factory-dependencies.md

title	Factory dependencies

Factory dependencies

Factory functions and static factory methods can be registered with injector.setFactory():

class Foo {}

function createFoo(): Foo {
    return new Foo();
}

injector.setFactory(Foo, createFoo);

Static factory methods work the same way:

class Bar {
    private constructor() {}

    public static create(): Bar {
        return new Bar();
    }
}

injector.setFactory(Bar, Bar.create);

Static factory methods can also be registered with the injectable decorator. Classes instantiated with a factory method can even have a private constructor:

class Service {
    private constructor() {}

    @injectable
    public static create(): Service {
        return new Service();
    }
}

The injectable decorator on static factory methods accepts the same options object as injector.setFactory().

Factory injection

If a factory function or method has parameters then the parameter types must be specified with the inject option:

class Component {
    private constructor(service: Service) {}

    @injectable({ inject: [ Service ] })
    public static create(service: Service): Component {
        return new Component(service);
    }
}

Factory tokens

Factories can also be qualified with one or more injection tokens:

import { InjectionToken, injectable } from "@kayahr/di";

const componentToken = new InjectionToken<Component>("some-component");

class Component {
    @injectable({ token: componentToken })
    public static create(): Component {
        return new Component();
    }
}

Just like with classes, tokens are also useful for interfaces because interfaces do not exist at runtime.

Factory lifetimes

Factories support the same lifetimes as classes:

import { Lifetime, injectable } from "@kayahr/di";

class Component {
    @injectable({ lifetime: Lifetime.TRANSIENT })
    public static create(): Component {
        return new Component();
    }
}

Asynchronous dependencies

Factory functions and methods can also return promises:

class UserDAO {
    private constructor(private readonly db: Database) {}

    @injectable({ inject: [ DBService ] })
    public static async create(dbService: DBService): Promise<UserDAO> {
        return new UserDAO(await dbService.connect());
    }
}

If a dependency depends on an asynchronous dependency then it automatically also becomes asynchronous:

@injectable({ inject: [ UserDAO ] })
class Component {
    public constructor(userDAO: UserDAO) {}
}

injector.get(Component) now returns a promise on the first call because instance creation must wait for UserDAO.

Pass-through parameters

Transient factory dependencies also support pass-through parameters:

import { Lifetime, injectable, injector } from "@kayahr/di";

class Component {
    @injectable({ inject: [ null, Service, null ], lifetime: Lifetime.TRANSIENT })
    public static create(name: string, service: Service, id: number): Component {
        return new Component();
    }
}

Resolve such a dependency like this:

const component = injector.getSync(Component, { args: [ "test", 32 ] });

Because the pass-through positions are only known from the runtime registration, args can not be typed precisely. TypeScript therefore does not validate the number or types of the passed values here.