[Xaml] allow the Previewer to provide their own Xaml files for any type (#262)

* [Xaml] allow the Previewer to provide their own Xaml files for any type

* [Xaml] use a Func instead of an interface, easier to use by reflection. Add tests

* [XamlC] move the InitializeComponent duplication to XamlC task

* [XamlC] generate branching code

* [XamlC] fix the XamlC issue

* [XamlC] make the API public

* [docs] fix docs

2 files changed, 53 insertions, 11 deletions

diff --git a/Xamarin.Forms.Build.Tasks/DebugXamlCTask.cs b/Xamarin.Forms.Build.Tasks/DebugXamlCTask.cs
index bec6386d..e0f81972 100644
--- a/Xamarin.Forms.Build.Tasks/DebugXamlCTask.cs
+++ b/Xamarin.Forms.Build.Tasks/DebugXamlCTask.cs
@@ -67,18 +67,11 @@ namespace Xamarin.Forms.Build.Tasks
 						LogLine(2, "no InitializeComponent found... skipped.");
 						continue;
 					}
-					if (typeDef.Methods.FirstOrDefault(md => md.Name == "InitCompRuntime") != null)
-					{
-						LogLine(2, "InitCompRuntime already exists... skipped");
+					var initCompRuntime = typeDef.Methods.FirstOrDefault(md => md.Name == "__InitComponentRuntime");
+					if (initCompRuntime == null) {
+						LogLine(2, "no __InitComponentRuntime found... skipped.");
 						continue;
 					}
-					LogLine(2, "");
-
-					LogString(2, "   Duplicating {0}.InitializeComponent () into {0}.InitCompRuntime ... ", typeDef.Name);
-					var initCompRuntime = new MethodDefinition("InitCompRuntime", initComp.Attributes, initComp.ReturnType);
-					initCompRuntime.Body = initComp.Body;
-					typeDef.Methods.Add(initCompRuntime);
-					LogLine(2, "done.");
 
 					//					IL_0000:  ldarg.0 
 					//					IL_0001:  callvirt instance void class [Xamarin.Forms.Core]Xamarin.Forms.ContentPage::'.ctor'()
@@ -92,7 +85,7 @@ namespace Xamarin.Forms.Build.Tasks
 					//					IL_0013:  br IL_001e
 					//
 					//					IL_0018:  ldarg.0 
-					//					IL_0019:  callvirt instance void class Xamarin.Forms.Xaml.XamlcTests.MyPage::InitCompRuntime()
+					//					IL_0019:  callvirt instance void class Xamarin.Forms.Xaml.XamlcTests.MyPage::__InitComponentRuntime()
 					//					IL_001e:  ret 
 
 					var altCtor =
diff --git a/Xamarin.Forms.Build.Tasks/XamlCTask.cs b/Xamarin.Forms.Build.Tasks/XamlCTask.cs
index 19548b80..b6387c0f 100644
--- a/Xamarin.Forms.Build.Tasks/XamlCTask.cs
+++ b/Xamarin.Forms.Build.Tasks/XamlCTask.cs
@@ -232,6 +232,16 @@ namespace Xamarin.Forms.Build.Tasks
 					}
 					LogLine(2, "");
 
+					if (typeDef.Methods.FirstOrDefault(md => md.Name == "__InitComponentRuntime") != null) {
+						LogLine(2, "   __InitComponentRuntime already exists... not duplicating");
+					} else {
+						LogString(2, "   Duplicating {0}.InitializeComponent () into {0}.__InitComponentRuntime ... ", typeDef.Name);
+						var initCompRuntime = new MethodDefinition("__InitComponentRuntime", initComp.Attributes, initComp.ReturnType);
+						initCompRuntime.Body = initComp.Body;
+						typeDef.Methods.Add(initCompRuntime);
+						LogLine(2, "done.");
+					}
+
 					LogString(2, "   Parsing Xaml... ");
 					var rootnode = ParseXaml(resource.GetResourceStream(), typeDef);
 					if (rootnode == null)
@@ -249,6 +259,45 @@ namespace Xamarin.Forms.Build.Tasks
 						var body = new MethodBody(initComp);
 						var il = body.GetILProcessor();
 						il.Emit(OpCodes.Nop);
+
+						// Generating branching code for the Previewer
+						//	IL_0007:  call class [mscorlib]System.Func`2<class [mscorlib]System.Type,string> class [Xamarin.Forms.Xaml.Internals]Xamarin.Forms.Xaml.XamlLoader::get_XamlFileProvider()
+						//  IL_000c:  brfalse IL_0031
+						//  IL_0011:  call class [mscorlib]System.Func`2<class [mscorlib]System.Type,string> class [Xamarin.Forms.Xaml.Internals]Xamarin.Forms.Xaml.XamlLoader::get_XamlFileProvider()
+						//  IL_0016:  ldarg.0 
+						//  IL_0017:  call instance class [mscorlib]System.Type object::GetType()
+						//  IL_001c:  callvirt instance !1 class [mscorlib]System.Func`2<class [mscorlib]System.Type, string>::Invoke(!0)
+						//  IL_0021:  brfalse IL_0031
+						//  IL_0026:  ldarg.0 
+						//  IL_0027:  call instance void class Xamarin.Forms.Xaml.UnitTests.XamlLoaderGetXamlForTypeTests::__InitComponentRuntime()
+						//  IL_002c:  ret
+						//  IL_0031:  nop
+
+						var nop = Instruction.Create(OpCodes.Nop);
+						var getXamlFileProvider = body.Method.Module.Import(body.Method.Module.Import(typeof(Xamarin.Forms.Xaml.Internals.XamlLoader))
+								.Resolve()
+								.Properties.FirstOrDefault(pd => pd.Name == "XamlFileProvider")
+						        .GetMethod);
+						il.Emit(OpCodes.Call, getXamlFileProvider);
+						il.Emit(OpCodes.Brfalse, nop);
+						il.Emit(OpCodes.Call, getXamlFileProvider);
+						il.Emit(OpCodes.Ldarg_0);
+						var getType = body.Method.Module.Import(body.Method.Module.Import(typeof(object))
+										  .Resolve()
+						                  .Methods.FirstOrDefault(md => md.Name == "GetType"));
+						il.Emit(OpCodes.Call, getType);
+						var func = body.Method.Module.Import(body.Method.Module.Import(typeof(Func<Type, string>))
+								 .Resolve()
+								 .Methods.FirstOrDefault(md => md.Name == "Invoke"));
+						func = func.ResolveGenericParameters(body.Method.Module.Import(typeof(Func<Type, string>)), body.Method.Module);
+						il.Emit(OpCodes.Callvirt, func);
+						il.Emit(OpCodes.Brfalse, nop);
+						il.Emit(OpCodes.Ldarg_0);
+						var initCompRuntime = typeDef.Methods.FirstOrDefault(md => md.Name == "__InitComponentRuntime");
+						il.Emit(OpCodes.Call, initCompRuntime);
+						il.Emit(OpCodes.Ret);
+						il.Append(nop);
+
 						var visitorContext = new ILContext(il, body);
 
 						rootnode.Accept(new XamlNodeVisitor((node, parent) => node.Parent = parent), null);