Angular Preloading Strategy : Enhancing User Experience and Performance

Angular offers several features to enhance performance and improve the user experience. One of these features is preloading, a mechanism to load modules in the background after the application has been bootstrapped.At the heart of this feature is the PreloadStrategy, a powerful tool for optimizing Angular applications.

In this blog post, we’ll explore what PreloadStrategy is, its significance, how it works, and how to customize it to suit your needs.

What is PreloadStrategy?

In Angular applications, lazy loading is commonly used to load modules only when needed, reducing the initial bundle size and improving load time. However, lazy loading might cause a delay when users navigate to a route whose module hasn’t been loaded yet. Preloading bridges this gap by loading lazy-loaded modules in the background after the application is initialized.

PreloadStrategy is an Angular interface that defines the strategy for preloading these modules. By default, Angular provides two built-in strategies:

1. NoPreloading: Disables preloading entirely.
2. PreloadAllModules: Preloads all lazy-loaded modules as soon as possible.

Why Use Preloading?

Preloading enhances user experience by:

• Reducing Navigation Latency: Modules are preloaded before users navigate to them, making transitions smoother.
• Balancing Performance: By preloading selectively, you can optimize network usage and avoid unnecessary loads.

How Does Preloading Work?

Angular’s preloading mechanism relies on the router. When defining routes, you specify lazy-loaded modules with the loadChildren property. Preloading strategies dictate whether these modules should be loaded immediately after the application starts or deferred until navigation.

Here’s an example of enabling preloading with the PreloadAllModules strategy:

Step 1: Configuring the Router

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

const routes: Routes = [
  { path: '', redirectTo: 'home', pathMatch: 'full' },
  { path: 'home', loadChildren: () => import('./home/home.module').then(m => m.HomeModule) },
  { path: 'about', loadChildren: () => import('./about/about.module').then(m => m.AboutModule) },
];

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

Step 2: Including the Router in Your Application

Ensure your application’s AppModule imports the AppRoutingModule:

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppRoutingModule } from './app-routing.module';
import { AppComponent } from './app.component';

@NgModule({
  declarations: [AppComponent],
  imports: [BrowserModule, AppRoutingModule],
  bootstrap: [AppComponent]
})
export class AppModule {}

Customizing PreloadStrategy

Sometimes, preloading all modules might not be ideal. You may want to preload specific modules based on conditions like user roles or network bandwidth. For such cases, you can create a custom PreloadStrategy.

Creating a Custom PreloadStrategy

A custom strategy involves implementing the PreloadStrategy interface and overriding the preload method:

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

@Injectable({ providedIn: 'root' })
export class CustomPreloadStrategy implements PreloadingStrategy {
  preload(route: Route, load: () => Observable<any>): Observable<any> {
    // Check for a custom data property to decide whether to preload
    if (route.data && route.data['preload']) {
      return load();
    }
    return of(null);
  }
}

Using the Custom Strategy

To use your custom strategy, configure the router as follows:

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

And add data properties to your routes:

const routes: Routes = [
  { path: 'home', loadChildren: () => import('./home/home.module').then(m => m.HomeModule), data: { preload: true } },
  { path: 'about', loadChildren: () => import('./about/about.module').then(m => m.AboutModule), data: { preload: false } },
];

Conclusion

Preloading strategies in Angular are essential for balancing application performance and user experience. The default strategies—NoPreloading and PreloadAllModules—cover basic needs, but you can tailor preloading behavior by implementing custom strategies. By leveraging preloading effectively, you can create applications that are fast and responsive, delighting your users.

Find out more about Preloading here.

Happy learning!! 😊