Xamarin Forms Xaml

This gist aims to demystify how Xamarin Forms Xaml works.

Here is the call stack executed to render a Xaml File.

1.LoadFromXaml - BindableObject Extension

2.XamlLoader.Load - uses XmlReader to read the xaml file

using (XmlReader reader = XmlReader.Create((TextReader) new StringReader(xaml)))
{
  while (reader.Read())
  {
    if (reader.NodeType != XmlNodeType.Whitespace && reader.NodeType == XmlNodeType.Element)
    {
      XamlLoader.RuntimeRootNode runtimeRootNode = new XamlLoader.RuntimeRootNode(new XmlType(reader.NamespaceURI, reader.Name, (IList<XmlType>) null), (object) view);
      XamlParser.ParseXaml((RootNode) runtimeRootNode, reader);
      HydratationContext context = new HydratationContext()
      {
        RootElement = view
      };
      runtimeRootNode.Accept((IXamlNodeVisitor) new XamlNodeVisitor((Action<INode, INode>) ((node, parent) => node.Parent = parent), false), (INode) null);
      runtimeRootNode.Accept((IXamlNodeVisitor) new NamescopingVisitor(context), (INode) null);
      runtimeRootNode.Accept((IXamlNodeVisitor) new CreateValuesVisitor(context), (INode) null);
      runtimeRootNode.Accept((IXamlNodeVisitor) new RegisterXNamesVisitor(context), (INode) null);
      runtimeRootNode.Accept((IXamlNodeVisitor) new ParseMarkupsVisitor(context), (INode) null);
      runtimeRootNode.Accept((IXamlNodeVisitor) new ApplyPropertiesVisitor(context), (INode) null);
      break;
    }
  }
}

3.XamlParser.ParseXaml

public static void ParseXaml(RootNode rootNode, XmlReader reader)
{
  IList<KeyValuePair<XmlName, INode>> list = XamlParser.ParseXamlAttributes(reader);
  IDictionaryExtensions.AddRange<XmlName, INode>((IDictionary<XmlName, INode>) rootNode.Properties, (IEnumerable<KeyValuePair<XmlName, INode>>) list);
  XamlParser.ParseXamlElementFor((IElementNode) rootNode, reader);
}

4.RootNode.Accept

public override void Accept(IXamlNodeVisitor visitor, INode parentNode)
{
  if (!visitor.VisitChildrenFirst)
    visitor.Visit(this, parentNode);
  foreach (INode node in this.Properties.Values)
    node.Accept(visitor, (INode) this);
  foreach (INode node in this.CollectionItems)
    node.Accept(visitor, (INode) this);
  if (!visitor.VisitChildrenFirst)
    return;
  visitor.Visit(this, parentNode);
}

5.ApplyPropertiesVisitor.Visit

public void Visit(ValueNode node, INode parentNode)
 {
   IElementNode index = parentNode as IElementNode;
   object obj = this.Values[(INode) node];
   object xamlelement = this.Values[parentNode];
   XmlName name;
   if (ApplyPropertiesVisitor.TryGetPropertyName((INode) node, parentNode, out name))
   {
     if (this.skips.Contains(name))
       return;
     ApplyPropertiesVisitor.SetPropertyValue(xamlelement, name, obj, this.Context.RootElement, (INode) node, this.Context, (IXmlLineInfo) node);
   }
   else
   {
     ContentPropertyAttribute customAttribute;
     if (!ApplyPropertiesVisitor.IsCollectionItem((INode) node, parentNode) || !(parentNode is IElementNode) || (customAttribute = CustomAttributeExtensions.GetCustomAttribute<ContentPropertyAttribute>((MemberInfo) IntrospectionExtensions.GetTypeInfo(this.Context.Types[index]))) == null)
       return;
     XmlName propertyName = new XmlName(((ElementNode) parentNode).NamespaceURI, customAttribute.Name);
     if (this.skips.Contains(propertyName))
       return;
     ApplyPropertiesVisitor.SetPropertyValue(xamlelement, propertyName, obj, this.Context.RootElement, (INode) node, this.Context, (IXmlLineInfo) node);
   }
 }

The source comes from decompiling Xamarin.Forms.Xaml and may not be 100% accurate

paulpatarinski/XamFormsXaml.md