FormsCancelButton.cs

using System;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media;

namespace Xamarin.Forms.Platform.UWP
{
	internal class FormsCancelButton : Windows.UI.Xaml.Controls.Button
	{
		TextBlock _cancelButtonGlyph;
		Border _cancelButtonBackground;

		public Brush ForegroundBrush 
		{
			get => _cancelButtonGlyph.Foreground;
			set => _cancelButtonGlyph.Foreground = value;
		}

		public Brush BackgroundBrush
		{
			get => _cancelButtonBackground.Background;
			set => _cancelButtonBackground.Background = value;
		}

		public bool IsReady { get; private set; }

		public event EventHandler ReadyChanged;

		protected override void OnApplyTemplate()
		{
			base.OnApplyTemplate();

			_cancelButtonGlyph = (TextBlock)GetTemplateChild("GlyphElement");
			_cancelButtonBackground = (Border)GetTemplateChild("BorderElement");

			if (_cancelButtonGlyph != null && _cancelButtonBackground != null)
			{
				// The SearchBarRenderer needs to be able to check whether we're ready to have the colors set
				// (we won't be until the first time the button actually appears, which requires the search bar
				// to be focused and have text in it)
				IsReady = true;

				// And we need to inform the SearchBarRenderer of this so it can run the button color update method
				OnReadyChanged();
			}
		}

		protected virtual void OnReadyChanged()
		{
			ReadyChanged?.Invoke(this, EventArgs.Empty);
		}
	}
}