What Are Resolvers in Angular Routing
💡 Concept Name
Route Resolver – A service in Angular that retrieves data before a route is activated, ensuring required data is available to the component immediately.
📘 Quick Intro
Resolvers in Angular routing are used to pre-fetch data before a component loads. They enhance the user experience by avoiding empty states or extra loading indicators after navigation.
🧠 Analogy / Short Story
Imagine ordering food in a restaurant and waiting until your meal is ready before being seated. Similarly, Angular waits for resolver data to load before entering a route.
🔧 Technical Explanation
- 🛠 A resolver is a service implementing the
Resolve<T>interface. - 🚦 It is configured in the
resolveproperty of route definitions. - 📡 Angular calls the resolver before navigating to the route.
- 📁 Once resolved, data is injected via
ActivatedRoute.data. - 🧩 Useful for fetching configuration, user profile, or preloading resources.
🎯 Purpose & Use Case
- ✅ Pre-fetch user details before profile page loads.
- ✅ Load configuration settings or app metadata before navigation.
- ✅ Improve perceived performance by avoiding UI flashes or loading spinners.
💻 Real Code Example
// 1. Create resolver service
import { Injectable } from '@angular/core';
import { Resolve } from '@angular/router';
import { Observable } from 'rxjs';
import { UserService } from './user.service';
@Injectable({ providedIn: 'root' })
export class UserResolver implements Resolve<any> {
constructor(private userService: UserService) {}
resolve(): Observable<any> {
return this.userService.getUser();
}
}
// 2. Define resolver in route
const routes: Routes = [
{
path: 'profile',
component: ProfileComponent,
resolve: { user: UserResolver }
}
];
// 3. Access resolved data in component
constructor(private route: ActivatedRoute) {
this.route.data.subscribe(data => {
console.log(data['user']);
});
}
❓ Interview Q&A
Q1: What is a resolver in Angular routing?
A: A resolver is a service that retrieves data before a route is activated.
Q2: Which interface must a resolver implement?
A: Resolve<T>
Q3: Where is a resolver defined in the route config?
A: Inside the resolve property of the route object.
Q4: How do components access resolved data?
A: Using ActivatedRoute.data subscription.
Q5: When is a resolver executed?
A: Before the component is loaded during navigation.
📝 MCQs
Q1. What does a resolver in Angular do?
- Guards against route changes
- Fetches data before route activation
- Creates routes dynamically
- Binds data after route loads
Q2. Which interface is used for resolvers?
- OnInit
- Resolve<T>
- ActivatedRoute
- CanActivate
Q3. Where is a resolver defined?
- In constructor
- Inside template
- In the resolve property of route
- In ngOnInit
Q4. When does Angular call a resolver?
- After route loads
- During component init
- Before route activation
- On page refresh only
Q5. How to access resolved data?
- Using @Input
- Using resolve()
- Using ActivatedRoute.data
- Using HttpClient
Q6. Which method must be implemented in a resolver?
- transform()
- activate()
- resolve()
- guard()
Q7. What does the resolve() method return?
- Boolean
- Promise only
- Observable or data
- Route definition
Q8. What happens if resolver fails?
- Component still loads
- Nothing
- Navigation can be canceled
- Route is ignored
Q9. Can you use multiple resolvers in one route?
- No
- Yes
- Only one per module
- Only with guards
Q10. Where are resolvers typically provided?
- In component
- In HTML
- In root or module
- In route params
💡 Bonus Insight
Resolvers can help avoid flickering UIs and improve performance by ensuring data is ready before component instantiation. They work well in tandem with guards for secure and efficient routing.
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