Angular Preloading Strategy for Lazy-Loaded Modules

In modern Angular applications, lazy loading modules is a common practice to improve initial load times. However, users might experience a delay when navigating to a lazily loaded route for the first time. This challenge focuses on implementing a preloading strategy to fetch these modules in the background, improving the user experience by making navigation feel instantaneous after the initial load.

Problem Description

Your task is to implement a custom preloading strategy for Angular's lazy-loaded modules. This strategy should define when and how lazily loaded modules are fetched. Specifically, you need to create a strategy that preloads modules after a specified delay and only if the network connection is considered "fast".

What needs to be achieved:

Implement a custom PreloadingStrategy service in Angular that intercepts route configurations with data: { preload: true }. This strategy will determine when to preload the associated module.

Key requirements:

1. Custom Preloading Strategy: Create a TypeScript class that implements the PreloadAllModules interface from @angular/router.
2. Conditional Preloading: The strategy should only preload a module if:
   • The route has data: { preload: true } configured.
   • A network connection is considered "fast".
   • A specified delay has passed since the application initialized.
3. Delay Mechanism: Implement a mechanism to introduce a configurable delay before preloading starts.
4. Network Speed Check: Implement a simple check to determine if the network connection is "fast" (e.g., using navigator.connection.effectiveType).
5. Module Loading: Use loadChildren from the route configuration to load the module. The PreloadingStrategy should return an observable that emits the loaded module.

Expected behavior:

When a route is configured with data: { preload: true }, the custom preloading strategy will monitor this route. After the defined delay and if the network is fast, the strategy will trigger the loading of the module associated with that route. The user should not experience a loading spinner when navigating to a preloaded route for the first time.

Important edge cases:

• No preload data: Routes without data: { preload: true } should not be preloaded.
• Slow network: If the network is detected as "slow", preloading should be skipped.
• Delay not passed: If the delay hasn't elapsed, preloading should be skipped.
• Multiple routes with preload: true: The strategy should handle multiple routes that are configured for preloading.
• Error handling during module load: While not the primary focus, consider how an error during preloading might be handled (though for this challenge, simply not loading is acceptable).

Examples

Example 1: Route Configuration

Imagine you have the following route configuration:

const routes: Routes = [
  {
    path: 'dashboard',
    loadChildren: () => import('./dashboard/dashboard.module').then(m => m.DashboardModule),
    data: { preload: true } // This route should be considered for preloading
  },
  {
    path: 'settings',
    loadChildren: () => import('./settings/settings.module').then(m => m.SettingsModule)
    // This route does NOT have { preload: true } and should NOT be preloaded
  }
];

Example 2: Preloading Logic (Conceptual)

Let's assume your custom strategy is configured with a delay = 3000 milliseconds and your navigator.connection.effectiveType is '4g'.

1. Application loads.
2. The router iterates through the routes.
3. It encounters the 'dashboard' route with data: { preload: true }.
4. The strategy checks if 3000ms have passed. (Initially, no).
5. The strategy checks navigator.connection.effectiveType. If it's '4g' (considered fast), it proceeds.
6. After 3000ms have passed, the strategy triggers the loadChildren observable for 'dashboard'.
7. The DashboardModule is loaded in the background.
8. When the user navigates to /dashboard, the module is already loaded, and navigation is instantaneous.
9. The 'settings' route is ignored because it lacks data: { preload: true }.

Example 3: Edge Case - Slow Network

If navigator.connection.effectiveType is '2g', even if data: { preload: true } is present and the delay has passed, the DashboardModule will not be preloaded.

Constraints

• The custom preloading strategy must be implemented in TypeScript.
• The delay for preloading should be configurable, defaulting to a reasonable value (e.g., 3 seconds).
• The "fast" network type should be based on navigator.connection.effectiveType. Consider '4g', '5g', 'ethernet', 'wifi' as "fast".
• The implementation should integrate with Angular's routing module configuration.
• Performance: The preloading mechanism itself should not significantly impact the initial application load performance. It should operate asynchronously.

Notes

• You will need to configure your application's RouterModule to use your custom preloading strategy.
• Consider using RxJS operators like delay, filter, and mergeMap within your preloading strategy.
• Accessing navigator.connection might require checking if the API is available in the browser environment.
• The goal is to implement the strategy itself. You'll likely need to mock or simulate navigator.connection for testing purposes if running outside a browser environment or if you want to precisely control the "network speed" for testing.
• The PreloadingStrategy interface requires a method named preload.