Angular Interview Questions

A resolver in Angular is a feature used to pre-fetch data before a route is activated. It helps in ensuring that the necessary data for a component is available as soon as the component is loaded. This can prevent situations where a component is rendered without the required data, thereby improving the user experience by avoiding loading indicators or incomplete displays.

Key Concepts of a Resolver:

1. Pre-fetching Data:

   • Resolvers fetch data before the route gets activated and the component is instantiated. This ensures that the component has all the required data when it is rendered.

2. Route Guards:

   • Resolvers are a type of route guard in Angular. They implement the Resolve interface and are used in the route configuration.

3. Observables:

   • Resolvers return an Observable, Promise, or any value that can be resolved asynchronously.

Implementing a Resolver in Angular:

Here’s a step-by-step guide to implementing a resolver:

1. Generate a Resolver:

   • Use the Angular CLI to generate a resolver.

     ng generate service data

2. Implement the Resolver Logic:

   • Implement the resolver by creating a service that implements the Resolve interface from @angular/router.
   • Example:

     import { Injectable } from '@angular/core';
     import { Resolve, ActivatedRouteSnapshot, RouterStateSnapshot } from '@angular/router';
     import { Observable } from 'rxjs';
     import { DataService } from './data.service';
     
     @Injectable({
       providedIn: 'root'
     })
     export class DataResolverService implements Resolve<any> {
     
       constructor(private dataService: DataService) {}
     
       resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<any> {
         return this.dataService.getData();
       }
     }

3. Define Routes Using the Resolver:

   • Use the resolver in the route configuration to pre-fetch data.
   • Example:

     import { NgModule } from '@angular/core';
     import { RouterModule, Routes } from '@angular/router';
     import { HomeComponent } from './home/home.component';
     import { DetailsComponent } from './details/details.component';
     import { DataResolverService } from './data-resolver.service';
     
     const routes: Routes = [
       { path: '', component: HomeComponent },
       {
         path: 'details/:id',
         component: DetailsComponent,
         resolve: { data: DataResolverService }
       }
     ];
     
     @NgModule({
       imports: [RouterModule.forRoot(routes)],
       exports: [RouterModule]
     })
     export class AppRoutingModule { }

4. Access the Resolved Data in the Component:

   • Access the resolved data in the target component using ActivatedRoute.
   • Example:

     import { Component, OnInit } from '@angular/core';
     import { ActivatedRoute } from '@angular/router';
     
     @Component({
       selector: 'app-details',
       templateUrl: './details.component.html'
     })
     export class DetailsComponent implements OnInit {
       data: any;
     
       constructor(private route: ActivatedRoute) {}
     
       ngOnInit(): void {
         this.route.data.subscribe((data: { data: any }) => {
           this.data = data.data;
         });
       }
     }

Example Scenario:

1. Generating a Data Service:

   ng generate service data

2. Implementing the Data Service:

   import { Injectable } from '@angular/core';
   import { Observable, of } from 'rxjs';
   
   @Injectable({
     providedIn: 'root'
   })
   export class DataService {
     getData(): Observable<any> {
       // Replace with actual data fetching logic
       return of({ id: 1, name: 'Test Data' });
     }
   }

3. Implementing the Data Resolver:

   import { Injectable } from '@angular/core';
   import { Resolve, ActivatedRouteSnapshot, RouterStateSnapshot } from '@angular/router';
   import { Observable } from 'rxjs';
   import { DataService } from './data.service';
   
   @Injectable({
     providedIn: 'root'
   })
   export class DataResolverService implements Resolve<any> {
   
     constructor(private dataService: DataService) {}
   
     resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<any> {
       return this.dataService.getData();
     }
   }

4. Configuring Routes with the Resolver:

   import { NgModule } from '@angular/core';
   import { RouterModule, Routes } from '@angular/router';
   import { HomeComponent } from './home/home.component';
   import { DetailsComponent } from './details/details.component';
   import { DataResolverService } from './data-resolver.service';
   
   const routes: Routes = [
     { path: '', component: HomeComponent },
     {
       path: 'details/:id',
       component: DetailsComponent,
       resolve: { data: DataResolverService }
     }
   ];
   
   @NgModule({
     imports: [RouterModule.forRoot(routes)],
     exports: [RouterModule]
   })
   export class AppRoutingModule { }

5. Accessing the Resolved Data in the Component:

   import { Component, OnInit } from '@angular/core';
   import { ActivatedRoute } from '@angular/router';
   
   @Component({
     selector: 'app-details',
     templateUrl: './details.component.html'
   })
   export class DetailsComponent implements OnInit {
     data: any;
   
     constructor(private route: ActivatedRoute) {}
   
     ngOnInit(): void {
       this.route.data.subscribe((data: { data: any }) => {
         this.data = data.data;
       });
     }
   }

In Summary:

A resolver in Angular is used to pre-fetch data before a route is activated, ensuring that the component has all the necessary data when it is rendered. This is implemented by creating a service that implements the Resolve interface, configuring the route to use the resolver, and accessing the resolved data in the component using ActivatedRoute. Resolvers enhance the user experience by ensuring that components are fully populated with data as soon as they are displayed.