Inheritance and dependency injection

Question

I have a set of angular2 components that should all get some service injected. My first thought was that it would be best to create a super class and inject the service ther

Answer 1

Instead of injecting a service that has all the other services as dependencies, like so:

class ProviderService {
    constructor(private service1: Service1, private service2: Service2) {}
}

class BaseComponent {
    constructor(protected providerService: ProviderService) {}

    ngOnInit() {
        // Access to all application services with providerService
        this.providerService.service1
    }
}

class DerivedComponent extends BaseComponent {
    ngOnInit() {
        // Access to all application services with providerService
        this.providerService.service1
    }
}

I would skip this extra step and simply add inject all the services in the BaseComponent, like so:

class BaseComponent {
    constructor(protected service1: Service1, protected service2: Service2) {}
}

class DerivedComponent extends BaseComponent {
    ngOnInit() {
        this.service1;
        this.service2;
    }
}

This technique assumes 2 things:

1. Your concern is entirely related to components inheritance. Most likely, the reason you landed on this question is because of the overwhelming amount of non-dry (WET?) code you need to repeat in each derived class. If you want to benefits of a single entry point for all your components and services, you will need to do the extra step.

2. Every component extends the BaseComponent

There is also a disadvantage if you decide use the constructor of a derived class, as you will need to call super() and pass in all the dependencies. Although I don't really see a use case that necessitates the use of constructor instead of ngOnInit, it is entirely possible that such a use case exists.