Platform.cs

using Gtk;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Xamarin.Forms.Internals;
using Xamarin.Forms.Platform.GTK.Helpers;
using Xamarin.Forms.Platform.GTK.Renderers;

namespace Xamarin.Forms.Platform.GTK
{
	public class Platform : BindableObject, INavigation, IDisposable
#pragma warning disable CS0618
		, IPlatform
#pragma warning restore
	{
		private bool _disposed;
		readonly List<Page> _modals;
		private readonly PlatformRenderer _renderer;

		internal static readonly BindableProperty RendererProperty =
			BindableProperty.CreateAttached("Renderer", typeof(IVisualElementRenderer),
				typeof(Platform), default(IVisualElementRenderer),
			propertyChanged: (bindable, oldvalue, newvalue) =>
			{
				var view = bindable as VisualElement;
				if (view != null)
					view.IsPlatformEnabled = newvalue != null;
			});

		internal PlatformRenderer PlatformRenderer => _renderer;

		Page Page { get; set; }

		IReadOnlyList<Page> INavigation.ModalStack
		{
			get { return _modals; }
		}

		IReadOnlyList<Page> INavigation.NavigationStack
		{
			get { return new List<Page>(); }
		}

		internal Platform()
		{
			_renderer = new PlatformRenderer(this);
			_modals = new List<Page>();
			Application.Current.NavigationProxy.Inner = this;

			MessagingCenter.Subscribe(this, Page.AlertSignalName, (Page sender, AlertArguments arguments) => DialogHelper.ShowAlert(PlatformRenderer, arguments));
			MessagingCenter.Subscribe(this, Page.ActionSheetSignalName, (Page sender, ActionSheetArguments arguments) => DialogHelper.ShowActionSheet(PlatformRenderer, arguments));
		}

		internal static void DisposeModelAndChildrenRenderers(Element view)
		{
			IVisualElementRenderer renderer;

			foreach (VisualElement child in view.Descendants())
				DisposeModelAndChildrenRenderers(child);

			renderer = GetRenderer((VisualElement)view);

			(renderer as Widget)?.Destroy();
			view.ClearValue(RendererProperty);
		}

		public static SizeRequest GetNativeSize(VisualElement view, double widthConstraint, double heightConstraint)
		{
			var renderView = GetRenderer(view);

			if (renderView == null || renderView.Container == null)
				return new SizeRequest(Size.Zero);

			return renderView.GetDesiredSize(widthConstraint, heightConstraint);
		}

		public static IVisualElementRenderer GetRenderer(VisualElement element)
		{
			return (IVisualElementRenderer)element.GetValue(RendererProperty);
		}

		public static void SetRenderer(VisualElement element, IVisualElementRenderer value)
		{
			if (element != null)
			{
				element.SetValue(RendererProperty, value);
				element.IsPlatformEnabled = value != null;
			}
		}

		public static IVisualElementRenderer CreateRenderer(VisualElement element)
		{
			var renderer = Registrar.Registered.GetHandlerForObject<IVisualElementRenderer>(element) ?? new DefaultRenderer();
			renderer.SetElement(element);

			return renderer;
		}

		void IDisposable.Dispose()
		{
			if (_disposed)
				return;

			_disposed = true;

			MessagingCenter.Unsubscribe<Page, ActionSheetArguments>(this, Page.ActionSheetSignalName);
			MessagingCenter.Unsubscribe<Page, AlertArguments>(this, Page.AlertSignalName);
			MessagingCenter.Unsubscribe<Page, bool>(this, Page.BusySetSignalName);

			foreach (var modal in _modals)
				DisposeModelAndChildrenRenderers(modal);
			DisposeModelAndChildrenRenderers(Page);

			PlatformRenderer.Destroy();
		}

		internal void SetPage(Page newRoot)
		{
			if (newRoot == null)
				return;

			if (Page != null)
				throw new NotImplementedException();

			Page = newRoot;

#pragma warning disable CS0618 // Type or member is obsolete
			// The Platform property is no longer necessary, but we have to set it because some third-party
			// library might still be retrieving it and using it
			Page.Platform = this;
#pragma warning restore CS0618 // Type or member is obsolete

			AddChild(Page);

			Application.Current.NavigationProxy.Inner = this;
		}

		private void AddChild(Page mainPage)
		{
			var viewRenderer = GetRenderer(mainPage);

			if (viewRenderer == null)
			{
				viewRenderer = CreateRenderer(mainPage);
				SetRenderer(mainPage, viewRenderer);

				PlatformRenderer.Add(viewRenderer.Container);
				PlatformRenderer.ShowAll();
			}
		}

		private void HandleChildRemoved(object sender, ElementEventArgs e)
		{
			var view = e.Element;
			DisposeModelAndChildrenRenderers(view);
		}

		void INavigation.InsertPageBefore(Page page, Page before)
		{
			throw new InvalidOperationException("InsertPageBefore is not supported globally on GTK, please use a NavigationPage.");
		}

		Task<Page> INavigation.PopAsync()
		{
			return ((INavigation)this).PopAsync(true);
		}

		Task<Page> INavigation.PopAsync(bool animated)
		{
			throw new InvalidOperationException("PopAsync is not supported globally on GTK, please use a NavigationPage.");
		}

		Task<Page> INavigation.PopModalAsync()
		{
			return ((INavigation)this).PopModalAsync(true);
		}

		Task<Page> INavigation.PopModalAsync(bool animated)
		{
			var modal = _modals.Last();
			_modals.Remove(modal);
			modal.DescendantRemoved -= HandleChildRemoved;

			var modalPage = GetRenderer(modal) as Container;

			var pageControl = PlatformRenderer.Child as IPageControl;

			Device.BeginInvokeOnMainThread(() =>
			{
				pageControl?.Control?.PopModal(modalPage);

				DisposeModelAndChildrenRenderers(modal);
			});

			return Task.FromResult<Page>(modal);
		}

		Task INavigation.PopToRootAsync()
		{
			return ((INavigation)this).PopToRootAsync(true);
		}

		Task INavigation.PopToRootAsync(bool animated)
		{
			throw new InvalidOperationException("PopToRootAsync is not supported globally on GTK, please use a NavigationPage.");
		}

		Task INavigation.PushAsync(Page root)
		{
			return ((INavigation)this).PushAsync(root, true);
		}

		Task INavigation.PushAsync(Page root, bool animated)
		{
			throw new InvalidOperationException("PushAsync is not supported globally on GTK, please use a NavigationPage.");
		}

		Task INavigation.PushModalAsync(Page modal)
		{
			return ((INavigation)this).PushModalAsync(modal, true);
		}

		Task INavigation.PushModalAsync(Page modal, bool animated)
		{
			_modals.Add(modal);

#pragma warning disable CS0618 // Type or member is obsolete
			// The Platform property is no longer necessary, but we have to set it because some third-party
			// library might still be retrieving it and using it
			modal.Platform = this;
#pragma warning restore CS0618 // Type or member is obsolete

			modal.DescendantRemoved += HandleChildRemoved;

			var modalRenderer = GetRenderer(modal);
			if (modalRenderer == null)
			{
				modalRenderer = CreateRenderer(modal);
				SetRenderer(modal, modalRenderer);
			}

			var pageControl = PlatformRenderer.Child as IPageControl;

			Device.BeginInvokeOnMainThread(() =>
			{
				if (pageControl != null)
				{
					var page = pageControl.Control;

					if (page != null)
					{
						page.PushModal(modalRenderer.Container);
					}
				}
			});

			return Task.FromResult<object>(null);
		}

		void INavigation.RemovePage(Page page)
		{
			throw new InvalidOperationException("RemovePage is not supported globally on GTK, please use a NavigationPage.");
		}

		internal class DefaultRenderer : VisualElementRenderer<VisualElement, Widget>
		{

		}

		#region Obsolete 

		SizeRequest IPlatform.GetNativeSize(VisualElement view, double widthConstraint, double heightConstraint)
		{
			return GetNativeSize(view, widthConstraint, heightConstraint);
		}

		#endregion
	}
}