如何修改Xamarin Forms中Frame控件的边框厚度？

方案1：自定义Frame渲染器（推荐） #

直接在Android端自定义Frame的渲染逻辑，完全控制边框厚度、圆角和背景，无需嵌套控件。

1. 共享项目中定义自定义Frame：

public class BorderedFrame : Frame
{
    public static readonly BindableProperty BorderThicknessProperty =
        BindableProperty.Create(nameof(BorderThickness), typeof(int), typeof(BorderedFrame), 1);

    public int BorderThickness
    {
        get => (int)GetValue(BorderThicknessProperty);
        set => SetValue(BorderThicknessProperty, value);
    }
}

2. Android项目中实现自定义渲染器：

[assembly: ExportRenderer(typeof(BorderedFrame), typeof(BorderedFrameRenderer))]
namespace YourAppName.Droid.Renderers
{
    public class BorderedFrameRenderer : FrameRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<Frame> e)
        {
            base.OnElementChanged(e);
            if (Element is BorderedFrame borderedFrame)
            {
                SetBackgroundColor(borderedFrame.BackgroundColor.ToAndroid());
                
                var radius = TypedValue.ApplyDimension(ComplexUnitType.Dip, borderedFrame.CornerRadius, Context.Resources.DisplayMetrics);
                var shape = new GradientDrawable();
                shape.SetColor(borderedFrame.BackgroundColor.ToAndroid());
                shape.SetStroke(borderedFrame.BorderThickness, borderedFrame.OutlineColor.ToAndroid());
                shape.SetCornerRadius(radius);

                if (Build.VERSION.SdkInt >= BuildVersionCodes.JellyBean)
                {
                    View.Background = shape;
                }
                else
                {
                    View.SetBackgroundDrawable(shape);
                }
            }
        }

        protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);
            if (e.PropertyName == BorderedFrame.BorderThicknessProperty.PropertyName ||
                e.PropertyName == Frame.CornerRadiusProperty.PropertyName ||
                e.PropertyName == Frame.OutlineColorProperty.PropertyName)
            {
                OnElementChanged(new ElementChangedEventArgs<Frame>(Element, Element));
            }
        }
    }
}

3. XAML中直接使用：

<local:BorderedFrame 
    BackgroundColor="White"
    OutlineColor="#2196F3"
    CornerRadius="10"
    BorderThickness="3"
    Padding="16">
    <Label Text="带自定义边框的内容" />
</local:BorderedFrame>

方案2：BoxView+布局组合（无需自定义渲染器） #

用BoxView做外层边框，内层布局承载内容，通过padding控制边框厚度：

<Grid>
    <!-- 外层BoxView作为边框 -->
    <BoxView 
        BackgroundColor="#2196F3"
        CornerRadius="10" />
    <!-- 内层布局，padding等于边框厚度，背景为内容背景色 -->
    <StackLayout 
        Padding="3"
        BackgroundColor="White"
        CornerRadius="10"
        Margin="0">
        <Label Text="带边框的内容" Margin="16" />
    </StackLayout>
</Grid>

这里内层布局的CornerRadius和外层保持一致即可，padding留出的边框空间不会导致圆角冲突。

方案3：使用Effect（效果）复用边框逻辑 #

不想写自定义渲染器的话，可以创建Effect来统一处理边框：

1. 共享项目中定义Effect：

public class BorderEffect : RoutingEffect
{
    public BorderEffect() : base("YourAppName.BorderEffect") { }

    public int BorderThickness { get; set; } = 1;
    public Color BorderColor { get; set; } = Color.Black;
}

2. Android项目中实现Effect：

[assembly: ResolutionGroupName("YourAppName")]
[assembly: ExportEffect(typeof(BorderEffectDroid), "BorderEffect")]
namespace YourAppName.Droid.Effects
{
    public class BorderEffectDroid : PlatformEffect
    {
        protected override void OnAttached()
        {
            if (Control is Android.Views.View view && Element is Frame frame)
            {
                var effect = (BorderEffect)Element.Effects.FirstOrDefault(e => e is BorderEffect);
                if (effect != null)
                {
                    var radius = TypedValue.ApplyDimension(ComplexUnitType.Dip, frame.CornerRadius, Context.Resources.DisplayMetrics);
                    var shape = new GradientDrawable();
                    shape.SetColor(frame.BackgroundColor.ToAndroid());
                    shape.SetStroke(effect.BorderThickness, effect.BorderColor.ToAndroid());
                    shape.SetCornerRadius(radius);
                    view.Background = shape;
                }
            }
        }

        protected override void OnDetached()
        {
            // 清理资源
        }
    }
}

3. XAML中使用：

<Frame 
    BackgroundColor="White"
    CornerRadius="10"
    Padding="16">
    <Frame.Effects>
        <local:BorderEffect BorderThickness="3" BorderColor="#2196F3" />
    </Frame.Effects>
    <Label Text="带边框的内容" />
</Frame>

自定义渲染器方案功能最完整，适合频繁使用场景；BoxView组合方案轻量化，适合简单需求；Effect方案兼顾复用性和灵活性，可根据实际场景选择。

内容的提问来源于stack exchange，提问作者Microkid