Initialize Xaml on demand (#15407)

* Initialize Xaml if requested by a module
* Drop XamlHost component
  - Exposing this means both app and module authors can use any Xaml
    but we want to enable Xaml hosting only for module authors for 0.82
    and for this we don’t need XamlHost.
* Initialize `XamlApplication`, which loads Xaml framework (costly),
  when there’s at least one component requesting Xaml support through
  `ReactNativeComponentBuilder::XamlSupport`
* Fix ReactWindows-Desktop build break

`//vnect/Shared/Shared.vcxitems` is included by two project:

1. `//vnext/Microsoft.ReactNative/Microsoft.ReactNative.vcxproj`
2. `//vnext/Desktop/React.Windows.Desktop.vcxproj`

(2) is a static library that breaks compiling `ReactNativeHost.cpp` as
it includes `XamlApplication.h` which in turn includes
`Xaml.XamlApplication.g.*` which requires `XamlApplication.idl` to be
generating them.  This won’t happen due to the project type of (2).
Wrap `XamlApplication`-related code in a macro defined only for (1).

Author: sundaramramaswamy

change/react-native-windows-73f0d63e-63d7-4322-b402-83e1c0c092e8.json

@@ -0,0 +1,7 @@
+{
+  "type": "none",
+  "comment": "Initialize Xaml on demand only for module authors",
+  "packageName": "react-native-windows",
+  "email": "[email protected]",
+  "dependentChangeType": "none"
+}

vnext/Microsoft.ReactNative/Fabric/Composition/ReactCompositionViewComponentBuilder.cpp

@@ -227,6 +227,14 @@ void ReactCompositionViewComponentBuilder::SetUnmountChildComponentViewHandler(
   m_unmountChildComponentViewHandler = impl;
 }
 
+bool ReactCompositionViewComponentBuilder::XamlSupport() const noexcept {
+  return m_xamlSupport;
+}
+
+void ReactCompositionViewComponentBuilder::XamlSupport(bool isRequired) noexcept {
+  m_xamlSupport = isRequired;
+}
+
 const UnmountChildComponentViewDelegate &ReactCompositionViewComponentBuilder::UnmountChildComponentViewHandler()
     const noexcept {
   return m_unmountChildComponentViewHandler;

vnext/Microsoft.ReactNative/Fabric/Composition/ReactCompositionViewComponentBuilder.h

@@ -39,6 +39,8 @@ struct ReactCompositionViewComponentBuilder
   void SetUpdateEventEmitterHandler(UpdateEventEmitterDelegate impl) noexcept;
   void SetMountChildComponentViewHandler(MountChildComponentViewDelegate impl) noexcept;
   void SetUnmountChildComponentViewHandler(UnmountChildComponentViewDelegate impl) noexcept;
+  bool XamlSupport() const noexcept;
+  void XamlSupport(bool isRequired) noexcept;
 
  public: // Composition::IReactCompositionViewComponentBuilder
   void SetViewComponentViewInitializer(const ViewComponentViewInitializer &initializer) noexcept;
@@ -108,6 +110,7 @@ struct ReactCompositionViewComponentBuilder
   winrt::Microsoft::ReactNative::Composition::Experimental::IVisualToMountChildrenIntoDelegate
       m_visualToMountChildrenIntoHandler;
   UpdateLayoutMetricsDelegate m_updateLayoutMetricsHandler;
+  bool m_xamlSupport{false};
 };
 
 } // namespace winrt::Microsoft::ReactNative::Composition

vnext/Microsoft.ReactNative/Fabric/WindowsComponentDescriptorRegistry.cpp

@@ -54,6 +54,14 @@ bool WindowsComponentDescriptorRegistry::hasComponentProvider(const std::string
   return std::find(m_componentNames.begin(), m_componentNames.end(), name) != m_componentNames.end();
 }
 
+bool WindowsComponentDescriptorRegistry::isXamlSupportRequired() const noexcept {
+  return std::any_of(m_builderByName.cbegin(), m_builderByName.cend(), [](const auto &pair) -> bool {
+    return winrt::get_self<winrt::Microsoft::ReactNative::Composition::ReactCompositionViewComponentBuilder>(
+               pair.second)
+        ->XamlSupport();
+  });
+}
+
 void WindowsComponentDescriptorRegistry::Add(
     winrt::hstring componentName,
     winrt::Microsoft::ReactNative::ReactViewComponentProvider const &provider) noexcept {

vnext/Microsoft.ReactNative/Fabric/WindowsComponentDescriptorRegistry.h

@@ -34,6 +34,9 @@ struct WindowsComponentDescriptorRegistry {
 
   bool hasComponentProvider(const std::string &name) noexcept;
 
+  // Returns true if any component requested for XAML support.
+  bool isXamlSupportRequired() const noexcept;
+
  private:
   void add(const facebook::react::ComponentDescriptorProvider &provider) noexcept;
 

vnext/Microsoft.ReactNative/IReactViewComponentBuilder.idl

@@ -124,6 +124,7 @@ namespace Microsoft.ReactNative
     void SetUpdateEventEmitterHandler(UpdateEventEmitterDelegate impl);
     void SetMountChildComponentViewHandler(MountChildComponentViewDelegate impl);
     void SetUnmountChildComponentViewHandler(UnmountChildComponentViewDelegate impl);
+    Boolean XamlSupport { get; set; };
   };
 
   // [exclusiveto(ShadowNode)]

vnext/Microsoft.ReactNative/IXamlProvider.idl

@@ -124,6 +124,7 @@
         DISABLE_XAML_GENERATED_MAIN;
         REACTWINDOWS_BUILD;
         RN_PLATFORM=windows;
+        RNW_XAML_ISLAND;
         NOMINMAX;
         FOLLY_CFG_NO_COROUTINES;
         FOLLY_NO_CONFIG;

vnext/Microsoft.ReactNative/Microsoft.ReactNative.vcxproj

@@ -13,6 +13,9 @@
 #include <winrt/Windows.Foundation.Collections.h>
 #include "IReactContext.h"
 #include "ReactInstanceSettings.h"
+#ifdef RNW_XAML_ISLAND
+#include "XamlApplication.h"
+#endif // RNW_XAML_ISLAND
 
 #include <Fabric/Composition/Modal/WindowsModalHostViewComponentView.h>
 #include <Fabric/WindowsComponentDescriptorRegistry.h>
@@ -102,6 +105,12 @@ IAsyncAction ReactNativeHost::ReloadInstance() noexcept {
     }
   }
 
+#ifdef RNW_XAML_ISLAND
+  if (componentregistry->isXamlSupportRequired()) {
+    winrt::Microsoft::ReactNative::Xaml::implementation::XamlApplication::EnsureCreated();
+  }
+#endif // RNW_XAML_ISLAND
+
   ReactPropertyBag(m_instanceSettings.Properties()).Set(ReactNativeHostProperty(), get_weak());
 
   Mso::React::ReactOptions reactOptions{};