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

* [Xaml] rename VisitChildrenFirst

* [Xaml] rework SkipChildren in XamlNode

* [Xaml] fix 45179

* fix

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)
 		{

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)
 		{

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)
 		{

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)
 		{

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)
 			{

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;

Xamarin.Forms.Xaml.UnitTests/Issues/Bz27863.xaml.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using Xamarin.Forms;
 
 using NUnit.Framework;
+using Xamarin.Forms.Core.UnitTests;
 
 namespace Xamarin.Forms.Xaml.UnitTests
 {
@@ -22,6 +23,18 @@ namespace Xamarin.Forms.Xaml.UnitTests
 		[TestFixture]
 		class Tests
 		{
+			[SetUp]
+			public void Setup()
+			{
+				Device.PlatformServices = new MockPlatformServices();
+			}
+
+			[TearDown]
+			public void TearDown()
+			{
+				Device.PlatformServices = null;
+			}
+
 			[TestCase(true)]
 			[TestCase(false)]
 			public void DataTemplateInResourceDictionaries (bool useCompiledXaml)

Xamarin.Forms.Xaml.UnitTests/Issues/Bz27968.xaml.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using Xamarin.Forms;
 
 using NUnit.Framework;
+using Xamarin.Forms.Core.UnitTests;
 
 namespace Xamarin.Forms.Xaml.UnitTests
 {
@@ -26,6 +27,18 @@ namespace Xamarin.Forms.Xaml.UnitTests
 		[TestFixture]
 		class Tests
 		{
+			[SetUp]
+			public void Setup()
+			{
+				Device.PlatformServices = new MockPlatformServices();
+			}
+
+			[TearDown]
+			public void TearDown()
+			{
+				Device.PlatformServices = null;
+			}
+
 			[TestCase(true)]
 			[TestCase(false)]
 			public void BaseClassIdentifiersAreValidForResources (bool useCompiledXaml)

Xamarin.Forms.Xaml.UnitTests/Issues/Bz45179.xaml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage 
+	xmlns="http://xamarin.com/schemas/2014/forms"
+	xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+	xmlns:local="clr-namespace:Xamarin.Forms.Xaml.UnitTests"
+	x:Class="Xamarin.Forms.Xaml.UnitTests.Bz45179">
+	<ContentPage.Resources>
+		<ResourceDictionary>
+			<DataTemplate x:Key="dt0">
+				<local:Bz45179_0/>
+			</DataTemplate>
+		</ResourceDictionary>
+	</ContentPage.Resources>
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Bz45179.xaml.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Collections.Generic;
+using NUnit.Framework;
+using Xamarin.Forms;
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	public class Bz45179_0 : ContentView {
+		public static int creator_count;
+		public Bz45179_0()
+		{
+			creator_count++;
+		}
+
+	}
+	public partial class Bz45179 : ContentPage
+	{
+		public Bz45179()
+		{
+			InitializeComponent();
+		}
+
+		public Bz45179(bool useCompiledXaml)
+		{
+			//this stub will be replaced at compile time
+		}
+
+		[TestFixture]
+		class Tests
+		{
+			[SetUp]
+			public void Setup()
+			{
+				Device.PlatformServices = new MockPlatformServices();
+			}
+
+			[TearDown]
+			public void TearDown()
+			{
+				Device.PlatformServices = null;
+			}
+
+			[TestCase(true)]
+			[TestCase(false)]
+			public void DTDoNotInstantiateTheirContent(bool useCompiledXaml)
+			{
+				Bz45179_0.creator_count = 0;
+				Assume.That(Bz45179_0.creator_count, Is.EqualTo(0));
+				var page = new Bz45179(useCompiledXaml);
+				Assert.That(Bz45179_0.creator_count, Is.EqualTo(0));
+			}
+		}
+	}
+}

Xamarin.Forms.Xaml.UnitTests/Xamarin.Forms.Xaml.UnitTests.csproj

@@ -406,6 +406,9 @@
     <Compile Include="Issues\Unreported008.xaml.cs">
       <DependentUpon>Unreported008.xaml</DependentUpon>
     </Compile>
+    <Compile Include="Issues\Bz45179.xaml.cs">
+      <DependentUpon>Bz45179.xaml</DependentUpon>
+    </Compile>
   </ItemGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
   <Import Project="..\.nuspec\Xamarin.Forms.Debug.targets" />
@@ -733,6 +736,9 @@
     <EmbeddedResource Include="Issues\Unreported008.xaml">
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
+    <EmbeddedResource Include="Issues\Bz45179.xaml">
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+    </EmbeddedResource>
   </ItemGroup>
   <ItemGroup>
     <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />

Xamarin.Forms.Xaml/ApplyPropertiesVisitor.cs

@@ -35,15 +35,10 @@ namespace Xamarin.Forms.Xaml
 
 		HydratationContext Context { get; }
 
-		public bool VisitChildrenFirst {
-			get { return true; }
-		}
-
-		public bool StopOnDataTemplate {
-			get { return true; }
-		}
-
+		public TreeVisitingMode VisitingMode => TreeVisitingMode.BottomUp;
+		public bool StopOnDataTemplate => true;
 		public bool StopOnResourceDictionary { get; }
+		public bool VisitNodeOnDataTemplate => true;
 
 		public void Visit(ValueNode node, INode parentNode)
 		{
@@ -81,6 +76,15 @@ namespace Xamarin.Forms.Xaml
 
 		public void Visit(ElementNode node, INode parentNode)
 		{
+			var propertyName = XmlName.Empty;
+			if (TryGetPropertyName(node, parentNode, out propertyName) && propertyName == XmlName._CreateContent){
+				var s0 = Values[parentNode];
+				if (s0 is ElementTemplate) {
+					SetTemplate(s0 as ElementTemplate, node);
+					return;
+				}
+			}
+
 			var value = Values [node];
 			var parentElement = parentNode as IElementNode;
 			var markupExtension = value as IMarkupExtension;
@@ -96,7 +100,7 @@ namespace Xamarin.Forms.Xaml
 				value = valueProvider.ProvideValue(serviceProvider);
 			}
 
-			XmlName propertyName = XmlName.Empty;
+			propertyName = XmlName.Empty;
 
 			//Simplify ListNodes with single elements
 			var pList = parentNode as ListNode;
@@ -113,11 +117,7 @@ namespace Xamarin.Forms.Xaml
 					return;
 
 				var source = Values [parentNode];
-
-				if (propertyName == XmlName._CreateContent && source is ElementTemplate)
-					SetTemplate(source as ElementTemplate, node);
-				else
-					SetPropertyValue(source, propertyName, value, Context.RootElement, node, Context, node);
+				SetPropertyValue(source, propertyName, value, Context.RootElement, node, Context, node);
 			} else if (IsCollectionItem(node, parentNode) && parentNode is IElementNode) {
 				// Collection element, implicit content, or implicit collection element.
 				string contentProperty;

Xamarin.Forms.Xaml/CreateValuesVisitor.cs

@@ -23,20 +23,10 @@ namespace Xamarin.Forms.Xaml
 
 		HydratationContext Context { get; }
 
-		public bool VisitChildrenFirst
-		{
-			get { return true; }
-		}
-
-		public bool StopOnDataTemplate
-		{
-			get { return true; }
-		}
-
-		public bool StopOnResourceDictionary
-		{
-			get { return false; }
-		}
+		public TreeVisitingMode VisitingMode => TreeVisitingMode.BottomUp;
+		public bool StopOnDataTemplate => true;
+		public bool StopOnResourceDictionary => false;
+		public bool VisitNodeOnDataTemplate => false;
 
 		public void Visit(ValueNode node, INode parentNode)
 		{

Xamarin.Forms.Xaml/ExpandMarkupsVisitor.cs

@@ -28,20 +28,10 @@ namespace Xamarin.Forms.Xaml
 
 		HydratationContext 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)
 		{

Xamarin.Forms.Xaml/FillResourceDictionariesVisitor.cs

@@ -20,20 +20,10 @@ namespace Xamarin.Forms.Xaml
 			get { return Context.Values; }
 		}
 
-		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)
 		{

Xamarin.Forms.Xaml/MarkupExtensions/StaticResourceExtension.cs

@@ -34,7 +34,7 @@ namespace Xamarin.Forms.Xaml
 			}
 			if (resource == null && (Application.Current == null || Application.Current.Resources == null ||
 									 !Application.Current.Resources.TryGetMergedValue(Key, out resource)))
-				throw new XamlParseException($"StaticResource not found for key {Key}", xmlLineInfo);
+					throw new XamlParseException($"StaticResource not found for key {Key}", xmlLineInfo);
 
 			var bp = valueProvider.TargetProperty as BindableProperty;
 			var pi = valueProvider.TargetProperty as PropertyInfo;

Xamarin.Forms.Xaml/NamescopingVisitor.cs

@@ -14,20 +14,10 @@ namespace Xamarin.Forms.Xaml
 
 		Dictionary<INode, object> Values { get; set; }
 
-		public bool VisitChildrenFirst
-		{
-			get { return false; }
-		}
-
-		public bool StopOnDataTemplate
-		{
-			get { return false; }
-		}
-
-		public bool StopOnResourceDictionary
-		{
-			get { return false; }
-		}
+		public TreeVisitingMode VisitingMode => TreeVisitingMode.TopDown;
+		public bool StopOnDataTemplate => false;
+		public bool StopOnResourceDictionary => false;
+		public bool VisitNodeOnDataTemplate => true;
 
 		public void Visit(ValueNode node, INode parentNode)
 		{

Xamarin.Forms.Xaml/PruneIgnoredNodesVisitor.cs

@@ -5,17 +5,10 @@ namespace Xamarin.Forms.Xaml
 {
 	class PruneIgnoredNodesVisitor : IXamlNodeVisitor
 	{
-		public bool StopOnDataTemplate {
-			get { return false; }
-		}
-
-		public bool StopOnResourceDictionary {
-			get { return false; }
-		}
-
-		public bool VisitChildrenFirst {
-			get { return false; }
-		}
+		public TreeVisitingMode VisitingMode => TreeVisitingMode.TopDown;
+		public bool StopOnDataTemplate => false;
+		public bool StopOnResourceDictionary => false;
+		public bool VisitNodeOnDataTemplate => true;
 
 		public void Visit(ElementNode node, INode parentNode)
 		{

Xamarin.Forms.Xaml/RegisterXNamesVisitor.cs

@@ -12,20 +12,10 @@ namespace Xamarin.Forms.Xaml
 
 		Dictionary<INode, object> Values { 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)
 		{

Xamarin.Forms.Xaml/XamlNode.cs

@@ -1,4 +1,3 @@
-using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -7,36 +6,30 @@ using Xamarin.Forms.Internals;
 
 namespace Xamarin.Forms.Xaml
 {
-	internal interface INode
+	interface INode
 	{
 		List<string> IgnorablePrefixes { get; set; }
-
 		IXmlNamespaceResolver NamespaceResolver { get; }
-
 		INode Parent { get; set; }
 
 		void Accept(IXamlNodeVisitor visitor, INode parentNode);
 		INode Clone();
 	}
 
-	internal interface IValueNode : INode
+	interface IValueNode : INode
 	{
 	}
 
-	internal interface IElementNode : INode, IListNode
+	interface IElementNode : INode, IListNode
 	{
 		Dictionary<XmlName, INode> Properties { get; }
-
 		List<XmlName> SkipProperties { get; }
-
 		INameScope Namescope { get; }
-
 		XmlType XmlType { get; }
-
 		string NamespaceURI { get; }
 	}
 
-	internal interface IListNode : INode
+	interface IListNode : INode
 	{
 		List<INode> CollectionItems { get; }
 	}
@@ -56,7 +49,7 @@ namespace Xamarin.Forms.Xaml
 		public IList<XmlType> TypeArguments { get; }
 	}
 
-	internal abstract class BaseNode : IXmlLineInfo, INode
+	abstract class BaseNode : IXmlLineInfo, INode
 	{
 		protected BaseNode(IXmlNamespaceResolver namespaceResolver, int linenumber = -1, int lineposition = -1)
 		{
@@ -66,27 +59,19 @@ namespace Xamarin.Forms.Xaml
 		}
 
 		public IXmlNamespaceResolver NamespaceResolver { get; }
-
-		public abstract void Accept(IXamlNodeVisitor visitor, INode parentNode);
-
 		public INode Parent { get; set; }
-
 		public List<string> IgnorablePrefixes { get; set; }
-
-		public bool HasLineInfo()
-		{
-			return LineNumber >= 0 && LinePosition >= 0;
-		}
-
 		public int LineNumber { get; set; }
-
 		public int LinePosition { get; set; }
 
+		public bool HasLineInfo() => LineNumber >= 0 && LinePosition >= 0;
+
+		public abstract void Accept(IXamlNodeVisitor visitor, INode parentNode);
 		public abstract INode Clone();
 	}
 
 	[DebuggerDisplay("{Value}")]
-	internal class ValueNode : BaseNode, IValueNode
+	class ValueNode : BaseNode, IValueNode
 	{
 		public ValueNode(object value, IXmlNamespaceResolver namespaceResolver, int linenumber = -1, int lineposition = -1)
 			: base(namespaceResolver, linenumber, lineposition)
@@ -101,19 +86,15 @@ namespace Xamarin.Forms.Xaml
 			visitor.Visit(this, parentNode);
 		}
 
-		public override INode Clone()
-		{
-			return new ValueNode(Value, NamespaceResolver, LineNumber, LinePosition) {
-				IgnorablePrefixes = IgnorablePrefixes
-			};
-		}
+		public override INode Clone() => new ValueNode(Value, NamespaceResolver, LineNumber, LinePosition) {
+			IgnorablePrefixes = IgnorablePrefixes
+		};
 	}
 
 	[DebuggerDisplay("{MarkupString}")]
-	internal class MarkupNode : BaseNode, IValueNode
+	class MarkupNode : BaseNode, IValueNode
 	{
-		public MarkupNode(string markupString, IXmlNamespaceResolver namespaceResolver, int linenumber = -1,
-			int lineposition = -1)
+		public MarkupNode(string markupString, IXmlNamespaceResolver namespaceResolver, int linenumber = -1, int lineposition = -1)
 			: base(namespaceResolver, linenumber, lineposition)
 		{
 			MarkupString = markupString;
@@ -126,15 +107,13 @@ namespace Xamarin.Forms.Xaml
 			visitor.Visit(this, parentNode);
 		}
 
-		public override INode Clone()
-		{
-			return new MarkupNode(MarkupString, NamespaceResolver, LineNumber, LinePosition) {
-				IgnorablePrefixes = IgnorablePrefixes
-			};
-		}
+		public override INode Clone() => new MarkupNode(MarkupString, NamespaceResolver, LineNumber, LinePosition) {
+			IgnorablePrefixes = IgnorablePrefixes
+		};
 	}
 
-	internal class ElementNode : BaseNode, IValueNode, IElementNode
+	[DebuggerDisplay("{XmlType.Name}")]
+	class ElementNode : BaseNode, IValueNode, IElementNode
 	{
 		public ElementNode(XmlType type, string namespaceURI, IXmlNamespaceResolver namespaceResolver, int linenumber = -1,
 			int lineposition = -1)
@@ -148,48 +127,48 @@ namespace Xamarin.Forms.Xaml
 		}
 
 		public Dictionary<XmlName, INode> Properties { get; }
-
 		public List<XmlName> SkipProperties { get; }
-
 		public List<INode> CollectionItems { get; }
-
 		public XmlType XmlType { get; }
-
 		public string NamespaceURI { get; }
-
 		public INameScope Namescope { get; set; }
 
 		public override void Accept(IXamlNodeVisitor visitor, INode parentNode)
 		{
-			if (!visitor.VisitChildrenFirst)
+			if (!SkipVisitNode(visitor, parentNode) && visitor.VisitingMode == TreeVisitingMode.TopDown)
 				visitor.Visit(this, parentNode);
-			if ((!visitor.StopOnDataTemplate || !IsDataTemplate(this, parentNode)) &&
-			    (!visitor.StopOnResourceDictionary || !IsResourceDictionary(this, parentNode)))
-			{
+
+			if (!SkipChildren(visitor, parentNode)) {
 				foreach (var node in Properties.Values.ToList())
 					node.Accept(visitor, this);
 				foreach (var node in CollectionItems)
 					node.Accept(visitor, this);
 			}
-			if (visitor.VisitChildrenFirst)
+
+			if (!SkipVisitNode(visitor, parentNode) && visitor.VisitingMode == TreeVisitingMode.BottomUp)
 				visitor.Visit(this, parentNode);
+
 		}
 
-		internal static bool IsDataTemplate(INode node, INode parentNode)
+		bool IsDataTemplate(INode parentNode)
 		{
 			var parentElement = parentNode as IElementNode;
 			INode createContent;
-			if (parentElement != null && parentElement.Properties.TryGetValue(XmlName._CreateContent, out createContent) &&
-			    createContent == node)
+			if (parentElement != null &&
+				parentElement.Properties.TryGetValue(XmlName._CreateContent, out createContent) &&
+				createContent == this)
 				return true;
 			return false;
 		}
 
-		internal static bool IsResourceDictionary(INode node, INode parentNode)
-		{
-			var enode = node as ElementNode;
-			return enode.XmlType.Name == "ResourceDictionary";
-		}
+		bool IsResourceDictionary() => XmlType.Name == "ResourceDictionary";
+
+		protected bool SkipChildren(IXamlNodeVisitor visitor, INode parentNode) =>
+			(visitor.StopOnDataTemplate && IsDataTemplate(parentNode)) ||
+			(visitor.StopOnResourceDictionary && IsResourceDictionary());
+
+		protected bool SkipVisitNode(IXamlNodeVisitor visitor, INode parentNode) =>
+			!visitor.VisitNodeOnDataTemplate && IsDataTemplate(parentNode);
 
 		public override INode Clone()
 		{
@@ -206,7 +185,7 @@ namespace Xamarin.Forms.Xaml
 		}
 	}
 
-	internal abstract class RootNode : ElementNode
+	abstract class RootNode : ElementNode
 	{
 		protected RootNode(XmlType xmlType, IXmlNamespaceResolver nsResolver) : base(xmlType, xmlType.NamespaceUri, nsResolver)
 		{
@@ -214,40 +193,39 @@ namespace Xamarin.Forms.Xaml
 
 		public override void Accept(IXamlNodeVisitor visitor, INode parentNode)
 		{
-			if (!visitor.VisitChildrenFirst)
+			if (!SkipVisitNode(visitor, parentNode) && visitor.VisitingMode == TreeVisitingMode.TopDown)
 				visitor.Visit(this, parentNode);
-			if ((!visitor.StopOnDataTemplate || !IsDataTemplate(this, parentNode)) &&
-				(!visitor.StopOnResourceDictionary || !IsResourceDictionary(this, parentNode)))
-			{
+
+			if (!SkipChildren(visitor, parentNode)) {
 				foreach (var node in Properties.Values.ToList())
 					node.Accept(visitor, this);
 				foreach (var node in CollectionItems)
 					node.Accept(visitor, this);
 			}
-			if (visitor.VisitChildrenFirst)
+
+			if (!SkipVisitNode(visitor, parentNode) && visitor.VisitingMode == TreeVisitingMode.BottomUp)
 				visitor.Visit(this, parentNode);
 		}
 	}
 
-	internal class ListNode : BaseNode, IListNode, IValueNode
+	class ListNode : BaseNode, IListNode, IValueNode
 	{
-		public ListNode(IList<INode> nodes, IXmlNamespaceResolver namespaceResolver, int linenumber = -1,
-			int lineposition = -1) : base(namespaceResolver, linenumber, lineposition)
+		public ListNode(IList<INode> nodes, IXmlNamespaceResolver namespaceResolver, int linenumber = -1, int lineposition = -1)
+			: base(namespaceResolver, linenumber, lineposition)
 		{
 			CollectionItems = nodes.ToList();
 		}
 
 		public XmlName XmlName { get; set; }
-
 		public List<INode> CollectionItems { get; set; }
 
 		public override void Accept(IXamlNodeVisitor visitor, INode parentNode)
 		{
-			if (!visitor.VisitChildrenFirst)
+			if (visitor.VisitingMode == TreeVisitingMode.TopDown)
 				visitor.Visit(this, parentNode);
 			foreach (var node in CollectionItems)
 				node.Accept(visitor, this);
-			if (visitor.VisitChildrenFirst)
+			if (visitor.VisitingMode == TreeVisitingMode.BottomUp)
 				visitor.Visit(this, parentNode);
 		}
 
@@ -262,12 +240,11 @@ namespace Xamarin.Forms.Xaml
 		}
 	}
 
-	internal static class INodeExtensions
+	static class INodeExtensions
 	{
 		public static bool SkipPrefix(this INode node, string prefix)
 		{
-			do
-			{
+			do {
 				if (node.IgnorablePrefixes != null && node.IgnorablePrefixes.Contains(prefix))
 					return true;
 				node = node.Parent;

Xamarin.Forms.Xaml/XamlNodeVisitor.cs

@@ -2,12 +2,11 @@
 
 namespace Xamarin.Forms.Xaml
 {
-	internal interface IXamlNodeVisitor
+	interface IXamlNodeVisitor
 	{
-		bool VisitChildrenFirst { get; }
-
+		TreeVisitingMode VisitingMode { get; }
 		bool StopOnDataTemplate { get; }
-
+		bool VisitNodeOnDataTemplate { get; }
 		bool StopOnResourceDictionary { get; }
 
 		void Visit(ValueNode node, INode parentNode);
@@ -17,22 +16,27 @@ namespace Xamarin.Forms.Xaml
 		void Visit(ListNode node, INode parentNode);
 	}
 
-	internal class XamlNodeVisitor : IXamlNodeVisitor
+	enum TreeVisitingMode {
+		TopDown,
+		BottomUp
+	}
+
+	class XamlNodeVisitor : IXamlNodeVisitor
 	{
 		readonly Action<INode, INode> action;
 
-		public XamlNodeVisitor(Action<INode, INode> action, bool visitChildrenFirst = false, bool stopOnDataTemplate = false)
+		public XamlNodeVisitor(Action<INode, INode> action, TreeVisitingMode visitingMode = TreeVisitingMode.TopDown, bool stopOnDataTemplate = false, bool visitNodeOnDataTemplate = true)
 		{
 			this.action = action;
-			VisitChildrenFirst = visitChildrenFirst;
+			VisitingMode = visitingMode;
 			StopOnDataTemplate = stopOnDataTemplate;
+			VisitNodeOnDataTemplate = visitNodeOnDataTemplate;
 		}
 
-		public bool VisitChildrenFirst { get; }
-
+		public TreeVisitingMode VisitingMode { get; }
 		public bool StopOnDataTemplate { get; }
-
 		public bool StopOnResourceDictionary { get; private set; }
+		public bool VisitNodeOnDataTemplate { get; }
 
 		public void Visit(ValueNode node, INode parentNode)
 		{