[Xaml] OnPlatform.Default value (#873)

* [Xaml] OnPlatform.Default value

* docs

Author: StephaneDelcroix

Xamarin.Forms.Core/OnPlatform.cs

@@ -43,9 +43,21 @@ public T WinPhone {
 			}
 		}
 
+		bool hasDefault;
+		T @default;
+		public T Default {
+			get { return @default; }
+			set {
+				hasDefault = true;
+				@default = value;
+			}
+		}
+
 		public IList<On> Platforms { get; private set; }
 
+#pragma warning disable RECS0108 // Warns about static fields in generic types
 		static readonly IValueConverterProvider s_valueConverter = DependencyService.Get<IValueConverterProvider>();
+#pragma warning restore RECS0108 // Warns about static fields in generic types
 
 		public static implicit operator T(OnPlatform<T> onPlatform)
 		{
@@ -60,7 +72,7 @@ public static implicit operator T(OnPlatform<T> onPlatform)
 			}
 
 			if (!onPlatform.useLegacyFallback)
-				return default(T);
+				return onPlatform.hasDefault ? onPlatform.@default : default(T);
 
 			//legacy fallback
 #pragma warning disable 0618, 0612

Xamarin.Forms.Xaml.UnitTests/OnPlatform.xaml

@@ -37,7 +37,7 @@
 				</OnPlatform>
 			</Label.Text>
 			<Label.HeightRequest>
-				<OnPlatform x:TypeArguments="x:Double">
+				<OnPlatform x:TypeArguments="x:Double" Default="63">
 					<On Platform="iOS">21</On>
 					<On Platform="Android, FooBar">42</On>
 				</OnPlatform>

Xamarin.Forms.Xaml.UnitTests/OnPlatform.xaml.cs

@@ -121,7 +121,15 @@ public void OnPlatform2Syntax(bool useCompiledXaml)
 				((MockPlatformServices)Device.PlatformServices).RuntimePlatform = "FooBar";
 				layout = new OnPlatform(useCompiledXaml);
 				Assert.AreEqual(42, layout.label0.HeightRequest);
+			}
 
+			[TestCase(false)]
+			[TestCase(true)]
+			public void OnPlatformDefault(bool useCompiledXaml)
+			{
+				((MockPlatformServices)Device.PlatformServices).RuntimePlatform = "\ud83d\ude80";
+				var layout = new OnPlatform(useCompiledXaml);
+				Assert.AreEqual(63, layout.label0.HeightRequest);
 			}
 		}
 	}

docs/Xamarin.Forms.Core/Xamarin.Forms/OnPlatform`1.xml

@@ -75,6 +75,22 @@
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
+    <Member MemberName="Default">
+      <MemberSignature Language="C#" Value="public T Default { get; set; }" />
+      <MemberSignature Language="ILAsm" Value=".property instance !T Default" />
+      <MemberType>Property</MemberType>
+      <AssemblyInfo>
+        <AssemblyVersion>2.0.0.0</AssemblyVersion>
+      </AssemblyInfo>
+      <ReturnValue>
+        <ReturnType>T</ReturnType>
+      </ReturnValue>
+      <Docs>
+        <summary>To be added.</summary>
+        <value>To be added.</value>
+        <remarks>To be added.</remarks>
+      </Docs>
+    </Member>
     <Member MemberName="iOS">
       <MemberSignature Language="C#" Value="public T iOS { get; set; }" />
       <MemberSignature Language="ILAsm" Value=".property instance !T iOS" />