Why does ChildActivationStart event trigger eventhough we don't have a child route in Angular?

问题

I defined a router for an Angular project. My router configuration is as follows:

export const routes: Routes = [
  {
    path: '',
    component: AppComponent,
  },
  {
    path: 'hello',
    component: HelloComponent,
  },
];

export const routing: ModuleWithProviders = RouterModule.forRoot(routes, { enableTracing: true });

When I run the project, I see the router lifecycle events triggered in the following order:

1. Navigation Start,
2. Routers Recognized,
3. Guards Check Start,
4. Child Activation Start, <-----
5. Activation Start,
6. Guards Check End,
7. Resolve Start,
8. Resolve End,
9. Activation End,
10. Child Activation End, <-----
11. Navigation End

In this list, I see Child Activation Start and Child Activation End events, too. However, I do not have a child route. Why those events trigger anyways?

To see the router events in action please check the console output of the following link:

https://stackblitz.com/edit/angular-khjkwg?file=app%2Fapp.routing.ts

Thanks.

来源：https://stackoverflow.com/questions/49866278/why-does-childactivationstart-event-trigger-eventhough-we-dont-have-a-child-rou