[Xaml[C]] Do not instantiate DataTemplate Content at parsing time (#683)

* [Xaml] rename VisitChildrenFirst

* [Xaml] rework SkipChildren in XamlNode

* [Xaml] fix 45179

* fix

6 files changed, 31 insertions, 79 deletions

diff --git a/Xamarin.Forms.Build.Tasks/CreateObjectVisitor.cs b/Xamarin.Forms.Build.Tasks/CreateObjectVisitor.cs
index ff8f84f9..3e3d1093 100644
--- a/Xamarin.Forms.Build.Tasks/CreateObjectVisitor.cs
+++ b/Xamarin.Forms.Build.Tasks/CreateObjectVisitor.cs
@@ -21,20 +21,11 @@ namespace Xamarin.Forms.Build.Tasks
 
 		ModuleDefinition Module { get; }
 
-		public bool VisitChildrenFirst
-		{
-			get { return true; }
-		}
+		public TreeVisitingMode VisitingMode => TreeVisitingMode.BottomUp;
+		public bool StopOnDataTemplate => true;
+		public bool StopOnResourceDictionary => false;
+		public bool VisitNodeOnDataTemplate => false;
 
-		public bool StopOnDataTemplate
-		{
-			get { return true; }
-		}
-
-		public bool StopOnResourceDictionary
-		{
-			get { return false; }
-		}
 
 		public void Visit(ValueNode node, INode parentNode)
 		{
diff --git a/Xamarin.Forms.Build.Tasks/ExpandMarkupsVisitor.cs b/Xamarin.Forms.Build.Tasks/ExpandMarkupsVisitor.cs
index e647f6c2..ff83de7a 100644
--- a/Xamarin.Forms.Build.Tasks/ExpandMarkupsVisitor.cs
+++ b/Xamarin.Forms.Build.Tasks/ExpandMarkupsVisitor.cs
@@ -24,20 +24,10 @@ namespace Xamarin.Forms.Build.Tasks
 
 		ILContext Context { get; }
 
-		public bool VisitChildrenFirst
-		{
-			get { return true; }
-		}
-
-		public bool StopOnDataTemplate
-		{
-			get { return false; }
-		}
-
-		public bool StopOnResourceDictionary
-		{
-			get { return false; }
-		}
+		public TreeVisitingMode VisitingMode => TreeVisitingMode.BottomUp;
+		public bool StopOnDataTemplate => false;
+		public bool StopOnResourceDictionary => false;
+		public bool VisitNodeOnDataTemplate => true;
 
 		public void Visit(ValueNode node, INode parentNode)
 		{
diff --git a/Xamarin.Forms.Build.Tasks/SetFieldVisitor.cs b/Xamarin.Forms.Build.Tasks/SetFieldVisitor.cs
index 1839cf7b..9a5b7444 100644
--- a/Xamarin.Forms.Build.Tasks/SetFieldVisitor.cs
+++ b/Xamarin.Forms.Build.Tasks/SetFieldVisitor.cs
@@ -13,20 +13,10 @@ namespace Xamarin.Forms.Build.Tasks
 
 		public ILContext Context { get; }
 
-		public bool VisitChildrenFirst
-		{
-			get { return false; }
-		}
-
-		public bool StopOnDataTemplate
-		{
-			get { return true; }
-		}
-
-		public bool StopOnResourceDictionary
-		{
-			get { return false; }
-		}
+		public TreeVisitingMode VisitingMode => TreeVisitingMode.TopDown;
+		public bool StopOnDataTemplate => true;
+		public bool StopOnResourceDictionary => false;
+		public bool VisitNodeOnDataTemplate => false;
 
 		public void Visit(ValueNode node, INode parentNode)
 		{
diff --git a/Xamarin.Forms.Build.Tasks/SetNamescopesAndRegisterNamesVisitor.cs b/Xamarin.Forms.Build.Tasks/SetNamescopesAndRegisterNamesVisitor.cs
index 860ff44e..674b1f35 100644
--- a/Xamarin.Forms.Build.Tasks/SetNamescopesAndRegisterNamesVisitor.cs
+++ b/Xamarin.Forms.Build.Tasks/SetNamescopesAndRegisterNamesVisitor.cs
@@ -16,20 +16,10 @@ namespace Xamarin.Forms.Build.Tasks
 
 		ILContext Context { get; }
 
-		public bool VisitChildrenFirst
-		{
-			get { return false; }
-		}
-
-		public bool StopOnDataTemplate
-		{
-			get { return true; }
-		}
-
-		public bool StopOnResourceDictionary
-		{
-			get { return false; }
-		}
+		public TreeVisitingMode VisitingMode => TreeVisitingMode.TopDown;
+		public bool StopOnDataTemplate => true;
+		public bool StopOnResourceDictionary => false;
+		public bool VisitNodeOnDataTemplate => false;
 
 		public void Visit(ValueNode node, INode parentNode)
 		{
diff --git a/Xamarin.Forms.Build.Tasks/SetPropertiesVisitor.cs b/Xamarin.Forms.Build.Tasks/SetPropertiesVisitor.cs
index 9cf62221..4a8fd70c 100644
--- a/Xamarin.Forms.Build.Tasks/SetPropertiesVisitor.cs
+++ b/Xamarin.Forms.Build.Tasks/SetPropertiesVisitor.cs
@@ -37,8 +37,9 @@ namespace Xamarin.Forms.Build.Tasks
 
 		public ILContext Context { get; }
 		public bool StopOnResourceDictionary { get; }
-		public bool VisitChildrenFirst { get; } = true;
-		public bool StopOnDataTemplate { get; } = true;
+		public TreeVisitingMode VisitingMode => TreeVisitingMode.BottomUp;
+		public bool StopOnDataTemplate => true;
+		public bool VisitNodeOnDataTemplate => true;
 
 		ModuleDefinition Module { get; }
 
@@ -88,6 +89,11 @@ namespace Xamarin.Forms.Build.Tasks
 			if ((propertyName != XmlName.Empty || TryGetPropertyName(node, parentNode, out propertyName)) && skips.Contains(propertyName))
 				return;
 
+			if (propertyName == XmlName._CreateContent) {
+				SetDataTemplate((IElementNode)parentNode, node, Context, node);
+				return;
+			}
+
 			//if this node is an IMarkupExtension, invoke ProvideValue() and replace the variable
 			var vardef = Context.Variables[node];
 			var vardefref = new VariableDefinitionReference(vardef);
@@ -114,11 +120,8 @@ namespace Xamarin.Forms.Build.Tasks
 					return;
 				if (parentNode is IElementNode && ((IElementNode)parentNode).SkipProperties.Contains (propertyName))
 					return;
-
-				if (propertyName == XmlName._CreateContent)
-					SetDataTemplate((IElementNode)parentNode, node, Context, node);
-				else
-					Context.IL.Append(SetPropertyValue(Context.Variables[(IElementNode)parentNode], propertyName, node, Context, node));
+				
+				Context.IL.Append(SetPropertyValue(Context.Variables[(IElementNode)parentNode], propertyName, node, Context, node));
 			}
 			else if (IsCollectionItem(node, parentNode) && parentNode is IElementNode)
 			{
diff --git a/Xamarin.Forms.Build.Tasks/SetResourcesVisitor.cs b/Xamarin.Forms.Build.Tasks/SetResourcesVisitor.cs
index 95a93393..4c2fc05c 100644
--- a/Xamarin.Forms.Build.Tasks/SetResourcesVisitor.cs
+++ b/Xamarin.Forms.Build.Tasks/SetResourcesVisitor.cs
@@ -1,6 +1,5 @@
 using System;
 using System.Collections;
-using System.Collections.Generic;
 using System.Linq;
 using Mono.Cecil;
 using Mono.Cecil.Cil;
@@ -20,26 +19,15 @@ namespace Xamarin.Forms.Build.Tasks
 
 		ModuleDefinition Module { get; }
 
-		public bool VisitChildrenFirst
-		{
-			get { return false; }
-		}
-
-		public bool StopOnDataTemplate
-		{
-			get { return true; }
-		}
-
-		public bool StopOnResourceDictionary
-		{
-			get { return false; }
-		}
+		public TreeVisitingMode VisitingMode => TreeVisitingMode.TopDown;
+		public bool StopOnDataTemplate => true;
+		public bool StopOnResourceDictionary => false;
+		public bool VisitNodeOnDataTemplate => false;
 
 		public void Visit(ValueNode node, INode parentNode)
 		{
 			XmlName propertyName;
-			if (!SetPropertiesVisitor.TryGetPropertyName(node, parentNode, out propertyName))
-			{
+			if (!SetPropertiesVisitor.TryGetPropertyName(node, parentNode, out propertyName)) {
 				if (!IsCollectionItem(node, parentNode))
 					return;
 				string contentProperty;