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canActivate

Interface that a class can implement to be a guard deciding if a route can be activated. If all guards return true, navigation will continue. If any guard returns false, navigation will be cancelled. If any guard returns a UrlTree, current navigation will be cancelled and a new navigation will be kicked off to the UrlTree returned from the guard.

The canActivate guard decides if route can be navigated to, which results in the creation of the route's component.

To implement the guard, create a new class and implement the CanActivate interface.
The interface can return a boolean (as a boolean, a promise, or an Observable) or the guard can navigate to another route.
If it returns a truthy value, the component will be created, otherwise it will not and the navigation gets canceled.

@Injectable({
  providedIn: "root",
})
export class OnlyDigitsGuard implements CanActivate {
  canActivate(
    next: ActivatedRouteSnapshot,
    state: RouterStateSnapshot
  ){
    return /^\d+$/.test(next.params.id);
  }

To guard a route, add the guard to the canActivate property while declaring the routes in the application.

const routes: Routes = [
  {
    path: 'parent',
    component: ParentComponent,
    canActivate: [OnlyDigitsGuard],
  },
]

canActivateChild

Interface that a class can implement to be a guard deciding if a child route can be activated. If all guards return true, navigation will continue. If any guard returns false, navigation will be cancelled. If any guard returns a UrlTree, current navigation will be cancelled and a new navigation will be kicked off to the UrlTree returned from the guard.

The canActivateChild guard, serves the same purpose as the canActivate guard and can prevent a route navigation.
The API to create the guard is the same, but for the canActivateChild guard you have to implement the CanActivateChild interface.

@Injectable({
  providedIn: "root",
})
export class OnlyDigitsGuard implements CanActivateChild {
  canActivateChild(
    next: ActivatedRouteSnapshot,
    state: RouterStateSnapshot
  ){
    return /^\d+$/.test(next.params.id);
  }

To add the guard to the routes, use the canActivateChild property.
Doing this will guard all the children's routes.

const routes: Routes = [
  {
    path: "parent",
    component: ParentComponent,
    canActivateChild: [OnlyDigitsGuard],
    children: [...],
  },
];

The differences

• canActivate will only execute when the parent component is not yet created. For example, if we navigate to the parent route it will be called, if we then navigate to a child route it will not. If we directly navigate to the child route, the canActivate guard will also be executed.
• canActivateChild will always be executed while navigating to/between child routes. For example, if we're at a child route child/1 and we navigate to child/2, the guard will get executed. If we directly navigate to a child route, the guard will also get called. If we navigate to the parent route, the canActivateChild guard will not be fired.
• because canActivate is guarding the parent route, the child parameters (and data) are not available on the ActivatedRouteSnapshot of the canActivate guard. To be able to access the child parameters, we have to drill down the child components on the RouterStateSnapshot.

Nice to knows

• because the parent component gets created first, the canActivate guard will always be called first.
• if we directly navigate to a child component and the child guard returns a falsy value then the parent component will also not be created, because the navigation is cancelled when one of the guards return a falsy value.
• when the canActivate guard returns a falsy value, then the canActivateChild guard will not be called.
• the canActivateChild guard can be rewritten as a canActivate guard on every child route.

Example application

Follow me on Twitter at @tim_deschryver | Subscribe to the Newsletter | Originally published on timdeschryver.dev.