[Xaml] Fallback to App.Current for DynResources (Previewer) (#793)

* [Xaml] Fallback to App.Current for DynResources (Previewer)

* [C] avoid NRE and ensure setting the style

Xamarin.Forms.Controls/SdxApp.xaml

@@ -0,0 +1,5 @@
+<?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="Xamarin.Forms.Controls.SdxApp">
+	<ContentPage.Content>
+	</ContentPage.Content>
+</ContentPage>

Xamarin.Forms.Controls/SdxApp.xaml.cs

@@ -0,0 +1,22 @@
+//
+// SdxApp.xaml.cs
+//
+// Author:
+//       Stephane Delcroix <stdelc@microsoft.com>
+//
+// Copyright (c) 2017 
+using System;
+using System.Collections.Generic;
+
+using Xamarin.Forms;
+
+namespace Xamarin.Forms.Controls
+{
+	public partial class SdxApp : ContentPage
+	{
+		public SdxApp()
+		{
+			InitializeComponent();
+		}
+	}
+}

Xamarin.Forms.Controls/SdxPage.xaml

@@ -0,0 +1,5 @@
+<?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="Xamarin.Forms.Controls.SdxPage">
+	<ContentPage.Content>
+	</ContentPage.Content>
+</ContentPage>

Xamarin.Forms.Controls/SdxPage.xaml.cs

@@ -0,0 +1,22 @@
+//
+// SdxPage.xaml.cs
+//
+// Author:
+//       Stephane Delcroix <stdelc@microsoft.com>
+//
+// Copyright (c) 2017 
+using System;
+using System.Collections.Generic;
+
+using Xamarin.Forms;
+
+namespace Xamarin.Forms.Controls
+{
+	public partial class SdxPage : ContentPage
+	{
+		public SdxPage()
+		{
+			InitializeComponent();
+		}
+	}
+}

Xamarin.Forms.Core.UnitTests/DynamicResourceTests.cs

@@ -10,6 +10,13 @@ namespace Xamarin.Forms.Core.UnitTests
 		{
 			base.Setup ();
 			Device.PlatformServices = new MockPlatformServices ();
+			Application.Current = new MockApplication();
+		}
+
+		[TearDown]
+		public override void TearDown()
+		{
+			Application.Current = null;
 		}
 
 		[Test]
@@ -143,11 +150,24 @@ namespace Xamarin.Forms.Core.UnitTests
 			label.SetDynamicResource (Label.TextProperty, "foo");
 			label.Resources = new ResourceDictionary { {"foo","FOO"}};
 
-			Assert.AreEqual ("FOO", label.Text);
+			Assume.That(label.Text, Is.EqualTo("FOO"));
 
 			label.Resources ["foo"] = "BAR";
 
 			Assert.AreEqual ("BAR", label.Text);
 		}
+
+		[Test]
+		public void FallbackToApplicationCurrent()
+		{
+			Application.Current.Resources = new ResourceDictionary { { "foo", "FOO" } };
+
+			var label = new Label();
+			label.BindingContext = new MockViewModel();
+			label.SetBinding(Label.TextProperty, "Text", BindingMode.TwoWay);
+			label.SetDynamicResource(Label.TextProperty, "foo");
+
+			Assert.That(label.Text, Is.EqualTo("FOO"));
+		}
 	}
 }

Xamarin.Forms.Core/MergedStyle.cs

@@ -125,12 +125,11 @@ namespace Xamarin.Forms
 			void RegisterImplicitStyles()
 			{
 				Type type = TargetType;
-				while (true)
-				{
+				while (true) {
 					BindableProperty implicitStyleProperty = BindableProperty.Create("ImplicitStyle", typeof(Style), typeof(VisualElement), default(Style),
-						propertyChanged: (bindable, oldvalue, newvalue) => ((VisualElement)bindable)._mergedStyle.OnImplicitStyleChanged());
-					Target.SetDynamicResource(implicitStyleProperty, type.FullName);
+						 propertyChanged: (bindable, oldvalue, newvalue) => OnImplicitStyleChanged());
 					_implicitStyles.Add(implicitStyleProperty);
+					Target.SetDynamicResource(implicitStyleProperty, type.FullName);
 					type = type.GetTypeInfo().BaseType;
 					if (s_stopAtTypes.Contains(type))
 						return;

Xamarin.Forms.Core/ResourcesExtensions.cs

@@ -55,6 +55,11 @@ namespace Xamarin.Forms
 					return true;
 				element = element.Parent;
 			}
+
+			//Fallback for the XF previewer
+			if (Application.Current != null && Application.Current.Resources != null && Application.Current.Resources.TryGetValue(key, out value))
+				return true;
+
 			value = null;
 			return false;
 		}