How does @Host decorator work in injector tree

Question

I'm trying to understand how the @Host decorator works uncoupled from view-components. So I created the following injector tree:

class Dependency {
}

@Injectable()
class NeedsDependency {
  constructor(@Host() public dependency: Dependency) {
    console.log(this.dependency); // outputs 'A', but I expected the error
  }
}

const parent = ReflectiveInjector.resolveAndCreate([{provide: Dependency, useValue: 'A'}]);
const child = parent.resolveAndCreateChild([]);
const grandchild = child.resolveAndCreateChild([NeedsDependency]);
grandchild.get(NeedsDependency);

I expected to get an error, because as I understand the host for grandchild injector is child, and there is no Dependency provided in the child injector. However, when I run this code, I get 'A' injected from the root injector. Why?

Answer 1

Host doesn't have semantic meaning in the context of plain ReflectiveInjector.resolveAndCreate injector.

It is just ignored when being used outside the compiler.

For the desired behaviour,

I expected to get an error, because as I understand the host for grandchild injector is child, and there is no Dependency provided in the child injector.

consider using Self instead.

Answer 2

Specifies that an injector should retrieve a dependency from any injector until reaching the host element of the current component.

https://angular.io/docs/ts/latest/api/core/index/Host-decorator.html

Sounds to me it will look for providers and stop looking after it reached the injector of a component.