Understanding Resolvers in Angular

When building Angular applications, one of the common requirements is to fetch data before navigating to a route. For instance, consider a scenario where you want to display a user's details on a profile page. Instead of loading the route and showing a spinner while fetching data, wouldn't it be better to resolve the data before entering the route? This is where Angular Resolvers shine.

In this post, we'll explore what resolvers are, why they're essential, and how to use them effectively in your Angular applications

What is a Resolver in Angular?

A Resolver in Angular is a service that retrieves data before a route is activated. It ensures that your component is rendered with the required data already available, improving user experience by reducing the time spent waiting for asynchronous calls after the view is loaded.

Resolvers are part of Angular's Route Guards, which include CanActivate, CanDeactivate, CanLoad, and others. While these guards determine route activation, Resolvers focus specifically on fetching data.

Why Use Resolvers?

1. Optimized User Experience: By fetching data before rendering, you avoid rendering incomplete views with loading indicators.
2. Centralized Data Logic: Resolvers encapsulate data-fetching logic, keeping your components clean and focused on UI concerns.
3. Error Handling: Resolvers allow you to handle errors gracefully and redirect users if data retrieval fails.

Implementing a Resolver

Here’s a step-by-step guide to creating and using a Resolver in Angular

Step 1: Define the Resolver Function

Here’s how you can create a resolver using ResolveFn:

import { ResolveFn } from '@angular/router';
import { inject } from '@angular/core';
import { UserService } from './user.service';

export const userResolver: ResolveFn<User> = (route, state) => {
  const userService = inject(UserService);
  const userId = route.paramMap.get('id');
  return userService.getUserById(userId!);
};

In the above code we have:

1. Dependency Injection: The inject() function provides access to the UserService.
2. Route Parameters: Use route.paramMap.get('id') to fetch dynamic route parameters.
3. Return Value: Return an observable, promise, or synchronous value. Angular will wait for the observable to resolve before activating the route.

Step 2: Configure the Route

Add the resolver to the desired route in the RouterModule configuration.

import { Routes } from '@angular/router';
import { UserComponent } from './user.component';
import { userResolver } from './user.resolver';

const routes: Routes = [
  {
    path: 'user/:id',
    component: UserComponent,
    resolve: { userData: userResolver },
  },
];

Here, the resolve property maps the resolver (userResolver) to a key (userData) that can be accessed in the component.

Step 3: Access Resolved Data in the Component

We can access the resolved data in component by using ActivatedRoute’s data property.

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

@Component({
  selector: 'app-user',
  template: `
User Details
             
{{ user | json }}
`,
})
export class UserComponent implements OnInit {
  user: User | undefined;

  constructor(private route: ActivatedRoute) {}

  ngOnInit() {
    this.user = this.route.snapshot.data['userData'];
  }
}

Conclusion

Resolvers in Angular are a powerful tool for managing data preloading and improving user experience. By combining resolvers with Angular's routing and lazy-loading capabilities, you can build robust, user-friendly applications.

Happy learning!! 😊