angular-docs-cn/aio/content/guide/lazy-loading-ngmodules.md

Lazy-loading feature modules

By default, NgModules are eagerly loaded, which means that as soon as the app loads, so do all the NgModules, whether or not they are immediately necessary. For large apps with lots of routes, consider lazy loading—a design pattern that loads NgModules as needed. Lazy loading helps keep initial bundle sizes smaller, which in turn helps decrease load times.

For the final sample app with two lazy-loaded modules that this page describes, see the .

{@a lazy-loading}

Lazy loading basics

This section introduces the basic procedure for configuring a lazy-loaded route. For a step-by-step example, see the step-by-step setup section on this page.

To lazy load Angular modules, use loadchildren (instead of component) in your AppRoutingModule routes configuration as follows.

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

In the lazy-loaded module's routing module, add a route for the component.

const routes: Routes = [ { path: '', component: ItemsComponent } ];

Also be sure to remove the ItemsModule from the AppModule. For step-by-step instructions on lazy loading modules, continue with the following sections of this page.

{@a step-by-step}

Step-by-step setup

There are two main steps to setting up a lazy-loaded feature module:

1. Create the feature module with the CLI, using the --route flag.
2. Configure the routes.

Set up an app

If you don’t already have an app, you can follow the steps below to create one with the CLI. If you already have an app, skip to Configure the routes. Enter the following command where customer-app is the name of your app:

ng new customer-app --routing

This creates an app called customer-app and the --routing flag generates a file called app-routing.module.ts, which is one of the files you need for setting up lazy loading for your feature module. Navigate into the project by issuing the command cd customer-app.

The --routing option requires Angular/CLI version 8.1 or higher. See Keeping Up to Date.

Create a feature module with routing

Next, you’ll need a feature module with a component to route to. To make one, enter the following command in the terminal, where customers is the name of the feature module. The path for loading the customers feature modules is also customers because it is specified with the --route option:

ng generate module customers --route customers --module app.module

This creates a customers folder having the new lazy-loadable feature module CustomersModule defined in the customers.module.ts file and the routing module CustomersRoutingModule defined in the customers-routing.module.ts file. The command automatically declares the CustomersComponent and imports CustomersRoutingModule inside the new feature module.

Because the new module is meant to be lazy-loaded, the command does NOT add a reference to the new feature module in the application's root module file, app.module.ts. Instead, it adds the declared route, customers to the routes array declared in the module provided as the --module option.

Notice that the lazy-loading syntax uses loadChildren followed by a function that uses the browser's built-in import('...') syntax for dynamic imports. The import path is the relative path to the module.

Add another feature module

Use the same command to create a second lazy-loaded feature module with routing, along with its stub component.

ng generate module orders --route orders --module app.module

This creates a new folder called orders containing the OrdersModule and OrdersRoutingModule, along with the new OrdersComponent source files. The orders route, specified with the --route option, is added to the routes array inside the app-routing.module.ts file, using the lazy-loading syntax.

Set up the UI

Though you can type the URL into the address bar, a navigation UI is easier for the user and more common. Replace the default placeholder markup in app.component.html with a custom nav so you can easily navigate to your modules in the browser:

To see your app in the browser so far, enter the following command in the terminal window:

ng serve

Then go to localhost:4200 where you should see “customer-app” and three buttons.

These buttons work, because the CLI automatically added the routes to the feature modules to the routes array in app.module.ts.

{@a config-routes}

Imports and route configuration

The CLI automatically added each feature module to the routes map at the application level. Finish this off by adding the default route. In the app-routing.module.ts file, update the routes array with the following:

The first two paths are the routes to the CustomersModule and the OrdersModule. The final entry defines a default route. The empty path matches everything that doesn't match an earlier path.

Inside the feature module

Next, take a look at the customers.module.ts file. If you’re using the CLI and following the steps outlined in this page, you don’t have to do anything here.

The customers.module.ts file imports the customers-routing.module.ts and customers.component.ts files. CustomersRoutingModule is listed in the @NgModule imports array giving CustomersModule access to its own routing module. CustomersComponent is in the declarations array, which means CustomersComponent belongs to the CustomersModule.

The app-routing.module.ts then imports the feature module, customers.module.ts using JavaScript's dynamic import.

The feature-specific route definition file customers-routing.module.ts imports its own feature component defined in the customers.component.ts file, along with the other JavaScript import statements. It then maps the empty path to the CustomersComponent.

The path here is set to an empty string because the path in AppRoutingModule is already set to customers, so this route in the CustomersRoutingModule, is already within the customers context. Every route in this routing module is a child route.

The other feature module's routing module is configured similarly.

Verify lazy loading

You can check to see that a module is indeed being lazy loaded with the Chrome developer tools. In Chrome, open the dev tools by pressing Cmd+Option+i on a Mac or Ctrl+Shift+j on a PC and go to the Network Tab.

Click on the Orders or Customers button. If you see a chunk appear, everything is wired up properly and the feature module is being lazy loaded. A chunk should appear for Orders and for Customers but will only appear once for each.

To see it again, or to test after working in the project, clear everything out by clicking the circle with a line through it in the upper left of the Network Tab:

Then reload with Cmd+r or Ctrl+r, depending on your platform.

forRoot() and forChild()

You might have noticed that the CLI adds RouterModule.forRoot(routes) to the AppRoutingModule imports array. This lets Angular know that the AppRoutingModule is a routing module and forRoot() specifies that this is the root routing module. It configures all the routes you pass to it, gives you access to the router directives, and registers the Router service. Use forRoot() only once in the application, inside the AppRoutingModule.

The CLI also adds RouterModule.forChild(routes) to feature routing modules. This way, Angular knows that the route list is only responsible for providing additional routes and is intended for feature modules. You can use forChild() in multiple modules.

The forRoot() method takes care of the global injector configuration for the Router. The forChild() method has no injector configuration. It uses directives such as RouterOutlet and RouterLink. For more information, see the forRoot() pattern section of the Singleton Services guide.

{@a preloading}

Preloading

Preloading improves UX by loading parts of your app in the background. You can preload modules or component data.

Preloading modules

Preloading modules improves UX by loading parts of your app in the background so users don't have to wait for the elements to download when they activate a route.

To enable preloading of all lazy loaded modules, import the PreloadAllModules token from the Angular router.

import { PreloadAllModules } from '@angular/router';

Still in the AppRoutingModule, specify your preloading strategy in forRoot().

RouterModule.forRoot( appRoutes, { preloadingStrategy: PreloadAllModules } )

Preloading component data

To preload component data, you can use a resolver. Resolvers improve UX by blocking the page load until all necessary data is available to fully display the page.

Resolvers

Create a resolver service. With the CLI, the command to generate a service is as follows:

ng generate service

In your service, import the following router members, implement Resolve, and inject the Router service:

import { Resolve } from '@angular/router';

...

export class CrisisDetailResolverService implements Resolve<> { resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<> { // your logic goes here } }

Import this resolver into your module's routing module.

import { YourResolverService } from './your-resolver.service';

Add a resolve object to the component's route configuration.

{ path: '/your-path', component: YourComponent, resolve: { crisis: YourResolverService } }

In the component, use an Observable to get the data from the ActivatedRoute.

ngOnInit() { this.route.data .subscribe((your-parameters) => { // your data-specific code goes here }); }

For more information with a working example, see the routing tutorial section on preloading.

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More on NgModules and routing

You may also be interested in the following:

• Routing and Navigation.
• Providers.
• Types of Feature Modules.
• Route-level code-splitting in Angular
• Route preloading strategies in Angular