Safe provide

A typesafe way to inject implementations for Typescript interfaces in Angular 2.

Why is this needed?

Angular's allows the user to configure how the injector will instantiate a token. This is commonly used to inject an implementation class when a component requires an interface to be injected. However, Angular does not check that the implementation satisfies the given interface. Consider the following example:

{provide: UserProviderToken, useClass: HttpUserProvider}

Interface information is lost after Typescript compilation. This means that no check is performed that HttpUserProvider implements UserProvider. The error only shows up in integration tests (or at runtime if test coverage is poor).

When using safe-provide's safeProvide function we get a compile-time check that HttpUserProvider implements UserProvider.

safeProvide(UserProviderToken).useClass(HttpUserProvider);

Usage

In the following example we configure angular to inject an instance of the HttpUserProvider class for the UserProvider interface.

1. Install package.

    $ npm install safe-provide --save

2. Create a SafeToken for your interface.

    // userProvider.ts
       
    import {SafeToken} from "safe-provide";
       
    export const UserProviderToken = new SafeToken<UserProvider>("UserProvider");
       

3. Configure a provider using safeProvide.

    // main.ts
       
    import {safeProvide, SafeToken} from "safe-provide";
       
    bootstrap(AppComponent, [
        safeProvide(UserProviderToken).useClass(HttpUserProvider)
    ]);

   The safeProvide function can be used anywhere angular allows provider configuration. Unlike with the Angular provide function, compilation will fail if the HttpUserProvider does not implicitly or explicitly implement UserProvider. We also have the option of providing a value

    safeProvide(UserProviderToken).useValue(new HttpUserProvider())
       

   or a factory function that returns an HttpUserProvider.

    safeProvide(UserProviderToken).useFactory(userProviderFactory)

4. Inject the UserProvider by passing the SafeToken to @Inject.

    // userComponent.ts
       
    @Component({
        // configuration
    })
    export class UsersComponent {
        constructor(@Inject(UserProviderToken) private userProvider:UserProvider) {
        }
    }