When does the constructor of an injected service in angular run?

Question

In Angular 7, if we have a service declared as @Injectable({providedIn: 'root'}), when does it's constructor execute? Does it execute upon the construction of a component that uses it (as a dependency) or does it wait until a method in that service is first called?

Note: Since the service is a singleton, I use the service's constructor to initialize some values. I could create a "Initialize()" method and call that in the component's constructor instead but I find this aproach a little messy.

Answer 1

Instances of injected services are created at the point a component that requires them is first created or when another service that requires them is first instantiated.

When a component (or another service) that requires a Service as a dependency is created (or instantiated), Angular scans the Dependency Injection tree, and looks for a definition (and an instance of the injected service). If no instance of the injected service is found, Angular creates an instance of the injected service, and only then does its constructor run.

Another way to instantiate a service before an object (or another service) that require it are created, is to add it as a dependency to the APP_INITIALIZER in the deps array, like this:

{ provide: APP_INITIALIZER, useFactory: () => () => null, deps: [MyService], multi: true }

This will create an instance of MyService when the app is initialized regardless of when other components/services that require it are created. More details here.