Make WinRT/UWP platform classes more maintainable (#652)

* Split Platform.cs into partial classes

* Split the NavigationPageRenderer into partial classes for easier maintenance

* Simplify GetCommandBarAsync, remove unneeded comments

* Correct broken GetCommandBarAsync method

* Fix OSX build

1 files changed, 97 insertions, 0 deletions

diff --git a/Xamarin.Forms.Platform.UAP/NavigationPageRendererUWP.cs b/Xamarin.Forms.Platform.UAP/NavigationPageRendererUWP.cs
new file mode 100644
index 00000000..a56ce903
--- /dev/null
+++ b/Xamarin.Forms.Platform.UAP/NavigationPageRendererUWP.cs
@@ -0,0 +1,97 @@
+using Windows.UI.Core;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Data;
+using Xamarin.Forms.PlatformConfiguration.WindowsSpecific;
+
+namespace Xamarin.Forms.Platform.UWP
+{
+	public partial class NavigationPageRenderer : IToolBarForegroundBinder
+	{
+		SystemNavigationManager _navManager;
+
+		public void BindForegroundColor(AppBar appBar)
+		{
+			SetAppBarForegroundBinding(appBar);
+		}
+
+		public void BindForegroundColor(AppBarButton button)
+		{
+			SetAppBarForegroundBinding(button);
+		}
+
+		void SetAppBarForegroundBinding(FrameworkElement element)
+		{
+			element.SetBinding(Control.ForegroundProperty,
+				new Windows.UI.Xaml.Data.Binding { Path = new PropertyPath("TitleBrush"), Source = _container, RelativeSource = new RelativeSource { Mode = RelativeSourceMode.TemplatedParent } });
+		}
+
+		void UpdateToolbarPlacement()
+		{
+			if (_container == null)
+			{
+				return;
+			}
+
+			_container.ToolbarPlacement = Element.OnThisPlatform().GetToolbarPlacement();
+		}
+
+		void UpdateShowTitle()
+		{
+			((ITitleProvider)this).ShowTitle = _parentTabbedPage == null && _parentMasterDetailPage == null;
+		}
+
+		static object GetDefaultColor()
+		{
+			return Windows.UI.Xaml.Application.Current.Resources["SystemControlBackgroundChromeMediumLowBrush"];
+		}
+
+		void UpdateBackButton()
+		{
+			bool showBackButton = PageController.InternalChildren.Count > 1 && NavigationPage.GetHasBackButton(_currentPage);
+			_container.ShowBackButton = showBackButton;
+
+			if (_navManager != null)
+			{
+				_navManager.AppViewBackButtonVisibility = showBackButton ? AppViewBackButtonVisibility.Visible : AppViewBackButtonVisibility.Collapsed;
+			}
+		}
+
+		async void UpdateTitleOnParents()
+		{
+			if (Element == null)
+				return;
+
+			ITitleProvider render = null;
+			if (_parentTabbedPage != null)
+			{
+				render = Platform.GetRenderer(_parentTabbedPage) as ITitleProvider;
+				if (render != null)
+					render.ShowTitle = (_parentTabbedPage.CurrentPage == Element) && NavigationPage.GetHasNavigationBar(_currentPage);
+			}
+
+			if (_parentMasterDetailPage != null)
+			{
+				render = Platform.GetRenderer(_parentMasterDetailPage) as ITitleProvider;
+				if (render != null)
+					render.ShowTitle = (_parentMasterDetailPage.Detail == Element) && NavigationPage.GetHasNavigationBar(_currentPage);
+			}
+
+			if (render != null && render.ShowTitle)
+			{
+				render.Title = _currentPage.Title;
+				render.BarBackgroundBrush = GetBarBackgroundBrush();
+				render.BarForegroundBrush = GetBarForegroundBrush();
+			}
+
+			if (_showTitle || (render != null && render.ShowTitle))
+			{
+				var platform = Element.Platform as Platform;
+				if (platform != null)
+				{
+					await platform.UpdateToolbarItems();
+				}
+			}
+		}
+	}
+}