angular-di

Angular Dependency Injection

Configure and use dependency injection in Angular v20+ with inject() and providers.

Basic Injection

Using inject()

Prefer inject() over constructor injection:

import { Component, inject } from "@angular/core";
import { HttpClient } from "@angular/common/http";
import { User } from "./user.service";

@Component({ selector: "app-user-list", template: ... , }) export class UserList { // Inject dependencies private http = inject(HttpClient); private userService = inject(User);

// Can use immediately users = this.userService.getUsers(); }

### Injectable Services

```typescript
import { Injectable, inject, signal } from "@angular/core";
import { HttpClient } from "@angular/common/http";

@Injectable({
  providedIn: "root", // Singleton at root level
})
export class User {
  private http = inject(HttpClient);

  private users = signal<User[]>([]);
  readonly users$ = this.users.asReadonly();

  async loadUsers() {
    const users = await firstValueFrom(this.http.get<User[]>("/api/users"));
    this.users.set(users);
  }
}

Provider Scopes

Root Level (Singleton)

// Recommended: providedIn
@Injectable({
  providedIn: "root",
})
export class Auth {}

// Alternative: in app.config.ts
export const appConfig: ApplicationConfig = {
  providers: [Auth],
};

Component Level (Instance per Component)

@Component({
  selector: "app-editor",
  providers: [EditorState], // New instance for each component
  template: `
    ...
  `,
})
export class Editor {
  private editorState = inject(EditorState);
}

Route Level

export const routes: Routes = [
  {
    path: "admin",
    providers: [Admin], // Shared within this route tree
    children: [
      { path: "", component: AdminDashboard },
      { path: "users", component: AdminUsers },
    ],
  },
];

Injection Tokens

Creating Tokens

import { InjectionToken } from "@angular/core";

// Simple value token
export const API_URL = new InjectionToken<string>("API_URL");

// Object token
export interface AppConfig {
  apiUrl: string;
  features: {
    darkMode: boolean;
    analytics: boolean;
  };
}

export const APP_CONFIG = new InjectionToken<AppConfig>("APP_CONFIG");

// Token with factory (self-providing)
export const WINDOW = new InjectionToken<Window>("Window", {
  providedIn: "root",
  factory: () => window,
});

export const LOCAL_STORAGE = new InjectionToken<Storage>("LocalStorage", {
  providedIn: "root",
  factory: () => localStorage,
});

Providing Token Values

// app.config.ts
export const appConfig: ApplicationConfig = {
  providers: [
    { provide: API_URL, useValue: "https://api.example.com" },
    {
      provide: APP_CONFIG,
      useValue: {
        apiUrl: "https://api.example.com",
        features: { darkMode: true, analytics: true },
      },
    },
  ],
};

Injecting Tokens

@Injectable({ providedIn: "root" })
export class Api {
  private apiUrl = inject(API_URL);
  private config = inject(APP_CONFIG);
  private window = inject(WINDOW);

  getBaseUrl(): string {
    return this.apiUrl;
  }
}

Provider Types

useClass

// Provide implementation
{ provide: Logger, useClass: ConsoleLogger }

// Conditional implementation
{
  provide: Logger,
  useClass: environment.production
    ? ProductionLogger
    : ConsoleLogger,
}

useValue

// Static values
{ provide: API_URL, useValue: 'https://api.example.com' }

// Configuration objects
{ provide: APP_CONFIG, useValue: { theme: 'dark', language: 'en' } }

useFactory

// Factory with dependencies
{
  provide: User,
  useFactory: (http: HttpClient, config: AppConfig) => {
    return new User(http, config.apiUrl);
  },
  deps: [HttpClient, APP_CONFIG],
}

// Async factory (not recommended - use provideAppInitializer)
{
  provide: CONFIG,
  useFactory: () => fetch('/config.json').then(r => r.json()),
}

useExisting

// Alias to existing provider
{ provide: AbstractLogger, useExisting: ConsoleLogger }

// Multiple tokens pointing to same instance
providers: [
  ConsoleLogger,
  { provide: Logger, useExisting: ConsoleLogger },
  { provide: ErrorLogger, useExisting: ConsoleLogger },
]

Injection Options

Optional Injection

@Component({...})
export class My {
  // Returns null if not provided
  private analytics = inject(Analytics, { optional: true });

  trackEvent(name: string) {
    this.analytics?.track(name);
  }
}

Self, SkipSelf, Host

@Component({
  providers: [Local],
})
export class Parent {
  // Only look in this component's injector
  private local = inject(Local, { self: true });
}

@Component({...})
export class Child {
  // Skip this component, look in parent
  private parentService = inject(ParentSvc, { skipSelf: true });

  // Only look up to host component
  private hostService = inject(Host, { host: true });
}

Multi Providers

Collect multiple values for same token:

// Token for multiple validators
export const VALIDATORS = new InjectionToken<Validator[]>("Validators");

// Provide multiple values
providers: [
  { provide: VALIDATORS, useClass: RequiredValidator, multi: true },
  { provide: VALIDATORS, useClass: EmailValidator, multi: true },
  { provide: VALIDATORS, useClass: MinLengthValidator, multi: true },
];

// Inject as array
@Injectable()
export class Validation {
  private validators = inject(VALIDATORS); // Validator[]

  validate(value: string): ValidationError[] {
    return this.validators.map((v) => v.validate(value)).filter(Boolean);
  }
}

HTTP Interceptors (Multi Provider)

// Interceptors use multi providers internally
export const appConfig: ApplicationConfig = {
  providers: [
    provideHttpClient(
      withInterceptors([authInterceptor, loggingInterceptor, errorInterceptor]),
    ),
  ],
};

App Initializers

Run async code before app starts using provideAppInitializer:

import { provideAppInitializer, inject } from "@angular/core";

export const appConfig: ApplicationConfig = {
  providers: [
    Config,
    provideAppInitializer(() => {
      const configService = inject(Config);
      return configService.loadConfig();
    }),
  ],
};

Multiple Initializers

providers: [
  provideAppInitializer(() => {
    const config = inject(Config);
    return config.load();
  }),
  provideAppInitializer(() => {
    const auth = inject(Auth);
    return auth.checkSession();
  }),
];

Environment Injector

Create injectors programmatically:

import {
  createEnvironmentInjector,
  EnvironmentInjector,
  inject,
} from "@angular/core";

@Injectable({ providedIn: "root" })
export class Plugin {
  private parentInjector = inject(EnvironmentInjector);

  loadPlugin(providers: Provider[]): EnvironmentInjector {
    return createEnvironmentInjector(providers, this.parentInjector);
  }
}

runInInjectionContext

Run code with injection context:

import {
  runInInjectionContext,
  EnvironmentInjector,
  inject,
} from "@angular/core";

@Injectable({ providedIn: "root" })
export class Utility {
  private injector = inject(EnvironmentInjector);

  executeWithDI<T>(fn: () => T): T {
    return runInInjectionContext(this.injector, fn);
  }
}

// Usage
utilityService.executeWithDI(() => {
  const http = inject(HttpClient);
  // Use http...
});

For advanced patterns, see references/di-patterns.md.