GuidesDependency Relationship

Dependency relationship

After creating identifiers and dependency items, you need to declare dependency relationship among dependencies. There are two types of dependency items that could dependent on others: class items and factory function items.

On a class

To declare dependencies of a class, you could use the Injector decorator:

class MapService {
  constructor(
    @Inject(SatelliteService) private readonly satellite: SatelliteService,
  ) {}
}

or an IdentifierDecorator:

interface IPlatformService {}
 
const IPlatformService = createIdentifier<IPlatformService>("platform");
 
class DialogService {
  constructor(
    @IPlatformService private readonly platformSrc: IPlatformService,
  ) {}
}

On a factory

You should list all its dependencies in the deps property:

const item = [
  I18NNumberTranspiler,
  {
    useFactory: (i18nService: I18NService) => {
      return i18nService.isChinese()
        ? new ChineseNumberTranspiler()
        : new EnglishNumberTranspiler();
    },
    deps: [I18NService],
  },
];

Optional

When a dependency is not held by an injector, the injector would throw an error when you want to get that dependency.

class MapService {
  constructor(
    @Inject(SatelliteService) private readonly satellite: SatelliteService,
  ) {}
}
 
const injector = new Injector([[MapService]]);
injector.get(MapService); // ERROR!

You could use Optional instead Inject, to mark SatelliteService as an optional dependency not a required one.

class MapService {
  constructor(
    @Optional(SatelliteService) private readonly satellite?: SatelliteService,
  ) {}
}
 
const injector = new Injector([MapService]);
injector.get(MapService);

In this case, MapService could construct, but its property satellite would be undefined.

Multiple

Similarly, you could use Many to mark the dependency could be injected with multiple instances:

class MapService {
  constructor(
    @Many(ISatelliteService) private readonly satellites: ISatelliteService[],
  ) {}
}
 
const injector = new Injector([
  [MapService],
  [ISatelliteService, { useClass: GPSSatellite }],
  [ISatelliteService, { useClass: BeidouSatellite }],
]);
injector.get(MapService);
BindingInjector