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Dependency Injection in Angular Marko Stanimirović

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Marko Stanimirović @MarkoStDev ★ Sr. Frontend Engineer at JobCloud ★ NgRx Core Team Member ★ Angular Belgrade Organizer ★ Hobby Musician

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Marko Stanimirović @MarkoStDev ★ Sr. Frontend Engineer at JobCloud ★ NgRx Core Team Member ★ Angular Belgrade Organizer ★ Hobby Musician ★ Google Developer Expert in Angular

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What is Dependency Injection?

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“DI is a design pattern that makes an object independent of its dependencies by decoupling its usage from its creation.”

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Improves testability, scalability and maintainability Reduces tight coupling

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class UsersService { private usersResource = new HttpUsersResource(); private logger = new ServerLogger(); } Without Dependency Injection

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class UsersService { private usersResource = new HttpUsersResource(); private logger = new ServerLogger(); } Tightly coupled Without Dependency Injection

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class UsersService { constructor( private usersResource: UsersResource, private logger: Logger ) {} } With Dependency Injection

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class UsersService { constructor( private usersResource: UsersResource, private logger: Logger ) {} } With Dependency Injection Loosely coupled

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Injection Scopes in Angular

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Injection Scopes in Standalone Angular Apps

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Injection Scopes in Standalone Angular Apps Root

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Injection Scopes in Standalone Angular Apps Route Root

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Injection Scopes in Standalone Angular Apps Element Route Root (Component / Directive)

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Root Providers

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Providing Service at the Root Level - #1 Way class UsersService { ./ ... }

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Providing Service at the Root Level - #1 Way @Injectable({ providedIn: 'root' }) class UsersService { ./ ... }

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Providing Service at the Root Level - #1 Way @Injectable({ providedIn: 'root' }) class UsersService { ./ ... } @Component(** **. */) class UserDetailsComponent { constructor(private usersService: UsersService) {} }

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Providing Service at the Root Level - #2 Way @Injectable() class UsersService { ./ ... } bootstrapApplication(AppComponent, { providers: [UsersService], });

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Route Providers

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Providing Service at the Route Level const userRoutes: Route[] = [ { path: '', component: UsersShellComponent, children: [ { path: '', component: UserListComponent }, { path: ':id', component: UserDetailsComponent }, ], }, ];

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Providing Service at the Route Level const userRoutes: Route[] = [ { path: '', component: UsersShellComponent, providers: [UsersService], children: [ { path: '', component: UserListComponent }, { path: ':id', component: UserDetailsComponent }, ], }, ];

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Providing Service at the Route Level const userRoutes: Route[] = [ { path: '', component: UsersShellComponent, providers: [UsersService], children: [ { path: '', component: UserListComponent }, { path: ':id', component: UserDetailsComponent }, ], }, ];

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Element Providers

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Providing Service at the Component Level @Component({ selector: 'app-user-details', standalone: true, template: ` ./app-user-form> ./ng-content> `, imports: [UserFormComponent], }) class UserDetailsComponent { constructor(readonly usersService: UsersService) {} }

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Providing Service at the Component Level @Component({ selector: 'app-user-details', standalone: true, template: ` ./app-user-form> ./ng-content> `, providers: [UsersService], imports: [UserFormComponent], }) class UserDetailsComponent { constructor(readonly usersService: UsersService) {} }

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Providing Service at the Component Level @Component({ selector: 'app-user-details', standalone: true, template: ` ./app-user-form> ./ng-content> `, providers: [UsersService], imports: [UserFormComponent], }) class UserDetailsComponent { constructor(readonly usersService: UsersService) {} }

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Providing Service at the Component Level @Component({ selector: 'app-user-details', standalone: true, template: ` ./app-user-form> ./ng-content> `, viewProviders: [UsersService], imports: [UserFormComponent], }) class UserDetailsComponent { constructor(readonly usersService: UsersService) {} }

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Providing Service at the Component Level @Component({ selector: 'app-user-details', standalone: true, template: ` ./app-user-form> ./ng-content> `, viewProviders: [UsersService], imports: [UserFormComponent], }) class UserDetailsComponent { constructor(readonly usersService: UsersService) {} }

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Dependency Resolution in Angular

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Element Injector viewProviders @Inject(UsersService) providers UserDetailsComponent

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Element Injector viewProviders @Inject(UsersService) providers UserDetailsComponent

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Element Injector viewProviders @Inject(UsersService) providers UserDetailsComponent

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Element Injector viewProviders @Inject(UsersService) providers Element Injector viewProviders providers AppComponent UserDetailsComponent

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Element Injector viewProviders @Inject(UsersService) providers Element Injector viewProviders providers AppComponent UserDetailsComponent

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Element Injector viewProviders @Inject(UsersService) providers Element Injector viewProviders providers AppComponent Route Injector providers UserDetailsComponent

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Element Injector viewProviders @Inject(UsersService) providers Root Injector providers Element Injector viewProviders providers AppComponent Route Injector providers UserDetailsComponent

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Element Injector viewProviders @Inject(UsersService) providers Root Injector providers Element Injector viewProviders providers AppComponent Route Injector providers UserDetailsComponent

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Resolution Modifiers

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Optional Modifier

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Optional Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Optional() private usersService.: UsersService) {} }

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Optional Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Optional() private usersService.: UsersService) {} } Element Injector viewProviders providers UserDetailsComponent @Optional() @Inject(UsersService)

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Optional Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Optional() private usersService.: UsersService) {} } Element Injector viewProviders providers UserDetailsComponent Element Injector viewProviders providers AppComponent Route Injector providers Root Injector providers @Optional() @Inject(UsersService) null

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SkipSelf Modifier

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SkipSelf Modifier @Component(** **. */) class UserDetailsComponent { constructor(@SkipSelf() private usersService: UsersService) {} }

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SkipSelf Modifier @Component(** **. */) class UserDetailsComponent { constructor(@SkipSelf() private usersService: UsersService) {} } Element Injector viewProviders providers UserDetailsComponent Element Injector viewProviders providers AppComponent Route Injector providers Root Injector providers @SkipSelf() @Inject(UsersService) search starts here

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Self Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Self() private usersService: UsersService) {} }

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Self Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Self() private usersService: UsersService) {} } Element Injector viewProviders providers UserDetailsComponent @Self() @Inject(UsersService)

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Self Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Self() private usersService: UsersService) {} } Element Injector viewProviders providers UserDetailsComponent @Self() @Inject(UsersService)

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Host Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Host() private usersService: UsersService) {} }

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Host Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Host() private usersService: UsersService) {} } Element Injector viewProviders providers UserDetailsComponent @Host() @Inject(UsersService)

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Host Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Host() private usersService: UsersService) {} } Element Injector viewProviders providers UserDetailsComponent @Host() @Inject(UsersService)

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Host Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Host() private usersService: UsersService) {} } Element Injector viewProviders providers UserDetailsComponent @Host() @Inject(UsersService) Element Injector viewProviders providers AppComponent

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Host Modifier @Component(** **. */) class UserDetailsComponent { constructor(@Host() private usersService: UsersService) {} } Element Injector viewProviders providers UserDetailsComponent @Host() @Inject(UsersService) Element Injector viewProviders providers AppComponent

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Injection Tokens

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Creating Injection Token const APP_THEME = new InjectionToken<'light' | 'dark'>('Application Theme');

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Creating Injection Token const APP_THEME = new InjectionToken<'light' | 'dark'>('Application Theme'); constant value type description

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Providing Injection Token const APP_THEME = new InjectionToken<'light' | 'dark'>('Application Theme'); bootstrapApplication(AppComponent, { providers: [ ], }); providing at the root level

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Providing Injection Token const APP_THEME = new InjectionToken<'light' | 'dark'>('Application Theme'); bootstrapApplication(AppComponent, { providers: [APP_THEME ], });

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Providing Injection Token const APP_THEME = new InjectionToken<'light' | 'dark'>('Application Theme'); bootstrapApplication(AppComponent, { providers: [{ }], });

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Providing Injection Token const APP_THEME = new InjectionToken<'light' | 'dark'>('Application Theme'); bootstrapApplication(AppComponent, { providers: [{ provide: APP_THEME }], });

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Providing Injection Token const APP_THEME = new InjectionToken<'light' | 'dark'>('Application Theme'); bootstrapApplication(AppComponent, { providers: [{ provide: APP_THEME, useValue: 'light' }], });

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Providing Injection Token const APP_THEME = new InjectionToken<'light' | 'dark'>('Application Theme'); bootstrapApplication(AppComponent, { providers: [{ provide: APP_THEME, useFactory: appThemeFactory }], }); function appThemeFactory() { return localStorage.getItem('theme') .? 'light'; }

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Injecting Injection Token const APP_THEME = new InjectionToken<'light' | 'dark'>('Application Theme'); bootstrapApplication(AppComponent, { providers: [{ provide: APP_THEME, useFactory: appThemeFactory }], }); @Component(** **. */) class SideMenuComponent { constructor( ) {} }

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Injecting Injection Token const APP_THEME = new InjectionToken<'light' | 'dark'>('Application Theme'); bootstrapApplication(AppComponent, { providers: [{ provide: APP_THEME, useFactory: appThemeFactory }], }); @Component(** **. */) class SideMenuComponent { constructor(@Inject(APP_THEME) theme: 'light' | 'dark') {} }

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Providing Injection Token - Tree Shakeable Way const APP_THEME = new InjectionToken('Application Theme', { });

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Providing Injection Token - Tree Shakeable Way const APP_THEME = new InjectionToken('Application Theme', { providedIn: 'root', factory: () .> localStorage.getItem('theme') .? 'light', });

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Providing Injection Token - Tree Shakeable Way const APP_THEME = new InjectionToken('Application Theme', { providedIn: 'root', factory: () .> localStorage.getItem('theme') .? 'light', }); @Component(** **. */) class SideMenuComponent { constructor(@Inject(APP_THEME) theme: 'light' | 'dark') {} }

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inject Function

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Constructor-Based DI vs inject @Component({ ** **. */ }) export class UserDetailsComponent { constructor( private usersService: UsersService, private route: ActivatedRoute ) {} }

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Constructor-Based DI vs inject @Component({ ** **. */ }) export class UserDetailsComponent { constructor( private usersService: UsersService, private route: ActivatedRoute ) {} } @Component({ ** **. */ }) export class UserDetailsComponent { private usersService = inject(UsersService); private route = inject(ActivatedRoute); }

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Constructor-Based DI vs inject @Component({ ** **. */ }) export class UserDetailsComponent { constructor( private usersService: UsersService, private route: ActivatedRoute ) {} } @Component({ ** **. */ }) export class UserDetailsComponent { private usersService = inject(UsersService); private route = inject(ActivatedRoute); } token

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Resolution Modifiers with inject @Component({ ** **. */ }) export class UserDetailsComponent { private usersService = inject(UsersService, { self: true, optional: true }); }

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Resolution Modifiers with inject @Component({ ** **. */ }) export class UserDetailsComponent { private usersService = inject(UsersService, { self: true, optional: true }); } config

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Factories with inject

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Factories with inject const ID_ROUTE_PARAM = new InjectionToken('ID Route Parameter Observable', { factory() { }, });

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Factories with inject const ID_ROUTE_PARAM = new InjectionToken('ID Route Parameter Observable', { factory() { const route = inject(ActivatedRoute); }, });

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Factories with inject const ID_ROUTE_PARAM = new InjectionToken('ID Route Parameter Observable', { factory() { const route = inject(ActivatedRoute); return route.params.pipe( map((params) .> params['id']), filter(Boolean), distinctUntilChanged() ); }, });

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Base Classes with inject

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Base Classes with Constructor-Based DI abstract class EntityResource { ./ ... }

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Base Classes with Constructor-Based DI abstract class EntityResource { protected constructor( protected http: HttpClient, protected apiUrl: string, protected uri: string ) {} ./ ... }

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Base Classes with Constructor-Based DI abstract class EntityResource { protected constructor( protected http: HttpClient, protected apiUrl: string, protected uri: string ) {} ./ ... } @Injectable({ providedIn: 'root' }) class UsersResource extends EntityResource { }

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Base Classes with Constructor-Based DI abstract class EntityResource { protected constructor( protected http: HttpClient, protected apiUrl: string, protected uri: string ) {} ./ ... } @Injectable({ providedIn: 'root' }) class UsersResource extends EntityResource { constructor( http: HttpClient, @Inject(API_URL) apiUrl: string ) { super(http, apiUrl, 'users'); } }

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Base Classes with inject abstract class EntityResource { ./ ... }

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Base Classes with inject abstract class EntityResource { protected http = inject(HttpClient); protected apiUrl = inject(API_URL); protected constructor(protected uri: string) {} ./ ... }

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Base Classes with inject abstract class EntityResource { protected http = inject(HttpClient); protected apiUrl = inject(API_URL); protected constructor(protected uri: string) {} ./ ... } @Injectable({ providedIn: 'root' }) class UsersResource extends EntityResource { constructor() { super('users'); } }

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Advanced DI Techniques

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Providing Logger Based on Environment

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Providing Logger Based on Environment @Injectable({ providedIn: 'root', }) abstract class Logger { abstract log(message: string): void; abstract error(message: string): void; }

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Providing Logger Based on Environment @Injectable({ providedIn: 'root', }) abstract class Logger { abstract log(message: string): void; abstract error(message: string): void; } @Injectable({ providedIn: 'root' }) class ServerLogger extends Logger { ./ ... } @Injectable({ providedIn: 'root' }) class ConsoleLogger extends Logger { ./ ... }

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Providing Logger Based on Environment @Injectable({ providedIn: 'root', useFactory: () .> environment.production ? inject(ServerLogger) : inject(ConsoleLogger), }) abstract class Logger { abstract log(message: string): void; abstract error(message: string): void; } @Injectable({ providedIn: 'root' }) class ServerLogger extends Logger { ./ ... } @Injectable({ providedIn: 'root' }) class ConsoleLogger extends Logger { ./ ... }

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Providing Logger Based on Environment @Injectable({ providedIn: 'root', useFactory: () .> environment.production ? inject(ServerLogger) : inject(ConsoleLogger), }) abstract class Logger { abstract log(message: string): void; abstract error(message: string): void; } @Injectable({ providedIn: 'root' }) class ServerLogger extends Logger { ./ ... } @Injectable({ providedIn: 'root' }) class ConsoleLogger extends Logger { ./ ... } @Injectable({ providedIn: 'root' }) class UsersService { constructor(private readonly logger: Logger) {} }

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Example from @ngrx/component package

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tick-scheduler.ts @Injectable({ providedIn: 'root', }) abstract class TickScheduler { abstract schedule(): void; }

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tick-scheduler.ts @Injectable({ providedIn: 'root', }) abstract class TickScheduler { abstract schedule(): void; } @Injectable({ providedIn: 'root' }) class AnimationFrameTickScheduler extends TickScheduler { ./ ... } class NoopTickScheduler extends TickScheduler { schedule(): void {} }

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tick-scheduler.ts @Injectable({ providedIn: 'root', useFactory: () .> { const zone = inject(NgZone); return isNgZone(zone) ? new NoopTickScheduler() : inject(AnimationFrameTickScheduler); }, }) abstract class TickScheduler { abstract schedule(): void; } @Injectable({ providedIn: 'root' }) class AnimationFrameTickScheduler extends TickScheduler { ./ ... } class NoopTickScheduler extends TickScheduler { schedule(): void {} }

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tick-scheduler.ts @Injectable({ providedIn: 'root', useFactory: () .> { const zone = inject(NgZone); return isNgZone(zone) ? new NoopTickScheduler() : inject(AnimationFrameTickScheduler); }, }) abstract class TickScheduler { abstract schedule(): void; } @Injectable({ providedIn: 'root' }) class AnimationFrameTickScheduler extends TickScheduler { ./ ... } class NoopTickScheduler extends TickScheduler { schedule(): void {} }

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Angular DI Under the Hood

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Scan project files

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Scan project files Find all classes with Angular-specific decorators @Injectable() @Component() @Directive() @Pipe()

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Scan project files Find all classes with Angular-specific decorators Preserve information about constructor parameter types @Injectable() @Component() @Directive() @Pipe()

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Scan project files Find all classes with Angular-specific decorators Preserve information about constructor parameter types Transpile TypeScript to JavaScript @Injectable() @Component() @Directive() @Pipe()

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users.service.ts @Injectable({ providedIn: 'root' }) class UsersService { constructor( private usersResource: UsersResource, private logger: Logger ) {} }

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users.service.ts @Injectable({ providedIn: 'root' }) class UsersService { constructor( private usersResource: UsersResource, private logger: Logger ) {} } class UsersService { constructor(usersResource, logger) { this.usersResource = usersResource; this.logger = logger; } } UsersService.ɵfac = function UsersService_Factory() { return new UsersService( ɵɵinject(UsersResource), ɵɵinject(Logger) ); }; UsersService.ɵprov = ɵɵdefineInjectable({ token: UsersService, factory: UsersService.ɵfac, providedIn: 'root' }); Compiled Output

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users.service.ts @Injectable({ providedIn: 'root' }) class UsersService { constructor( private usersResource: UsersResource, private logger: Logger ) {} } class UsersService { constructor(usersResource, logger) { this.usersResource = usersResource; this.logger = logger; } } UsersService.ɵfac = function UsersService_Factory() { return new UsersService( ɵɵinject(UsersResource), ɵɵinject(Logger) ); }; UsersService.ɵprov = ɵɵdefineInjectable({ token: UsersService, factory: UsersService.ɵfac, providedIn: 'root' }); Compiled Output

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users.service.ts @Injectable({ providedIn: 'root' }) class UsersService { constructor( private usersResource: UsersResource, private logger: Logger ) {} } class UsersService { constructor(usersResource, logger) { this.usersResource = usersResource; this.logger = logger; } } UsersService.ɵfac = function UsersService_Factory() { return new UsersService( ɵɵinject(UsersResource), ɵɵinject(Logger) ); }; UsersService.ɵprov = ɵɵdefineInjectable({ token: UsersService, factory: UsersService.ɵfac, providedIn: 'root' }); Compiled Output

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users.service.ts @Injectable({ providedIn: 'root' }) class UsersService { constructor( private usersResource: UsersResource, private logger: Logger ) {} } class UsersService { constructor(usersResource, logger) { this.usersResource = usersResource; this.logger = logger; } } UsersService.ɵfac = function UsersService_Factory() { return new UsersService( ɵɵinject(UsersResource), ɵɵinject(Logger) ); }; UsersService.ɵprov = ɵɵdefineInjectable({ token: UsersService, factory: UsersService.ɵfac, providedIn: 'root' }); Compiled Output

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Summary

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DI mechanism is one of the most powerful features of the Angular Framework! DI gives the ability to write loosely coupled and reusable code that is easier to test, scale, and maintain.

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Marko Stanimirović @MarkoStDev Thank You!