Gradient as a Buttons BorderColor?

问题

I want to set a gradient border color for a Xamarin.Forms button. How can I do that?

Current:

Desire:

A few options rise to the top through Google, but none seem to be platform independent AND work for Buttons.

To name a couple:
-Using a WebView and setting the gradient background using CSS (source: Xamarin forums)
-XFGloss looks to be a outstandingly well-designed solution but, unfortunately its scope, as written, appears to be Cell views and Layouts only. Likewise even if this does work for Buttons, it appears to be iOS & Android only.

The desire is an iOS & UWP compatible solution for applying gradients to a Button s BorderColor.

Did some preliminary digging and have yet to find anything for Buttons. A thousand apologies if I missed a post on SO regarding Button BorderColor property gradients.

回答1:

You could custom ButtomRender to realize gradient border in Xamarin Forms. Edit the BorderColor property in OnElementChanged override method. You could pass a LinearGradientBrush to Control.BorderBrush to realize this feature within UWP. For detail, please refer the following code.

UWP

public class MyButtonRenderer : ButtonRenderer
{
    protected override void OnElementChanged(ElementChangedEventArgs<Button> e)
    {
        base.OnElementChanged(e);
        if (e.NewElement != null)
        {
            if (Element.IsSet(Button.BorderColorProperty) && Element.BorderColor != (Color)Button.BorderColorProperty.DefaultValue)
            {
                UpdateBorderColor();
            }

        }
    }
    protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        base.OnElementPropertyChanged(sender, e);
        if (e.PropertyName == Button.BorderColorProperty.PropertyName)
        {
            UpdateBorderColor();
        }
    }
    void UpdateBorderColor()
    {
        Control.BorderBrush = Element.BorderColor != Color.Default ? Element.BorderColor.ToGradientBrush() : (Brush)Windows.UI.Xaml.Application.Current.Resources["ButtonBorderThemeBrush"];
    }

}
internal static class ConvertExtensions
{
    public static Brush ToGradientBrush(this Color color)
    {
        var GradientBrush = new LinearGradientBrush();
        GradientBrush.StartPoint = new Windows.Foundation.Point(0.5, 0);
        GradientBrush.EndPoint = new Windows.Foundation.Point(0.5, 1);
        GradientBrush.GradientStops.Add(new GradientStop() { Color = Windows.UI.Colors.LightGray, Offset = 0.0 });
        GradientBrush.GradientStops.Add(new GradientStop() { Color = color.ToWindowsColor(), Offset = 1.0 });
        return GradientBrush;
    }

    public static Windows.UI.Color ToWindowsColor(this Color color)
    {
        return Windows.UI.Color.FromArgb((byte)(color.A * 255), (byte)(color.R * 255), (byte)(color.G * 255), (byte)(color.B * 255));
    }

}

IOS

public class MyButtonRenderer : ButtonRenderer
{
   CAGradientLayer gradient;
   CAShapeLayer shape;
   protected override void OnElementChanged(ElementChangedEventArgs<Button> e)
    {
        base.OnElementChanged(e);

        gradient = new CAGradientLayer(); 
        // add start color          
        gradient.Colors = new CGColor[] { ((GradientButton)Element).StartColor.ToCGColor(), Element.BorderColor.ToCGColor() };

       shape = new CAShapeLayer();
       shape.LineWidth = (nfloat)(Element.BorderWidth);
       shape.StrokeColor = UIColor.Black.CGColor;
       shape.FillColor = UIColor.Clear.CGColor;
       gradient.Mask = shape;

       Control.Layer.AddSublayer(gradient);
       Control.Layer.BorderColor = UIColor.Clear.CGColor;
    }

     public override void Draw(CGRect rect)
     {
       base.Draw(rect);

       shape.Path = UIBezierPath.FromRect(rect).CGPath;
       gradient.Frame = rect;
     }
}

This is code sample that contain UWP and IOS platform please check.