What is resolver in Angular?

A Resolver in Angular is a service that provides a way to fetch data before navigating to a component. A Resolver acts as a middleware between the Router and the component, and is used to retrieve data that is required by the component to display its view.

Here’s an example of a basic Resolver implementation:

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

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

  constructor(private dataService: DataService) {}

  resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<any> {
    return this.dataService.getData();
  }
}

In this example, the DataResolver retrieves data from the DataService before navigating to the component. The Router will wait for the data to be retrieved before it continues with the navigation. The resolved data can then be passed to the component via the ActivatedRoute and used to display the view.

To use a Resolver in your application, you need to add it to the route configuration in your RoutingModule:

import { DataResolver } from './data.resolver';

const routes: Routes = [
  {
    path: 'data',
    component: DataComponent,
    resolve: {
      data: DataResolver
    }
  }
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule { }

In this example, the DataResolver is added to the resolve property of the data route, which means that it will be triggered every time the user navigates to the data route.