如何修改Xamarin Forms Picker弹出GUI的背景色及相关样式？

要修改Picker弹出的弹窗样式（背景色、标题、尺寸），需要通过自定义渲染器替换默认的AlertDialog，以下是具体实现步骤：

1. 共享项目中创建自定义Picker控件 #

定义绑定属性来传递弹窗的自定义参数：

public class CustomPicker : Picker
{
    public static readonly BindableProperty DialogBackgroundColorProperty =
        BindableProperty.Create(nameof(DialogBackgroundColor), typeof(Color), typeof(CustomPicker), Color.Default);

    public static readonly BindableProperty DialogTitleProperty =
        BindableProperty.Create(nameof(DialogTitle), typeof(string), typeof(CustomPicker), "请选择");

    public static readonly BindableProperty DialogWidthProperty =
        BindableProperty.Create(nameof(DialogWidth), typeof(int), typeof(CustomPicker), -1);

    public static readonly BindableProperty DialogHeightProperty =
        BindableProperty.Create(nameof(DialogHeight), typeof(int), typeof(CustomPicker), -1);

    public Color DialogBackgroundColor
    {
        get => (Color)GetValue(DialogBackgroundColorProperty);
        set => SetValue(DialogBackgroundColorProperty, value);
    }

    public string DialogTitle
    {
        get => (string)GetValue(DialogTitleProperty);
        set => SetValue(DialogTitleProperty, value);
    }

    public int DialogWidth
    {
        get => (int)GetValue(DialogWidthProperty);
        set => SetValue(DialogWidthProperty, value);
    }

    public int DialogHeight
    {
        get => (int)GetValue(DialogHeightProperty);
        set => SetValue(DialogHeightProperty, value);
    }
}

2. 安卓项目实现自定义渲染器 #

重写Picker的点击事件，创建自定义AlertDialog并应用样式：

[assembly: ExportRenderer(typeof(CustomPicker), typeof(CustomPickerRenderer))]
namespace YourAppNamespace.Droid.Renderers
{
    public class CustomPickerRenderer : PickerRenderer
    {
        private AlertDialog _dialog;

        public CustomPickerRenderer(Context context) : base(context)
        {
        }

        protected override void OnElementChanged(ElementChangedEventArgs<Picker> e)
        {
            base.OnElementChanged(e);

            if (Control == null || e.NewElement == null)
                return;

            Control.Click += Control_Click;
        }

        private void Control_Click(object sender, EventArgs e)
        {
            var customPicker = Element as CustomPicker;
            if (customPicker == null || customPicker.Items.Count == 0)
                return;

            var builder = new AlertDialog.Builder(Context);
            builder.SetTitle(customPicker.DialogTitle);
            builder.SetItems(customPicker.Items.ToArray(), (s, args) =>
            {
                customPicker.SelectedIndex = args.Which;
                _dialog?.Dismiss();
            });

            _dialog = builder.Create();
            _dialog.Show();

            // 设置弹窗背景色
            if (customPicker.DialogBackgroundColor != Color.Default)
            {
                var window = _dialog.Window;
                window.SetBackgroundDrawable(new ColorDrawable(customPicker.DialogBackgroundColor.ToAndroid()));
            }

            // 调整弹窗尺寸
            if (customPicker.DialogWidth != -1 || customPicker.DialogHeight != -1)
            {
                var window = _dialog.Window;
                var layoutParams = window.Attributes;

                if (customPicker.DialogWidth != -1)
                    layoutParams.Width = customPicker.DialogWidth;
                if (customPicker.DialogHeight != -1)
                    layoutParams.Height = customPicker.DialogHeight;

                window.Attributes = layoutParams;
            }
        }

        protected override void Dispose(bool disposing)
        {
            if (disposing)
            {
                Control.Click -= Control_Click;
                _dialog?.Dispose();
            }
            base.Dispose(disposing);
        }
    }
}

3. 在XAML中使用自定义Picker #

直接绑定自定义属性即可实现弹窗样式定制：

<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">

    <StackLayout>
        <local:CustomPicker 
            Title="点击选择"
            DialogTitle="自定义弹窗标题"
            DialogBackgroundColor="#34495E"
            DialogWidth="700"
            DialogHeight="500">
            <local:CustomPicker.Items>
                <x:String>选项A</x:String>
                <x:String>选项B</x:String>
                <x:String>选项C</x:String>
            </local:CustomPicker.Items>
        </local:CustomPicker>
    </StackLayout>
</ContentPage>

注意事项 #

• 弹窗尺寸参数为像素值，若需使用dp单位，可在CustomPicker中添加dp转px的转换方法，或者修改绑定属性类型为double后处理转换逻辑。
• 若需要更复杂的弹窗布局（如自定义列表项样式），可使用builder.SetView()方法加载自定义布局文件替代默认列表。
• 确保安卓项目中正确引用自定义渲染器的命名空间。

内容的提问来源于stack exchange，提问作者Vincent Bottini