Angular之RouterModule的forRoot與forChild

Angular 提供了一種方式來把服務提供商從模塊中分離出來，以便模塊既可以帶着 providers 被根模塊導入，也可以不帶 providers 被子模塊導入。

區別：

`forRoot` creates a module that contains all the directives, the given routes, and the router service itself

`forChild` creates a module that contains all the directives and the given routes, but does not include the router service.

RouterModule的元數據

@NgModule({
    declarations: ROUTER_DIRECTIVES, 
    exports: ROUTER_DIRECTIVES
})

RouterModule定義的指令

const ROUTER_DIRECTIVES = [RouterOutlet, RouterLink, RouterLinkWithHref, RouterLinkActive];