What is Angular Lazy-loading feature modules?
Lazy-loading is a technique in Angular that allows the application to load feature modules only when they are needed, rather than loading all of the modules at the start of the application. This can improve the initial load time and performance of the application, as well as reduce the amount of memory used.
In Angular, lazy-loading is achieved by using the loadChildren property in the @NgModule decorator of the routing module. The loadChildren property is used to specify the location of the module that should be loaded lazily.
For example, if you have a feature module called ProductModule that should be loaded lazily, you would update your routing module as follows:
const routes: Routes = [
{ path: 'products', loadChildren: () => import('./product/product.module').then(m => m.ProductModule) }
];
This tells the router to load the ProductModule when the user navigates to the /products route.
It’s worth mentioning that lazy-loading feature modules can improve the performance of your application by reducing the initial bundle size and loading the necessary modules based on the user interactions.
How to step by step setup Angular Lazy-loading feature modules?
Here are the general steps to set up lazy-loading of feature modules in an Angular application:
1. Create a new feature module: Use the Angular CLI to create a new feature module using the ng generate module command. For example, to create a module called ProductModule, you would run ng generate module product.
2. Create routes for the feature module: In the new feature module, create a new routing module using the ng generate module command. For example, ng generate module product –routing. This will create a new product-routing.module.ts file in the product module folder.
3. Define the lazy-loaded routes: In the newly created routing module, import the RouterModule and Routes from @angular/router. Then define the routes for the feature module using the Routes type. For example:
const routes: Routes = [
{ path: '', component: ProductListComponent },
{ path: ':id', component: ProductDetailComponent },
];
4. Update the app-routing module: In the app-routing module, import the new feature module’s routing module and add it to the imports array. Use the loadChildren property to specify the location of the module that should be loaded lazily.
const routes: Routes = [
{ path: 'products', loadChildren: () => import('./product/product.module').then(m => m.ProductModule) }
];
5. Use the feature module: Once you’ve set up the lazy-loaded routes, you can use the components from the feature module in the app.component.html file or any other component.
6. Run the application: Once you’ve completed these steps, you should be able to run your application and see that the feature module is only loaded when the user navigates to the corresponding route.
It’s worth mentioning that this is a simple example, you can customize the routes based on your application requirements and keep in mind that the loadChildren property can accept a string or a function that returns a string or a promise that resolves to a string.
Angular forRoot() and forChild()
In Angular, the forRoot() and forChild() methods are used when configuring a module’s routing. These methods are used to set up the router service with the routes for a module.
forRoot() is used when configuring the routes for the root module of the application. It is typically used in the app.module.ts file and is used to set up the router service with the routes for the entire application. The forRoot() method takes an object that contains the routes for the application and returns a module with the router service configured with those routes.
forChild() is used when configuring the routes for a feature module. It is typically used in the feature module’s routing module and is used to set up the router service with the routes for that specific feature module. The forChild() method takes an object that contains the routes for the feature module and returns a module with the router service configured with those routes.
Here’s an example of how to use forRoot() and forChild() in an Angular application:
// app-routing.module.ts
import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
const routes: Routes = [
{ path: '', redirectTo: 'home', pathMatch: 'full' },
{ path: 'home', component: HomeComponent },
{ path: 'about', component: AboutComponent },
{ path: 'products', loadChildren: './product/product.module#ProductModule' },
];
@NgModule({
imports: [RouterModule.forRoot(routes)],
exports: [RouterModule]
})
export class AppRoutingModule { }
// product-routing.module.ts
import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
const routes: Routes = [
{ path: '', component: ProductListComponent },
{ path: ':id', component: ProductDetailComponent },
];
@NgModule({
imports: [RouterModule.forChild(routes)],
exports: [RouterModule]
})
export class ProductRoutingModule { }
In this example, forRoot() method is used to configure the router service with the routes for the entire application in the app-routing.module.ts file, while forChild() method is used to configure the router service with the routes for the ProductModule in the product-routing.module.ts file.
What is Angular Preloading?
In Angular, preloading is a technique that allows the application to load certain feature modules in the background while the user is interacting with the application. This can improve the overall performance of the application by reducing the time it takes for the feature modules to load when the user navigates to them.
Angular provides a preloading strategy called PreloadAllModules that preloads all feature modules as soon as the application starts. This strategy loads all feature modules in parallel with the application, which can improve the application’s start-up time.
To enable preloading in an Angular application, you need to import the RouterModule and call the forRoot() method with the PreloadAllModules strategy. Here’s an example of how to enable preloading in the app-routing.module.ts file:
import { NgModule } from '@angular/core';
import { RouterModule, Routes, PreloadAllModules } from '@angular/router';
const routes: Routes = [
{ path: '', redirectTo: 'home', pathMatch: 'full' },
{ path: 'home', component: HomeComponent },
{ path: 'about', component: AboutComponent },
{ path: 'products', loadChildren: './product/product.module#ProductModule' },
];
@NgModule({
imports: [RouterModule.forRoot(routes, {preloadingStrategy: PreloadAllModules})],
exports: [RouterModule]
})
export class AppRoutingModule { }
Angular Troubleshooting lazy-loading modules
Lazy-loading feature modules in Angular can be a powerful technique for improving the performance of your application, but it can also cause some issues if not implemented correctly. Here are some common issues that you might encounter when lazy-loading feature modules and how to troubleshoot them:
1. 404 error when navigating to a lazy-loaded route: This issue can occur if the path specified in the loadChildren property is incorrect. Make sure that the path to the module is correct and that the module is being exported correctly.
2. Error: Unexpected value ‘undefined’ imported by the module: This issue can occur if the module being imported is not being exported correctly. Make sure that the module is being exported in the correct way.
3. Error: Can’t resolve all parameters for X: (?): This issue can occur if the components or services in the lazy-loaded module have dependencies that are not being provided. Make sure that all dependencies are being provided correctly.
4. Error: No provider for X: This issue can occur if the services provided in the root module are not being propagated to the lazy-loaded modules. Make sure that the services are being provided correctly and that they are being imported correctly in the lazy-loaded module.
5. Error: Template parse errors: This issue can occur if the components in the lazy-loaded module are not being recognized by the Angular compiler. Make sure that the components are being imported correctly in the lazy-loaded module and that they are being declared in the declarations array of the @NgModule decorator.
If you run into any of these issues, it’s recommended to check the path of the module, the imports/exports, dependencies, services provided and the components declared in the modules. Also, it’s recommended to check the browser’s developer console for any errors that might give you more details about the issue.
