Resolver In Angular

In Angular, a resolver is a type of service that helps to fetch data asynchronously before a route is activated. Resolvers are used to ensure that the data required by a component is available before the component is rendered, preventing empty or incomplete data from being displayed.

A resolver implements the Resolve interface from the @angular/router package and provides a resolve() method. The resolve() method returns an observable or a promise that resolves to the data that needs to be fetched. The Resolve interface requires the implementation of the resolve() method, which takes an ActivatedRouteSnapshot object as a parameter and returns a promise or an observable.

Here’s an example of a resolver that returns data as an observable:

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 DataResolver implements Resolve<any> {

  constructor(private dataService: DataService) { }

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

In this example, we’re creating a resolver called DataResolver that returns data as observable. The resolve() method calls a getData() method on a DataService to fetch the data.

To use a resolver in a route, you can add it to the resolve property of the route configuration in the routing module. Here's an example:

import { NgModule } from '@angular/core';
import { Routes, RouterModule } from '@angular/router';
import { DataComponent } from './data.component';
import { DataResolver } from './data.resolver';

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

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

In this example, we’re adding the DataResolver to the resolve property of the route configuration for a route that displays the DataComponent. The resolved data is then made available to the component through the ActivatedRoute service.

To access the resolved data in the component, you can inject the ActivatedRoute service and subscribe to the data property of the snapshot object. Here's an example:

import { Component } from '@angular/core';
import { ActivatedRoute } from '@angular/router';

@Component({
  selector: 'app-data',
  template: '<p>{{ data }}</p>'
})
export class DataComponent {
  data: any;

  constructor(private route: ActivatedRoute) { }

  ngOnInit() {
    this.route.data.subscribe((data: any) => {
      this.data = data.data;
    });
  }
}        

In this example, we’re injecting the ActivatedRoute service and subscribing to the data property of the snapshot object. The resolved data is then assigned to the data property of the component.

Overall, resolvers are a powerful tool in Angular for managing asynchronous data loading before a route is activated. By using resolvers effectively, we can ensure that our components always have the necessary data available, leading to a better user experience and more robust application behaviour.

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