
            var showMethod = typeof(PickerRenderer).GetMethod("ShowPicker", BindingFlags.NonPublic | BindingFlags.Instance);
            showMethod?.Invoke(this, null);
        }
    }
}

2. 共享代码中调用：

var renderer = Platform.GetRenderer(MyPicker) as CustomPickerRenderer;
renderer?.OpenPicker();

二、简易替代方案 #

如果不想编写平台特定代码，可以将Picker设为完全透明并覆盖在Entry上方，通过Entry的点击手势触发Picker交互：

<!-- XAML布局 -->
<Grid>
    <Entry x:Name="MyEntry" Text="{Binding SelectedOption}" />
    <Picker x:Name="MyPicker" Opacity="0" SelectedItem="{Binding SelectedOption}" />
</Grid>

// 代码后台添加点击手势
MyEntry.GestureRecognizers.Add(new TapGestureRecognizer
{
    Command = new Command(() => 
    {
        MyPicker.Focus();
        // 如果Focus仍失效，结合上面的平台方法调用
    })
});

内容的提问来源于stack exchange，提问作者Fardin Farnezhad