Angular Preloading Lazy Modules: An In-depth Guide

What is Angular Preloading?

In Angular, preloading is a technique used to load lazy-loaded modules in the background after the application has been initially loaded. This can improve the performance and user experience of an Angular application by making necessary parts of the app available sooner than they would be with on-demand loading alone.

Why Preload Lazy Modules?

While lazy loading is effective for reducing the initial load time of an Angular application, it can lead to delays when navigating to routes that require these modules. Preloading addresses this issue by loading these modules in the background, reducing wait times when users navigate to different parts of the app.

How to Implement Preloading in Angular

Angular provides several strategies for preloading, such as NoPreloading, PreloadAllModules, and custom preloading strategies.

Basic Preloading Strategy

To implement preloading, you need to set it up in your AppRoutingModule as follows:

Configure Preloading in AppRoutingModule:

import { NgModule } from '@angular/core';
import { RouterModule, Routes, PreloadAllModules } from '@angular/router';

const routes: Routes = [
  {
    path: 'feature',
    loadChildren: () => import('./feature/feature.module').then(m => m.FeatureModule)
  }
];

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

Custom Preloading Strategy

Create a custom strategy if you need more control.

import { Injectable } from '@angular/core';
import { PreloadingStrategy, Route } from '@angular/router';
import { Observable, of } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class CustomPreloadingStrategy implements PreloadingStrategy {
  preload(route: Route, load: () => Observable<any>): Observable<any> {
    return route.data && route.data.preload ? load() : of(null);
  }
}        

Using the Custom Preloading Strategy:

import { RouterModule, Routes } from '@angular/router';
import { CustomPreloadingStrategy } from './custom-preloading.strategy';

const routes: Routes = [
  {
    path: 'feature',
    loadChildren: () => import('./feature/feature.module').then(m => m.FeatureModule),
    data: { preload: true }
  }
];

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

Advantages of Preloading Lazy Modules

1. Improved User Experience: Reduces the wait time for users navigating to lazy-loaded routes.
2. Efficient Resource Utilization: Background preloading optimizes resource usage without blocking the initial loading of the application.
3. Better Performance: Helps in balancing the load time by spreading it across the user’s idle time.

Disadvantages of Preloading Lazy Modules

1. Increased Initial Network Load: Preloading modules can increase the overall initial network load.
2. Potential Unnecessary Loading: Modules that might not be used during a session are still loaded, which can be wasteful.
3. Complexity: Custom strategies can add complexity to the routing configuration.

Best Practices for Preloading Lazy Modules

1. Selective Preloading: Use custom strategies to preload only the necessary modules.
2. Monitor Performance: Regularly monitor the application’s performance to ensure that preloading is having the desired effect.
3. User Behavior Analysis: Analyze user navigation patterns to determine which modules are worth preloading.
4. Balancing Load: Avoid overloading the initial network load by preloading only what is necessary.

Using ngx-quicklink for Preloading

ngx-quicklink is a library that improves upon the standard preloading strategy by only preloading modules associated with links currently visible in the viewport.

Setting up ngx-quicklink

Install ngx-quicklink:

npm install @angular/router ngx-quicklink        

Configure ngx-quicklink in AppModule:

import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
import { QuicklinkModule, QuicklinkStrategy } from 'ngx-quicklink';

const routes: Routes = [
  {
    path: 'feature',
    loadChildren: () => import('./feature/feature.module').then(m => m.FeatureModule)
  }
];

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

Conclusion

Preloading lazy modules in Angular enhances application performance and user experience by ensuring necessary resources are loaded in the background. While it offers significant advantages, it’s essential to carefully consider its implementation to avoid unnecessary network load and complexity. Leveraging tools like ngx-quicklink can further optimize preloading, making it more efficient and context-aware. By following best practices and tailoring the strategy to your application's needs, you can achieve a balanced and performant Angular application.