Dependecy Injection

Example on how to test Dependency Injection

Overview

Dependency injection is providing the objects that an object needs (its dependencies) instead of having it construct them itself. It's a very useful technique for testing, since it allows dependencies to be mocked or stubbed out.

Dependencies can be injected into objects by constructor injection or setter injection.

Example

Test

export class NoDependencyInjection {
  protected restHost: RESTHost;

  // We can retrieve the RESTHost from a config element, but we have no way of creating mocks/spies/expectations
  init() {
    const restHostConfig = new ConfigElementAccessor('ing/testing/dependencyInjectionRestHost');
    this.restHost = restHostConfig.get<RESTHost>('restHost');
  }

  doApiCall() {
    const request = this.restHost.createRequest("GET", "/ping", "");

    return request.execute();
  }
}

// To test this, create a mock for restHostConfig
export class SomeDependencyInjection {
  private restHost: RESTHost;

  // Retrieve the RESTHost from a config element
  constructor(restHostConfig: ConfigElementAccessor) {
    this.restHost = restHostConfig.get<RESTHost>('restHost');
  }

  doApiCall() {
    const request = this.restHost.createRequest("GET", "/ping", "");

    return request.execute();
  }
}

// Even Better, we can test this by creating a mock for restHost directly, always delegate logic to the parent, not the child!
// Let the action/workflow that needs this class, fetch everything that is needed.
export class GoodDependencyInjection {
  constructor(private restHost: RESTHost) {
  }

  doApiCall() {
    const request = this.restHost.createRequest("GET", "/ping", "");

    return request.execute();
  }
}

// Test oriented dependency injection. Useful in a LOT of cases. In your normal execution, you want to retrieve the value
// from some config element, but in test cases, you have an out
export class TestRelatedDependencyInjection {
  // When we create this class in our workflows or actions, do not pass a parameter, but when writing tests, create a mocked restHost
  constructor(private restHost?: RESTHost) {
    if (!restHost) {
      const restHostConfig = new ConfigElementAccessor('ing/testing/dependencyInjectionRestHost');
      this.restHost = restHostConfig.get<RESTHost>('restHost');
    }
  }

  doApiCall() {
    const request = this.restHost.createRequest("GET", "/ping", "");

    return request.execute();
  }
}

export class NoDependencyInjectionTestDouble extends NoDependencyInjection {
  // @ts-ignore
  constructor(protected restHost: RESTHost) {
  }
}