在Xamarin Forms视觉工作室的布局中添加渐变背景 [英] Add Gradient background to layouts in Xamarin Forms visual studio

问题描述

我是Xamarin Forms中的新手，我为Menu创建一个ContentPage.我需要在其背景上使用线性渐变颜色.但是我找不到任何链接可以告诉我如何创建背景渐变颜色.我也希望将颜色放置在按钮背景上，例如android中的选择器.如果您有任何信息，请给予答复.

I am a newbie in Xamarin Forms, I create a ContentPage for Menu. I need linear gradient color at its background. But I can't find any link which tell me how to create background gradient color. I also want that color place at button background like selector in android. If you have any information please give reply.

推荐答案

在下面的代码中，我们可以为下面的任何布局设置水平和垂直渐变(我将其放在#region中)，如果要编写其他布局，则可以为StackLayout编写布局只需替换您在StackLayout上的布局即可.

In below code we can set Horizontal and Vertical Gradient(I put it in #region) for any Layout in below I write for StackLayout if you want to write another Layout just replace your Layout on StackLayout.

在PCL中:

using System;
using Xamarin.Forms;

namespace GradientColor
 {
   public class GradientColorStack : StackLayout
    {
      public Color StartColor { get; set; }
      public Color EndColor { get; set; }
    }
 }

Xamarin.Android:

  using System;
  using GradientColor;
  using GradientColor.Droid;
  using Xamarin.Forms;
  using Xamarin.Forms.Platform.Android;
  [assembly: ExportRenderer(typeof(GradientColorStack), typeof(GradientColorStackRenderer))]

 namespace GradientColor.Droid
  {
     public class GradientColorStackRenderer : VisualElementRenderer<StackLayout>
       {
    private Color StartColor { get; set; }
    private Color EndColor { get; set; }

    protected override void DispatchDraw(global::Android.Graphics.Canvas canvas)
      {
        #region for Vertical Gradient
        //var gradient = new Android.Graphics.LinearGradient(0, 0, 0, Height,
        #endregion

        #region for Horizontal Gradient
     var gradient = new Android.Graphics.LinearGradient(0, 0, Width, 0,
          #endregion

            this.StartColor.ToAndroid(),
            this.EndColor.ToAndroid(),
            Android.Graphics.Shader.TileMode.Mirror);

        var paint = new Android.Graphics.Paint()
        {
            Dither = true,
        };
        paint.SetShader(gradient);
        canvas.DrawPaint(paint);
        base.DispatchDraw(canvas);
    }

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

        if (e.OldElement != null || Element == null)
        {
            return;
        }
        try
        {
            var stack = e.NewElement as GradientColorStack;
            this.StartColor = stack.StartColor;
            this.EndColor = stack.EndColor;
        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(@"ERROR:", ex.Message);
        }
      }
   }
}

Xamarin.iOS:

  using System;
  using CoreAnimation;
  using CoreGraphics;
  using GradientColor;
  using GradientColor.iOS;
  using Xamarin.Forms;
  using Xamarin.Forms.Platform.iOS;
  [assembly: ExportRenderer(typeof(GradientColorStack), typeof(GradientColorStackRenderer))]

 namespace GradientColor.iOS
   {
public class GradientColorStackRenderer : VisualElementRenderer<StackLayout>
    {
      public override void Draw(CGRect rect)
      {
        base.Draw(rect);
        GradientColorStack stack = (GradientColorStack)this.Element;
        CGColor startColor = stack.StartColor.ToCGColor();

        CGColor endColor = stack.EndColor.ToCGColor();

        #region for Vertical Gradient

        var gradientLayer = new CAGradientLayer();

        #endregion

        #region for Horizontal Gradient

        //var gradientLayer = new CAGradientLayer()
        //{
        //  StartPoint = new CGPoint(0, 0.5),
        //  EndPoint = new CGPoint(1, 0.5)
        //};

        #endregion



        gradientLayer.Frame = rect;
        gradientLayer.Colors = new CGColor[] { startColor, endColor 
        };

        NativeView.Layer.InsertSublayer(gradientLayer, 0);
      }
    }
 }

在XAML中:

 <?xml version="1.0" encoding="utf-8"?>
 <ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:local="clr-namespace:GradientColor; assembly:GradientColor" x:Class="GradientColor.GradientColorPage">
 <Grid VerticalOptions="FillAndExpand" RowSpacing="0">
    <Grid.RowDefinitions>
        <RowDefinition Height="*"/>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
    <Grid Grid.Row="0" BackgroundColor="Olive">
         <StackLayout VerticalOptions="Center">
            <Label Text="Normal color for stacklayout" HorizontalTextAlignment="Center" VerticalTextAlignment="Center"  TextColor="White"/>
         </StackLayout>
    </Grid>
    <Grid Grid.Row="1">
         <local:GradientColorStack StartColor="Green" EndColor="Red">
               <StackLayout VerticalOptions="CenterAndExpand">
               <Label Text="Gradient color for StackLayout" HorizontalTextAlignment="Center" VerticalTextAlignment="Center"   TextColor="White"/>
               </StackLayout>

         </local:GradientColorStack>
     </Grid>
   </Grid>
  </ContentPage>

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