System.ArgumentException：“不是有效的默认值参数名称：defaultValue”（NControl）

Question

I am trying to implement a button with a gradient occupying the NControl and NGraphics plugin

To do this I created a folder named Controls in my shared project, within it I created a class called GradientButton which inherits from NControlView (using NControl.Abstractions;) and contains all the methods and commands to be binded in the view

GradientButton.CS:

using NControl.Abstractions;
using NGraphics;
using System.Collections.Generic;
using System.Windows.Input;
using Xamarin.Forms;
using Color = Xamarin.Forms.Color;
using Point = NGraphics.Point;
using TextAlignment = Xamarin.Forms.TextAlignment;

namespace SoftwareCore.Forms.Controls
{

    public class GradientButton : NControlView
    {
        private readonly Label _label;
        private readonly NControlView _background;

        public GradientButton()
        {
            HeightRequest = 44;
            WidthRequest = 100;

            _label = new Label
            {
                Text = Text,
                TextColor = TextColor,
                FontSize = 17,
                BackgroundColor = Color.Transparent,
                HorizontalTextAlignment = TextAlignment.Center,
                VerticalTextAlignment = TextAlignment.Center
            };

            _background = new NControlView
            {
                DrawingFunction = (canvas, rect) =>
                {
                    var brush = new LinearGradientBrush(
                        Point.Zero,
                        Point.OneX,
                        StartColor.ToNColor(),
                        EndColor.ToNColor());

                    var curveSize = BorderRadius;
                    var width = rect.Width;
                    var height = rect.Height;

                    canvas.DrawPath(new PathOp[]{
                        new MoveTo(curveSize, 0),
                        // Top Right corner
                        new LineTo(width-curveSize, 0),
                        new CurveTo(
                            new Point(width-curveSize, 0),
                            new Point(width, 0),
                            new Point(width, curveSize)
                        ),
                        new LineTo(width, height-curveSize),
                        // Bottom right corner
                        new CurveTo(
                            new Point(width, height-curveSize),
                            new Point(width, height),
                            new Point(width-curveSize, height)
                        ),
                        new LineTo(curveSize, height),
                        // Bottom left corner
                        new CurveTo(
                            new Point(curveSize, height),
                            new Point(0, height),
                            new Point(0, height-curveSize)
                        ),
                        new LineTo(0, curveSize),
                        new CurveTo(
                            new Point(0, curveSize),
                            new Point(0, 0),
                            new Point(curveSize, 0)
                        ),
                        new ClosePath()
                    }, null, brush);


                }
            };

            Content = new Grid
            {
                Children = {
                    _background,
                    _label
                }
            };
        }

        public Color StartColor
        {
            get => (Color)GetValue(StartColorProperty);
            set
            {
                SetValue(StartColorProperty, value);
                Invalidate();
            }
        }

        public Color EndColor
        {
            get => (Color)GetValue(EndColorProperty);
            set
            {
                SetValue(EndColorProperty, value);
                Invalidate();
            }
        }

        public static BindableProperty StartColorProperty =
            BindableProperty.Create(nameof(StartColor), typeof(Color), typeof(GradientButton), defaultBindingMode: BindingMode.OneWay,
                propertyChanged: (b, o, n) =>
                {
                    var ctrl = (GradientButton)b;
                    ctrl.StartColor = (Color)n;
                });
        public static BindableProperty EndColorProperty =
            BindableProperty.Create(nameof(EndColor), typeof(Color), typeof(GradientButton), defaultBindingMode: BindingMode.OneWay,
                propertyChanged: (b, o, n) =>
                {
                    var ctrl = (GradientButton)b;
                    ctrl.EndColor = (Color)n;
                });

        public int BorderRadius
        {
            get => (int)GetValue(BorderRadiusProperty);
            set
            {
                SetValue(BorderRadiusProperty, value);
                Invalidate();
            }
        }
        public const int DefaultRadius = 40;
        public static BindableProperty BorderRadiusProperty =
            BindableProperty.Create(nameof(BorderRadius), typeof(int), typeof(GradientButton), DefaultRadius, BindingMode.OneTime,
                propertyChanged: (b, o, n) =>
                {
                    var ctrl = (GradientButton)b;
                    ctrl.BorderRadius = (int)n;
                });

        public string Text
        {
            get => GetValue(TextProperty) as string;
            set
            {
                SetValue(TextProperty, value);
                _label.Text = value;
                Invalidate();
            }
        }

        public static BindableProperty TextProperty =
            BindableProperty.Create(nameof(Text), typeof(string), typeof(GradientButton), defaultBindingMode: BindingMode.OneWay,
                propertyChanged: (b, o, n) =>
                {
                    var ctrl = (GradientButton)b;
                    ctrl.Text = (string)n;
                });

        public Color TextColor
        {
            get => (Color)GetValue(TextColorProperty);
            set
            {
                SetValue(TextColorProperty, value);
                _label.TextColor = value;
                Invalidate();
            }
        }

        public static BindableProperty TextColorProperty =
            BindableProperty.Create(nameof(TextColor), typeof(Color), typeof(GradientButton), defaultBindingMode: BindingMode.OneWay,
                propertyChanged: (b, o, n) =>
                {
                    var ctrl = (GradientButton)b;
                    ctrl.TextColor = (Color)n;
                });

        public static BindableProperty CommandProperty =
            BindableProperty.Create(nameof(Command), typeof(ICommand), typeof(GradientButton), defaultBindingMode: BindingMode.OneWay,
                propertyChanged: (bindable, oldValue, newValue) =>
                {
                    var ctrl = (GradientButton)bindable;
                    ctrl.Command = (ICommand)newValue;
                });

        public ICommand Command
        {
            get => (ICommand)GetValue(CommandProperty);
            set => SetValue(CommandProperty, value);
        }

        public static BindableProperty CommandParameterProperty =
            BindableProperty.Create(nameof(CommandParameter), typeof(object), typeof(GradientButton), defaultBindingMode: BindingMode.TwoWay,
                propertyChanged: (bindable, oldValue, newValue) =>
                {
                    var ctrl = (GradientButton)bindable;
                    ctrl.CommandParameter = newValue;
                });

        //private readonly Label _label;
        //private readonly NControlView _background;

        public object CommandParameter
        {
            get => GetValue(CommandParameterProperty);
            set => SetValue(CommandParameterProperty, value);
        }

        public override bool TouchesBegan(IEnumerable<Point> points)
        {
            base.TouchesBegan(points);
            this.ScaleTo(0.96, 65, Easing.CubicInOut);
            return true;
        }

        public override bool TouchesCancelled(IEnumerable<Point> points)
        {
            base.TouchesCancelled(points);
            this.ScaleTo(1.0, 65, Easing.CubicInOut);
            return true;
        }

        public override bool TouchesEnded(IEnumerable<Point> points)
        {
            base.TouchesEnded(points);
            this.ScaleTo(1.0, 65, Easing.CubicInOut);
            if (Command != null && Command.CanExecute(CommandParameter))
                Command.Execute(CommandParameter);

            return true;
        }
    }

    public static class ColorHelpers
    {
        public static NGraphics.Color ToNColor(this Color color)
        {
            return new NGraphics.Color(color.R, color.G, color.B, color.A);
        }
    }
}

Then in my XAML view I declare the custom property to be able to use the control and use the button and its properties

LOGINVIEW.XAML:

<?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:controls="clr-namespace:SoftwareCore.Forms.Controls"
             BindingContext="{Binding Main, Source={StaticResource Locator}}"
             x:Class="SoftwareCore.Forms.Views.LoginView"
             Title="Login">


        <StackLayout
                BindingContext="{Binding Login}"
                Padding="8">

            <StackLayout
                    Orientation="Horizontal">

                    <controls:GradientButton
                                Margin="10"
                                BorderRadius="20"
                                EndColor="#5a41b9"
                                HorizontalOptions="FillAndExpand"
                                StartColor="#6C6FEC"
                                Text="Get started"
                                TextColor="White" />        

            </StackLayout>
        </StackLayout>
</ContentPage>

But when starting the project and trying to show the view my application bursts with the following error on the screen

System.ArgumentException: 'Not a valid default value Parameter name: defaultValue'

I do not understand what is the value that I am not initializing and when trying to see the StackTrace it gives me very little information about the error

When you continue debugging after the first error, it shows a second in the LoginView in the code behind

What am I doing wrong? What am I forgetting to initialize? Any help for me?

Answer 1

The error comes for the lack of a DefaultValue when creating the BindableProperties for the Colors: StartColorProperty and EndColorProperty .

Change the Create method of these two BindableProperties and add a DefaultValue . If you don't want to use anything just use the Color.Default , something like this:

public static BindableProperty StartColorProperty =
    BindableProperty.Create(nameof(StartColor), typeof(Color), typeof(GradientButton), defaultValue: Color.Default, defaultBindingMode: BindingMode.OneWay,
        propertyChanged: (b, o, n) =>
        {
            var ctrl = (GradientButton)b;
            ctrl.StartColor = (Color)n;
        });

public static BindableProperty EndColorProperty =
    BindableProperty.Create(nameof(EndColor), typeof(Color), typeof(GradientButton), defaultValue: Color.Default, defaultBindingMode: BindingMode.OneWay,
        propertyChanged: (b, o, n) =>
        {
            var ctrl = (GradientButton)b;
            ctrl.EndColor = (Color)n;
        });

Hope this helps.-