Service Seeker

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Service Locator TypeScript Library

Install

npm install service-seeker

Usage

Defining providers

The provider configuration is an object literal with two properties:

• The provide property holds the token that serves as the key for both locating a dependency value and configuring the injector.

• The second property is a provider definition object, which tells the injector how to create the dependency value. The provider-definition key can be useClass, as in the example. It can also be useValue, or useFactory. Each of these keys provides a different type of dependency, as discussed in the following section.

app.provider.ts

import { Injector } from '@nx-clean/core';

class SomeService {
  constructor() {}
}

const injector = Injector.create([
  { provide: SomeService, useClass: SomeService },
]);

const instance = injector.get<SomeService>(SomeService);

SomeService {}

Injecting a configuration object

A common use case for object literals is a configuration object. The following configuration object includes an address of a web API endpoint.

app.provider.ts

import { Inject, Injectable, InjectionToken, Injector } from '@nx-clean/core';

interface ApiConfig {
  url: string;
}

const API_CONFIG: ApiConfig = {
  url: 'https://api.clean-architecture.design',
};

To provide and inject the configuration object, specify the object in the Injector as providers array.

app.provider.ts

const injector = Injector.create([
  { provide: API_TOKEN, useValue: API_CONFIG },
]);

Using an InjectionToken object

Define and use an InjectionToken object for choosing a provider token for non-class dependencies. The following example defines a token, APP_CONFIG of the type InjectionToken.

The optional type parameter, <AppConfig>, and the token description, api.config, specify the token's purpose.

app.provider.ts

const API_TOKEN = new InjectionToken<ApiConfig>('api.config');

Next, register the dependency provider in the service using the InjectionToken object of API_CONFIG.

app.provider.ts

const injector = Injector.create([
  { provide: API_TOKEN, useValue: API_CONFIG },
]);

Now get the configuration object using Injector's get method

app.provider.ts

const config = injector.get<ApiConfig>(API_TOKEN);

{
  url: 'https://api.clean-architecture.design';
}

Now, inject the configuration object into the constructor with @Inject() parameter decorator.

app.provider.ts

@Injectable()
class SomeService {
  constructor(@Inject(API_TOKEN) public config: ApiConfig) {}
}

Specifying an alternative class provider

Different classes can provide the same service. For example, the following code tells the injector to return a BetterLogger instance when the component asks for a logger using the Logger token.

app.provider.ts

abstract class OneService {}

@Injectable()
class SomeService implements OneService {
  constructor(@Inject(API_TOKEN) public config: ApiConfig) {}
}

const injector = Injector.create([
  { provide: API_TOKEN, useValue: API_CONFIG },
  { provide: OneService, useClass: SomeService },
]);

const instance = injector.get<OneService>(OneService);

SomeService {config: {…}}
  config:
    url: "https://api.clean-architecture.design"

Configuring class providers with dependencies

If the alternative class providers have their own dependencies, specify both providers in the providers metadata property of the parent module or component.

app.provider.ts

@Injectable()
class InjectableClass {
  constructor(public oneService: OneService) {}
}

const injector = Injector.create([
  { provide: API_TOKEN, useValue: API_CONFIG },
  { provide: OneService, useClass: SomeService },
  { provide: InjectableClass, useClass: InjectableClass },
]);

const instance = injector.get<InjectableClass>(InjectableClass);

InjectableClass {oneService: SomeService}
  oneService: SomeService
    config:
      url: "https://api.clean-architecture.design"

Completed example code

app.provider.ts

import { Inject, Injectable, InjectionToken, Injector } from '@nx-clean/core';

interface ApiConfig {
  url: string;
}

const API_CONFIG: ApiConfig = {
  url: 'https://api.clean-architecture.design',
};

const API_TOKEN = new InjectionToken<ApiConfig>('api-token');

abstract class OneService {}

@Injectable()
class SomeService implements OneService {
  constructor(@Inject(API_TOKEN) public config: ApiConfig) {}
}

@Injectable()
class InjectableClass {
  constructor(public oneService: OneService) {}
}

const injector = Injector.create([
  { provide: API_TOKEN, useValue: API_CONFIG },
  { provide: OneService, useClass: SomeService },
  { provide: InjectableClass, useClass: InjectableClass },
]);

const instance = injector.get<InjectableClass>(InjectableClass);