Allow out of tree platforms to work without custom metro configs

docs/platforms.md

@@ -40,6 +40,7 @@ At the end, a map of available platforms is passed to the bundler (Metro) to mak
 
 ```ts
 type PlatformConfig<ProjectParams, ProjectConfig, DependencyConfig> = {
+  npmPackageName?: string;
   projectConfig: (string, ProjectParams) => ?ProjectConfig,
   dependencyConfig: (string, ProjectParams) => ?DependencyConfig,
   linkConfig: () => {
@@ -57,6 +58,13 @@ type PlatformConfig<ProjectParams, ProjectConfig, DependencyConfig> = {
 };
 ```
 
+### npmPackageName
+
+Returns the name of the npm package that should be used as the source for react-native JS code for platforms that provide platform specific overrides to core JS files.  This causes the default metro config to redirect imports of react-native to another package based when bundling for that platform.  The package specified should provide a complete react-native implementation for that platform.
+
+If this property is not specified, it is assumed that the code in core `react-native` works for the platform.
+
+
 ### projectConfig
 
 Returns a project configuration for a given platform or `null`, when no project found. This is later used inside `linkConfig` to perform linking and unlinking.

packages/cli-types/src/index.ts

@@ -63,6 +63,7 @@ interface PlatformConfig<
   DependencyConfig,
   DependencyParams
 > {
+  npmPackageName?: string;
   projectConfig: (
     projectRoot: string,
     projectParams: ProjectParams | void,

packages/cli/package.json

@@ -53,6 +53,7 @@
     "metro-config": "^0.58.0",
     "metro-core": "^0.58.0",
     "metro-react-native-babel-transformer": "^0.58.0",
+    "metro-resolver": "^0.58.0",
     "minimist": "^1.2.0",
     "mkdirp": "^0.5.1",
     "node-stream-zip": "^1.9.1",

packages/cli/src/tools/config/schema.ts

@@ -83,6 +83,7 @@ export const dependencyConfig = t
     platforms: map(
       t.string(),
       t.object({
+        npmPackageName: t.string().optional(),
         dependencyConfig: t.func(),
         projectConfig: t.func(),
         linkConfig: t.func(),
@@ -178,6 +179,7 @@ export const projectConfig = t
     platforms: map(
       t.string(),
       t.object({
+        npmPackageName: t.string().optional(),
         dependencyConfig: t.func(),
         projectConfig: t.func(),
         linkConfig: t.func(),

packages/cli/src/tools/loadMetroConfig.ts

@@ -5,6 +5,7 @@ import path from 'path';
 // @ts-ignore - no typed definition for the package
 import {loadConfig} from 'metro-config';
 import {Config} from '@react-native-community/cli-types';
+import {reactNativePlatformResolver} from './metroPlatformResolver';
 
 const INTERNAL_CALLSITES_REGEX = new RegExp(
   [
@@ -21,6 +22,12 @@ const INTERNAL_CALLSITES_REGEX = new RegExp(
 
 export interface MetroConfig {
   resolver: {
+    resolveRequest?: (
+      context: any,
+      realModuleName: string,
+      platform: string,
+      moduleName: string,
+    ) => any;
     resolverMainFields: string[];
     platforms: string[];
   };
@@ -48,16 +55,40 @@ export interface MetroConfig {
  * Default configuration
  */
 export const getDefaultConfig = (ctx: Config): MetroConfig => {
+  const outOfTreePlatforms = Object.keys(ctx.platforms).filter(
+    platform => ctx.platforms[platform].npmPackageName,
+  );
+
   return {
     resolver: {
+      resolveRequest:
+        outOfTreePlatforms.length === 0
+          ? undefined
+          : reactNativePlatformResolver(
+              outOfTreePlatforms.reduce<{[platform: string]: string}>(
+                (result, platform) => {
+                  result[platform] = ctx.platforms[platform].npmPackageName!;
+                  return result;
+                },
+                {},
+              ),
+            ),
       resolverMainFields: ['react-native', 'browser', 'main'],
       platforms: [...Object.keys(ctx.platforms), 'native'],
     },
     serializer: {
+      // We can include multiple copies of InitializeCore here because metro will
+      // only add ones that are already part of the bundle
       getModulesRunBeforeMainModule: () => [
         require.resolve(
           path.join(ctx.reactNativePath, 'Libraries/Core/InitializeCore'),
         ),
+        ...outOfTreePlatforms.map(platform =>
+          require.resolve(
+            `${ctx.platforms[platform]
+              .npmPackageName!}/Libraries/Core/InitializeCore`,
+          ),
+        ),
       ],
       getPolyfills: () =>
         require(path.join(ctx.reactNativePath, 'rn-get-polyfills'))(),
@@ -77,10 +108,7 @@ export const getDefaultConfig = (ctx: Config): MetroConfig => {
       babelTransformerPath: require.resolve(
         'metro-react-native-babel-transformer',
       ),
-      assetRegistryPath: path.join(
-        ctx.reactNativePath,
-        'Libraries/Image/AssetRegistry',
-      ),
+      assetRegistryPath: 'react-native/Libraries/Image/AssetRegistry',
     },
     watchFolders: [],
   };

packages/cli/src/tools/metroPlatformResolver.ts

@@ -0,0 +1,49 @@
+/**
+ * This is an implementation of a metro resolveRequest option which will remap react-native imports
+ * to different npm packages based on the platform requested.  This allows a single metro instance/config
+ * to produce bundles for multiple out of tree platforms at a time.
+ *
+ * @param platformImplementations
+ * A map of platform to npm package that implements that platform
+ *
+ * Ex:
+ * {
+ *    windows: 'react-native-windows'
+ *    macos: 'react-native-macos'
+ * }
+ */
+// @ts-ignore - no typed definition for the package
+import {resolve} from 'metro-resolver';
+
+export function reactNativePlatformResolver(platformImplementations: {
+  [platform: string]: string;
+}) {
+  return (
+    context: any,
+    _realModuleName: string,
+    platform: string,
+    moduleName: string,
+  ) => {
+    let backupResolveRequest = context.resolveRequest;
+    delete context.resolveRequest;
+
+    try {
+      let modifiedModuleName = moduleName;
+      if (platformImplementations[platform]) {
+        if (moduleName === 'react-native') {
+          modifiedModuleName = platformImplementations[platform];
+        } else if (moduleName.startsWith('react-native/')) {
+          modifiedModuleName = `${
+            platformImplementations[platform]
+          }/${modifiedModuleName.slice('react-native/'.length)}`;
+        }
+      }
+      let result = resolve(context, modifiedModuleName, platform);
+      return result;
+    } catch (e) {
+      throw e;
+    } finally {
+      context.resolveRequest = backupResolveRequest;
+    }
+  };
+}

yarn.lock

@@ -7752,7 +7752,7 @@ metro-react-native-babel-transformer@^0.58.0:
     metro-react-native-babel-preset "0.58.0"
     metro-source-map "0.58.0"
 
-metro-resolver@0.58.0:
+metro-resolver@0.58.0, metro-resolver@^0.58.0:
   version "0.58.0"
   resolved "https://registry.yarnpkg.com/metro-resolver/-/metro-resolver-0.58.0.tgz#4d03edc52e2e25d45f16688adf3b3f268ea60df9"
   integrity sha512-XFbAKvCHN2iWqKeiRARzEXn69eTDdJVJC7lu16S4dPQJ+Dy82dZBr5Es12iN+NmbJuFgrAuIHbpWrdnA9tOf6Q==