feat: install __r in runtime only if it wasn't yet provided

packages/metro-runtime/src/polyfills/__tests__/require-test.js

@@ -100,6 +100,7 @@ describe('require', () => {
     createModuleSystem(moduleSystem, false, '');
     expect(moduleSystem.__r).not.toBeUndefined();
     expect(moduleSystem.__d).not.toBeUndefined();
+    expect(moduleSystem.__e).toBeUndefined();
 
     const mockExports = {foo: 'bar'};
     const mockFactory = jest
@@ -128,6 +129,26 @@ describe('require', () => {
     expect(mockFactory.mock.calls[0][6]).toEqual([2, 3]);
   });
 
+  test('exposes metroRequire as __e when runtime owner has already installed __r', () => {
+    const runtimeOwnerRequire = jest.fn();
+    moduleSystem.__r = runtimeOwnerRequire;
+
+    createModuleSystem(moduleSystem, false, '');
+
+    expect(moduleSystem.__r).toBe(runtimeOwnerRequire);
+    expect(moduleSystem.__e).not.toBeUndefined();
+    expect(moduleSystem.__e).not.toBe(runtimeOwnerRequire);
+  });
+
+  test('does not overwrite __r if already provided by the runtime owner', () => {
+    const runtimeOwnerRequire = jest.fn();
+    moduleSystem.__r = runtimeOwnerRequire;
+
+    createModuleSystem(moduleSystem, false, '');
+
+    expect(moduleSystem.__r).toBe(runtimeOwnerRequire);
+  });
+
   test('properly prefixes __d with global prefix', () => {
     createModuleSystem(moduleSystem, false, '__metro');
     expect(moduleSystem.__d).toBeUndefined();

packages/metro-runtime/src/polyfills/require.js

@@ -80,7 +80,15 @@ export type DefineFn = (
 type VerboseModuleNameForDev = string;
 type ModuleDefiner = (moduleId: ModuleID) => void;
 
-global.__r = metroRequire as RequireFn;
+if (!global.__r) {
+  // Entry point require function.
+  // Install only if the runtime owner hasn't already provided one.
+  global.__r = metroRequire as RequireFn;
+} else {
+  // Install it as a separate binding.
+  // The runtime owner might e.g. use it to implement his own __r wrapper.
+  global.__e = metroRequire as RequireFn;
+}
 global[`${__METRO_GLOBAL_PREFIX__}__d`] = define as DefineFn;
 global.__c = clear;
 global.__registerSegment = registerSegment;