What are Angular Resolvers?

Angular resolvers are functions that resolve data before a component is loaded. They are used to fetch data from an API or other data sources and return it to the component. The benefit of using a resolver is that it ensures that the data is available before the component is loaded, preventing any issues with undefined data or errors.

Using Angular Resolvers

To use an Angular resolver, we need to follow these steps:

1. Define the resolver

2. Add the resolver to the route

3. Inject the resolved data into the component

Let's go through these steps in more detail.

1. Define the Resolver

To define a resolver, we need to create a new class and implement the Resolve interface from the @angular/router package. The Resolve interface has a single method called resolve, which takes a route and a state and returns an Observable or Promise of the resolved data.

Here's an example of a resolver that fetches data from an API:

import { Injectable } from '@angular/core';
import { Resolve, ActivatedRouteSnapshot } 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): Observable<any> {
    return this.dataService.getData(route.params.id);
  }
}

In this example, we're injecting a service called DataService, which is responsible for fetching data from the API. We're also implementing the Resolve interface and defining the resolve method, which takes the current route as a parameter and returns an Observable of the resolved data.

1. Add the Resolver to the Route

Once we've defined the resolver, we need to add it to the route that we want to use it for. We can do this by adding a resolve property to the route configuration.

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/:id',
    component: DataComponent,
    resolve: {
      data: DataResolver
    }
  }
];

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

In this example, we're defining a route for the DataComponent and adding a resolve property to the route configuration. We're mapping the data property to the DataResolver, which we defined earlier.

1. Inject the Resolved Data into the Component

Finally, we need to inject the resolved data into the component. We can do this by adding a parameter to the component's constructor with the same name that we used in the route configuration.

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

@Component({
  selector: 'app-data',
  template: '{{ data | json }}'
})
export class DataComponent {
  constructor(private route: ActivatedRoute) {}

  get data() {
    return this.route.snapshot.data.data;
  }
}

In this example, we're injecting the ActivatedRoute service into the component and using the snapshot property to get the resolved data from the route configuration. We're then using this data in the template to display it.

Using Angular resolvers can help to ensure that data is available before a component is loaded.