Builder pattern applied to InjectionToken factory functions

injection-token-factory-builder.ts

// ========================================
// app-module.ts
// ========================================
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { NumbersModule } from './numbers';

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    NumbersModule
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

// ========================================
// numbers.ts
// ========================================
import { InjectionToken, NgModule } from '@angular/core';

export const TWO = new InjectionToken<number>('TWO');
export const SEVEN = new InjectionToken<number>('SEVEN');
export const EIGHT = new InjectionToken<number>('EIGHT');
export const NINE = new InjectionToken<number>('NINE');

@NgModule({
  providers: [
    { provide: TWO, useValue: 2 },
    { provide: SEVEN, useValue: 7 },
    { provide: EIGHT, useFactory: () => 2 + 2 * 2 + 2 },
    { provide: NINE, useFactory: (two: number, seven: number) => two + seven, deps: [TWO, SEVEN] }
  ]
})
export class NumbersModule {

}

// ========================================
// app.component.ts
// ========================================
import { Component, Inject, InjectionToken, inject } from '@angular/core';
import { SEVEN, EIGHT, NINE } from './numbers';

export class Foo {
  public readonly data = { a: 1, b: 2, c: 3 };
}

export const SIX = new InjectionToken<number>('SIX', { providedIn: 'root', factory: () => 6 })

type FooFactory = () => Foo;

type FooBuilderFactory = FooFactory & {
  a(a: InjectionToken<number>): FooBuilderFactory;
  b(b: InjectionToken<number>): FooBuilderFactory;
  c(c: InjectionToken<number>): FooBuilderFactory;
}

export function fooFactoryFactory(a: InjectionToken<number>, b: InjectionToken<number>, c: InjectionToken<number>): FooBuilderFactory {
  const foo = new Foo();

  const injectionInputs = { a, b, c };

  const factory = (() => {

    foo.data.a = inject(injectionInputs.a);
    foo.data.b = inject(injectionInputs.b);
    foo.data.c = inject(injectionInputs.a) * inject(injectionInputs.b) * inject(injectionInputs.c);

    return foo;
  }) as FooBuilderFactory;

  factory.a = (_a: InjectionToken<number>) => { injectionInputs.a = _a; return factory; }
  factory.b = (_b: InjectionToken<number>) => { injectionInputs.b = _b; return factory; }
  factory.c = (_c: InjectionToken<number>) => { injectionInputs.c = _c; return factory; }

  return factory;
}

export const FOO = new InjectionToken<Foo>('FOO', {
  providedIn: 'root',
  factory: fooFactoryFactory(SEVEN, SIX, EIGHT).a(EIGHT).c(NINE)
});

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss']
})
export class AppComponent {
  title = 'injection-test';

  constructor(@Inject(FOO) foo: Foo) {
    console.log('foo =', foo);
  }
}