Fix init-windows template to automatically detect existing codegen spec files (#15096)

* Initial plan

* Fix init-windows template to use conditional codegen includes

Co-authored-by: anupriya13 <[email protected]>

* Automatically detect existing codegen spec files instead of using hardcoded names

Co-authored-by: anupriya13 <[email protected]>

* Add explanatory comments about Mustache template syntax for C++ developers

Co-authored-by: sundaramramaswamy <[email protected]>

* Change files

* Run yarn change prerelease and yarn lint:fix

Co-authored-by: anupriya13 <[email protected]>

* Revert unrelated changes to FlatList-multiColumn.windows.js

Co-authored-by: anupriya13 <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: anupriya13 <[email protected]>
Co-authored-by: sundaramramaswamy <[email protected]>
Co-authored-by: Sundaram Ramaswamy <[email protected]>

Author: 3 people

change/react-native-windows-d6398257-a0a3-43a6-a9df-7d9f9a41727d.json

@@ -0,0 +1,7 @@
+{
+  "comment": "Fix init-windows template to automatically detect existing codegen spec files",
+  "type": "prerelease",
+  "packageName": "react-native-windows",
+  "email": "[email protected]",
+  "dependentChangeType": "patch"
+}

vnext/templates/cpp-lib/template.config.js

@@ -93,6 +93,24 @@ async function getFileMappings(config = {}, options = {}) {
       .replace('}', '') ?? crypto.randomUUID();
   const currentUser = username.sync(); // Gets the current username depending on the platform.
 
+  // Check for existing codegen spec files
+  const codegenPath = path.join(projectRoot, 'windows', projectName, 'codegen');
+  let existingSpecFiles = [];
+  let firstSpecName = null;
+  if (existsSync(codegenPath)) {
+    try {
+      const specFiles = await glob('*Spec.g.h', {cwd: codegenPath});
+      existingSpecFiles = specFiles;
+      if (specFiles.length > 0) {
+        // Extract the spec name from filename (e.g., "NativeMyModuleSpec.g.h" -> "MyModuleSpec")
+        const firstFile = specFiles[0];
+        firstSpecName = firstFile.replace(/^Native/, '').replace(/\.g\.h$/, '');
+      }
+    } catch (e) {
+      // If we can't read the codegen directory, continue with empty array
+    }
+  }
+
   const cppNugetPackages = [];
 
   const replacements = {
@@ -104,6 +122,12 @@ async function getFileMappings(config = {}, options = {}) {
     namespace: namespace,
     namespaceCpp: namespaceCpp,
 
+    // Codegen spec files information
+    existingSpecFiles: existingSpecFiles,
+    hasExistingSpecFiles: existingSpecFiles.length > 0,
+    firstSpecFile: existingSpecFiles.length > 0 ? existingSpecFiles[0] : null,
+    firstSpecName: firstSpecName,
+
     rnwVersion: rnwVersion,
     rnwPathFromProjectRoot: path
       .relative(projectRoot, rnwPath)

vnext/templates/cpp-lib/windows/MyLib/MyLib.h

@@ -6,7 +6,17 @@
 #if __has_include("codegen/Native{{ pascalName }}DataTypes.g.h")
   #include "codegen/Native{{ pascalName }}DataTypes.g.h"
 #endif
-#include "codegen/Native{{ pascalName }}Spec.g.h"
+// Note: The following lines use Mustache template syntax which will be processed during
+// project generation to produce standard C++ code. If existing codegen spec files are found,
+// use the actual filename; otherwise use conditional includes.
+{{#hasExistingSpecFiles}}
+#include "codegen/{{ firstSpecFile }}"
+{{/hasExistingSpecFiles}}
+{{^hasExistingSpecFiles}}
+#if __has_include("codegen/Native{{ pascalName }}Spec.g.h")
+  #include "codegen/Native{{ pascalName }}Spec.g.h"
+#endif
+{{/hasExistingSpecFiles}}
 
 #include "NativeModules.h"
 
@@ -18,7 +28,16 @@ namespace winrt::{{ namespaceCpp }}
 REACT_MODULE({{ pascalName }})
 struct {{ pascalName }}
 {
+  // Note: Mustache template syntax below will be processed during project generation
+  // to produce standard C++ code based on detected codegen files.
+{{#hasExistingSpecFiles}}
+  using ModuleSpec = {{ namespaceCpp }}Codegen::{{ firstSpecName }};
+{{/hasExistingSpecFiles}}
+{{^hasExistingSpecFiles}}
+#if __has_include("codegen/Native{{ pascalName }}Spec.g.h")
   using ModuleSpec = {{ namespaceCpp }}Codegen::{{ pascalName }}Spec;
+#endif
+{{/hasExistingSpecFiles}}
 
   REACT_INIT(Initialize)
   void Initialize(React::ReactContext const &reactContext) noexcept;