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<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="App.Views.HandsOnRoundedBoxViewPage"
             xmlns:local="clr-namespace:App.Controls"
             xmlns:mvvm="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
             mvvm:ViewModelLocator.AutowireViewModel="true" >
  <StackLayout HorizontalOptions="Center"
                VerticalOptions="Center">
    <local:RoundedBoxView x:Name="roundedBox"
                          HeightRequest="100"
                          WidthRequest="100"
                          CornerRadius="{Binding MyCornerRadius}"
                          Clicked="{Binding ClickCommand}"/>
    <Button Text="Next Color" />
  </StackLayout>
</ContentPage>

using Prism.Mvvm;
using System.Diagnostics;

namespace App.ViewModels
{
	public class HandsOnRoundedBoxViewPageViewModel : BindableBase
	{ 
	
		public HandsOnRoundedBoxViewPageViewModel()
		{
			this.ClickCommand = new Prism.Commands.DelegateCommand(OnClick);
		}

		public Prism.Commands.DelegateCommand ClickCommand { get; }

		private int myCornerRadius = 20;

		public int MyCornerRadius
		{
			get {
				Debug.WriteLine("HandsOnRoundedBoxViewPageViewModel " + "MyCornerRadius Get ");
				return this.myCornerRadius;
			}
			set
			{
				Debug.WriteLine("HandsOnRoundedBoxViewPageViewModel " + "MyCornerRadius Set " + value);
				this.SetProperty(ref this.myCornerRadius, value);
			}
		}

		private void OnClick()
		{
			Debug.WriteLine("HandsOnRoundedBoxViewPageViewModel " + "OnClick()");

			this.MyCornerRadius = 40;
		}
	}
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;
using Xamarin.Forms;

namespace App.Controls
{
	public class RoundedBoxView : View
	{
		#region CornerRadius BindableProperty
		
		public static readonly BindableProperty CornerRadiusProperty =
			BindableProperty.Create(
				nameof(CornerRadius),
				typeof(double),
				typeof(RoundedBoxView),
				5.0,
				defaultBindingMode: BindingMode.TwoWay); 
				//ViewModelの値が変更されたら、を有効にするためTwoWay
		
		public double CornerRadius
		{
			get {
				System.Diagnostics.Debug.WriteLine("RoundedBoxView CornerRadius Get");

				return (double)GetValue(CornerRadiusProperty);
			} 
			set {
				System.Diagnostics.Debug.WriteLine("RoundedBoxView CornerRadius Set");

				SetValue(CornerRadiusProperty, value);
			}
		}
		
		#endregion

		#region Color BindableProperty
		public static readonly BindableProperty ColorProperty =
			BindableProperty.Create(
				nameof(Color), 
				typeof(Color), 
				typeof(RoundedBoxView), 
				Color.Accent);

		public Color Color
		{
			get { return (Color)GetValue(ColorProperty); }
			set { SetValue(ColorProperty, value); }
		}
		#endregion

		public static readonly BindableProperty ClickedProperty =
			BindableProperty.Create(
				nameof(Command), 
				typeof(ICommand), 
				typeof(RoundedBoxView));
		
		public ICommand Clicked
		{
			get { return (ICommand)GetValue(ClickedProperty); }
			set { SetValue(ClickedProperty, value); }
		}
		
		/// <summary>
		/// ネイティブ コントロールから呼び出される用
		/// </summary>
		internal void SendClick()
		{
			if (Clicked != null)
				Clicked.Execute(parameter: null);
		}
	}
}