alxhub · Jun 30, 2022 · alfaproject · Jul 1, 2022 · AndrewKushnir · Jun 30, 2022
diff --git a/goldens/public-api/core/index.md b/goldens/public-api/core/index.md
@@ -457,6 +457,7 @@ export abstract class EnvironmentInjector implements Injector {
     abstract get<T>(token: ProviderToken<T>, notFoundValue?: T, flags?: InjectFlags): T;
     // @deprecated (undocumented)
     abstract get(token: any, notFoundValue?: any): any;
+    abstract runInContext<ReturnT>(fn: () => ReturnT): ReturnT;
 }
 
 // @public

diff --git a/packages/core/src/di/r3_injector.ts b/packages/core/src/di/r3_injector.ts
@@ -89,6 +89,18 @@ export abstract class EnvironmentInjector implements Injector {
    */
   abstract get(token: any, notFoundValue?: any): any;
 
+  /**
+   * Runs the given function in the context of this `EnvironmentInjector`.
+   *
+   * Within the function's stack frame, `inject` can be used to inject dependencies from this
+   * injector. Note that `inject` is only usable synchronously, and cannot be used in any
+   * asynchronous callbacks or after any `await` points.
+   *
+   * @param fn the closure to be run in the context of this injector
+   * @returns the return value of the function, if any
+   */
+  abstract runInContext<ReturnT>(fn: () => ReturnT): ReturnT;
+
   abstract destroy(): void;
 
   /**
@@ -180,6 +192,19 @@ export class R3Injector extends EnvironmentInjector {
     this._onDestroyHooks.push(callback);
   }
 
+  override runInContext<ReturnT>(fn: () => ReturnT): ReturnT {
+    this.assertNotDestroyed();
+
+    const previousInjector = setCurrentInjector(this);
+    const previousInjectImplementation = setInjectImplementation(undefined);
+    try {
+      return fn();
+    } finally {
+      setCurrentInjector(previousInjector);
+      setInjectImplementation(previousInjectImplementation);
+    }
+  }
+
   override get<T>(
       token: ProviderToken<T>, notFoundValue: any = THROW_IF_NOT_FOUND,
       flags = InjectFlags.Default): T {

diff --git a/packages/core/src/render3/ng_module_ref.ts b/packages/core/src/render3/ng_module_ref.ts
@@ -86,6 +86,10 @@ export class NgModuleRef<T> extends viewEngine_NgModuleRef<T> implements Interna
     return this._r3Injector.get(token, notFoundValue, injectFlags);
   }
 
+  runInContext<ReturnT>(fn: () => ReturnT): ReturnT {
+    return this.injector.runInContext(fn);
+  }
+
   override destroy(): void {
     ngDevMode && assertDefined(this.destroyCbs, 'NgModule already destroyed');
     const injector = this._r3Injector;

diff --git a/packages/core/test/acceptance/environment_injector_spec.ts b/packages/core/test/acceptance/environment_injector_spec.ts
@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Component, ComponentFactoryResolver, createEnvironmentInjector, ENVIRONMENT_INITIALIZER, EnvironmentInjector, InjectionToken, INJECTOR, Injector, NgModuleRef} from '@angular/core';
+import {Component, ComponentFactoryResolver, createEnvironmentInjector, ENVIRONMENT_INITIALIZER, EnvironmentInjector, inject, InjectFlags, InjectionToken, INJECTOR, Injector, NgModuleRef, ViewContainerRef} from '@angular/core';
 import {R3Injector} from '@angular/core/src/di/r3_injector';
 import {TestBed} from '@angular/core/testing';
 
@@ -119,4 +119,94 @@ describe('environment injector', () => {
     });
     expect(injector.get(EnvScopedToken, false)).toBeTrue();
   });
+
+  describe('runInContext()', () => {
+    it('should return the function\'s return value', () => {
+      const injector = TestBed.inject(EnvironmentInjector);
+      const returnValue = injector.runInContext(() => 3);
+      expect(returnValue).toBe(3);
+    });
+
+    it('should make inject() available', () => {
+      const TOKEN = new InjectionToken<string>('TOKEN');
+      const injector = createEnvironmentInjector(
+          [{provide: TOKEN, useValue: 'from injector'}], TestBed.inject(EnvironmentInjector));
+
+      const result = injector.runInContext(() => inject(TOKEN));
+      expect(result).toEqual('from injector');
+    });
+
+    it('should properly clean up after the function returns', () => {
+      const TOKEN = new InjectionToken<string>('TOKEN');
+      const injector = TestBed.inject(EnvironmentInjector);
+      injector.runInContext(() => {});
+      expect(() => inject(TOKEN, InjectFlags.Optional)).toThrow();
+    });
+
+    it('should properly clean up after the function throws', () => {
+      const TOKEN = new InjectionToken<string>('TOKEN');
+      const injector = TestBed.inject(EnvironmentInjector);
+      expect(() => injector.runInContext(() => {
+        throw new Error('crashes!');
+      })).toThrow();
+      expect(() => inject(TOKEN, InjectFlags.Optional)).toThrow();
+    });
+
+    it('should set the correct inject implementation', () => {
+      const TOKEN = new InjectionToken<string>('TOKEN', {
+        providedIn: 'root',
+        factory: () => 'from root',
+      });
+
+      @Component({
+        standalone: true,
+        template: '',
+        providers: [{provide: TOKEN, useValue: 'from component'}],
+      })
+      class TestCmp {
+        envInjector = inject(EnvironmentInjector);
+
+        tokenFromComponent = inject(TOKEN);
+        tokenFromEnvContext = this.envInjector.runInContext(() => inject(TOKEN));
+
+        // Attempt to inject ViewContainerRef within the environment injector's context. This should
+        // not be available, so the result should be `null`.
+        vcrFromEnvContext =
+            this.envInjector.runInContext(() => inject(ViewContainerRef, InjectFlags.Optional));
+      }
+
+      const instance = TestBed.createComponent(TestCmp).componentInstance;
+      expect(instance.tokenFromComponent).toEqual('from component');
+      expect(instance.tokenFromEnvContext).toEqual('from root');
+      expect(instance.vcrFromEnvContext).toBeNull();
+    });
+
+    it('should be reentrant', () => {
+      const TOKEN = new InjectionToken<string>('TOKEN', {
+        providedIn: 'root',
+        factory: () => 'from root',
+      });
+
+      const parentInjector = TestBed.inject(EnvironmentInjector);
+      const childInjector =
+          createEnvironmentInjector([{provide: TOKEN, useValue: 'from child'}], parentInjector);
+
+      const results = parentInjector.runInContext(() => {
+        const fromParentBefore = inject(TOKEN);
+        const fromChild = childInjector.runInContext(() => inject(TOKEN));
+        const fromParentAfter = inject(TOKEN);
+        return {fromParentBefore, fromChild, fromParentAfter};
+      });
+
+      expect(results.fromParentBefore).toEqual('from root');
+      expect(results.fromChild).toEqual('from child');
+      expect(results.fromParentAfter).toEqual('from root');
+    });
+
+    it('should not function on a destroyed injector', () => {
+      const injector = createEnvironmentInjector([], TestBed.inject(EnvironmentInjector));
+      injector.destroy();
+      expect(() => injector.runInContext(() => {})).toThrow();
+    });
+  });
 });