Providers

Providers are a core concept in CHO's Dependency Injection (DI) system. They define how to create entities.

Factory Providers

CHO DI supports only a single type of provider: the factory provider. The provider is an object containing provide and factory properties.

The provide property is a token that identifies the entity. It can be a class, a string, or a symbol.

The factory property is an async function that returns a promise that resolves the entity. The factory function gets an injector as an argument. This injector can be used to resolve other dependencies.

Example:

import type { Provider } from "@chojs/core";

const myProvider: Provider = {
  provide: MyService,
  factory: async (injector) => {
    const dep1 = await injector.resolve(Dep1);
    const dep2 = await injector.resolve(Dep2);
    return new MyService(dep1, dep2);
  },
};

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Providing Techniques

Since there is only single type of provider, there are techniques to achieve different behaviors.

Value Providers

Value providers are used to provide a constant value. The factory function simply returns the value. Note that the factory function must be async and return a promise even for non-async values.

const valueProvider: Provider = {
  provide: "API_URL",
  factory: () => Promise.resolve("https://api.example.com"),
};

Class Providers

Class providers are used to create overrides for classes (create instance of class A as class B). In CHO, instead of using a class provider, create a provider that resolves the override instance.

const classProvider: Provider = {
  provide: MyService,
  factory: async (injector) => {
    return injector.resolve(OtherService);
  },
};

Existing Providers

Existing providers are used to create aliases for existing providers. In CHO, instead of using an existing provider, create a provider that resolves the existing provider. See the example of class provider above.