Prefetch Dynamic Data with the resolve Guard in Angular

One way to deal with getting and displaying data from an API is to route a user to a component, and then in that component’s ngOnInit hook call a method in a service to get the necessary data. While getting the data, perhaps the component can show a loading indicator. There’s another way however using what’s known as a route resolver, which allows you to get data before navigating to the new route.

server-resolver.service.ts

import {Injectable} from '@angular/core';
import {ActivatedRouteSnapshot, Resolve, RouterStateSnapshot} from '@angular/router';
import {Observable} from 'rxjs/Observable';
import {ServersService} from '../servers.service';

interface Server {
  id: number;
  name: string;
  status: string;
}

@Injectable({
  providedIn: 'root'
})
export class ServerResolverService implements Resolve<Server> {

  constructor(private serversService: ServersService) {
  }

  resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<Server> | Promise<Server> | Server {
    return this.serversService.getServer(+route.params['id']);
  }
}

app.module.ts

import {BrowserModule} from '@angular/platform-browser';
import {NgModule} from '@angular/core';
import {FormsModule} from '@angular/forms';
import {HttpModule} from '@angular/http';

import {AppComponent} from './app.component';
import {HomeComponent} from './home/home.component';
import {UsersComponent} from './users/users.component';
import {ServersComponent} from './servers/servers.component';
import {UserComponent} from './users/user/user.component';
import {EditServerComponent} from './servers/edit-server/edit-server.component';
import {ServerComponent} from './servers/server/server.component';
import {ServersService} from './servers/servers.service';
import {RouterModule, Routes} from '@angular/router';
import {PageNotFoundComponent} from './page-not-found/page-not-found.component';
import {AuthService} from './auth.service';
import {AuthGuard} from './auth.guard';
import {CanDeactivateGuard} from './can-deactivate.guard';
import {ErrorPageComponent} from './error-page/error-page.component';
import {ServerResolverService} from './servers/server/server-resolver.service';

const appRoutes: Routes = [
  {path: '', component: HomeComponent},
  {
    path: 'users', component: UsersComponent, children: [
      {path: ':id/:name', component: UserComponent}]
  },
  {
    path: 'servers', canActivateChild: [AuthGuard], component: ServersComponent, children: [
      {path: ':id', component: ServerComponent, resolve: {server: ServerResolverService}},
      {path: ':id/edit', component: EditServerComponent, canDeactivate: [CanDeactivateGuard]}
    ]
  },
  // {path: 'not-found', component: PageNotFoundComponent},
  {path: 'not-found', component: ErrorPageComponent, data: {message: 'Page not found'}},
  {path: '**', redirectTo: '/not-found'},
];

@NgModule({
  declarations: [
    AppComponent,
    HomeComponent,
    UsersComponent,
    ServersComponent,
    UserComponent,
    EditServerComponent,
    ServerComponent,
    PageNotFoundComponent,
    ErrorPageComponent
  ],
  imports: [
    BrowserModule,
    FormsModule,
    HttpModule,
    RouterModule.forRoot(appRoutes)
  ],
  providers: [ServersService, AuthGuard, AuthService, CanDeactivateGuard, ServerResolverService],
  bootstrap: [AppComponent]
})
export class AppModule {
}

server.component.ts

import {Component, OnInit} from '@angular/core';

import {ServersService} from '../servers.service';
import {ActivatedRoute, Data, Params, Router} from '@angular/router';

@Component({
  selector: 'app-server',
  templateUrl: './server.component.html',
  styleUrls: ['./server.component.css']
})
export class ServerComponent implements OnInit {
  server: { id: number, name: string, status: string };

  constructor(private serversService: ServersService,
              private route: ActivatedRoute,
              private router: Router) {
  }

  ngOnInit() {

    this.route.data.subscribe((value: Data) => {
      this.server = value['server'];
    });

    // const id = +this.route.snapshot.params['id'];
    // this.server = this.serversService.getServer(id);
    // this.route.params.subscribe((params: Params) => {
    //   this.server = this.serversService.getServer(+params['id']);
    // });
  }

  onEdit() {
    this.router.navigate(['edit'], {relativeTo: this.route, queryParamsHandling: 'preserve'});
  }
}

References
https://angular.io/guide/router#resolve-pre-fetching-component-data

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