How to write a service that requires constructor parameters?

You can make the parameters optional by adding @Optional() (DI) and ? (TypeScript), and @Inject(somekey) for primitive values that are not supported as provider keys

@Injectable()export class MetricsService {    constructor(        private http: Http,        @Inject('wsAuthKey') @Optional() public wsAuthKey?: string,        @Inject('wsHost') @Optional() public wsHost?: string        ) {        this.wsAuthKey  = wsAuthKey || "blahblahblahblahblahblahblahblah=";        this.wsHost     = wsHost    || "https://preprod-admin.myservice.ws";    }

providers: [  {provide: 'wsAuthKey', useValue: 'abc'},   {provide: 'wsHost', useValue: 'efg'}, ]

If they are provided, they are passed, otherwise they are ignored, but DI still can inject the MetricsService.

This is a common recipe that is described in this question in particular. It should be a service that holds the configuration:

@Injectable()export class MetricsConfig {  wsAuthKey = "blahblahblahblahblahblahblahblah=";  wsHost = "https://preprod-admin.myservice.ws";}@Injectable()export class MetricsService {    constructor(        private http: Http,        metricsConfig: MetricsConfig    ) {        this.wsAuthKey  = metricsConfig.wsAuthKey;        this.wsHost     = metricsConfig.wsHost;    }}

In the case when it needs to changed, it can be overridden or extended for entire module or for particular component:

@Component(  ...  { provide: MetricsConfig, useClass: class ExtendedMetricsConfig { ... } })export class DatavizComponent ...

There's no real need to make MetricsConfig a class in this case. It can be an OpaqueToken value provider as well. But a class can be conveniently extended, it is easier to inject and already provides an interface for typing.

From the official docs: https://angular.io/guide/dependency-injection-in-action#injectiontoken

Use an @Optional decorator in the constructor:

export class MyService {  constructor( @Optional() public var: type = value ) { }}