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Angular Lazy loading

When the development of an application starts, it just contains a small set of features so the app usually loads fast. However, as new features are added, the overall application size grows up and its loading speed decreases. It is in this context where Lazy loading finds its place. Lazy loading is a design pattern that defers initialization of objects until it is needed, so, for example, users that just access to a website’s home page do not need to have other areas loaded. Angular handles lazy loading through the routing module which redirects to requested pages. Those pages can be loaded at start or on demand.

An example with Angular

To explain how lazy loading is implemented using angular, a basic sample app is going to be developed. This app will consist in a window named "level 1" that contains two buttons that redirects to other windows in a "second level". It is a simple example, but useful to understand the relation between angular modules and lazy loading.

Figure 1. Levels app structure.

This graphic shows that modules acts as gates to access components "inside" them.

Because the objective of this guide is related mainly with logic, the html structure and SCSS styles are less relevant, but the complete code can be found as a sample here.

Implementation

First write in a console ng new level-app --routing, to generate a new project called level-app including an app-routing.module.ts file (--routing flag). If you are using Nx, the command would be nx generate @nrwl/angular:app level-app --routing in your Nx workspace.

In the file app.component.html delete all the content except the router-outlet tag.

File app.component.html

<router-outlet></router-outlet>

The next steps consists on creating features modules.

run ng generate module first --routing to generate a module named first.
run ng generate module first/second-left --routing to generate a module named second-left under first.
run ng generate module first/second-right --routing to generate a module second-right under first.
run ng generate component first/first to generate a component named first inside the module first.
run ng generate component first/second-left/content to generate a component content inside the module second-left.
run ng generate component first/second-right/content to generate a component content inside the module second-right.

If you are using Nx, you have to specify the project name (level-app) along with the --project flag. For example, command for generating the first module will be `ng generate module first --project=level-app --routing`

To move between components we have to configure the routes used:

In app-routing.module.ts add a path 'first' to FirstComponent and a redirection from '' to 'first'.

File app-routing.module.ts.

...
import { FirstComponent } from './first/first/first.component';

const routes: Routes = [
  {
    path: 'first',
    component: FirstComponent
  },
  {
    path: '',
    redirectTo: 'first',
    pathMatch: 'full',
  },
];

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

In app.module.ts import the module which includes FirstComponent.

File app.module.ts

....
import { FirstModule } from './first/first.module';

@NgModule({
  ...
  imports: [
    ....
    FirstModule
  ],
  ...
})
export class AppModule { }

In first-routing.module.ts add routes that direct to the content of SecondRightModule and SecondLeftModule. The content of both modules have the same name so, in order to avoid conflicts the name of the components are going to be changed using as ( original-name as new-name).

File first-routing.module.ts

...
import { ContentComponent as ContentLeft} from './second-left/content/content.component';
import { ContentComponent as ContentRight} from './second-right/content/content.component';
import { FirstComponent } from './first/first.component';

const routes: Routes = [
  {
    path: '',
    component: FirstComponent
  },
  {
    path: 'first/second-left',
    component: ContentLeft
  },
  {
    path: 'first/second-right',
    component: ContentRight
  }
];

@NgModule({
  imports: [RouterModule.forChild(routes)],
  exports: [RouterModule]
})
export class FirstRoutingModule { }

In first.module.ts import SecondLeftModule and SecondRightModule.

File first.module.ts

...
import { SecondLeftModule } from './second-left/second-left.module';
import { SecondRightModule } from './second-right/second-right.module';

@NgModule({
  ...
  imports: [
    ...
    SecondLeftModule,
    SecondRightModule,
  ]
})
export class FirstModule { }

Using the current configuration, we have a project that loads all the modules in a eager way. Run ng serve (with --project=level-app in an Nx workspace) to see what happens.

First, during the compilation we can see that just a main file is built.

Figure 2. Compile eager.

If we go to http://localhost:4200/first and open developer options (F12 on Chrome), it is found that a document named "first" is loaded.

Figure 3. First level eager.

If we click on [Go to right module] a second level module opens, but there is no 'second-right' document.

Figure 4. Second level right eager.

But, typing the URL directly will load 'second-right' but no 'first', even if we click on [Go back]

Figure 5. Second level right eager direct URL.

Modifying an angular application to load its modules lazily is easy, you have to change the routing configuration of the desired module (for example FirstModule).

File app-routing.module.ts.

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

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

Notice that instead of loading a component, you dynamically import it in a loadChildren attribute because modules acts as gates to access components "inside" them. Updating the app to load lazily has four consequences:

No component attribute.
No import of FirstComponent.
FirstModule import has to be removed from the imports array at app.module.ts.
Change of context.

If we check first-routing.module.ts again, we can see that the path for ContentLeft and ContentRight is set to 'first/second-left' and 'first/second-right' respectively, so writing http://localhost:4200/first/second-left will redirect us to ContentLeft. However, after loading a module with loadChildren setting the path to 'second-left' and 'second-right' is enough because it acquires the context set by AppRoutingModule.

File first-routing.module.ts

const routes: Routes = [
  {
    path: '',
    component: FirstComponent
  },
  {
    path: 'second-left',
    component: ContentLeft
  },
  {
    path: 'second-right',
    component: ContentRight
  }
];

If we go to 'first' then FirstModule is situated in '/first' but also its children ContentLeft and ContentRight, so it is not necessary to write in their path 'first/second-left' and 'first/second-right', because that will situate the components on 'first/first/second-left' and 'first/first/second-right'.

Figure 6. First level lazy wrong path.

When we compile an app with lazy loaded modules, files containing them will be generated

Figure 7. First level lazy compilation.

And if we go to developer tools → network, we can find those modules loaded (if they are needed).

Figure 8. First level lazy.

To load the component ContentComponent of SecondLeftModule lazily, we have to load SecondLeftModule as a children of FirstModule:

Change component to loadChildren and reference SecondLeftModule.

File first-routing.module.ts.

const routes: Routes = [
  {
    path: '',
    component: FirstComponent
  },
  {
    path: 'second-left',
    loadChildren: () => import('./second-left/second-left.module').then(m => m.SecondLeftModule),
  },
  {
    path: 'second-right',
    component: ContentRight
  }
];

Remove SecondLeftModule at first.component.ts
Route the components inside SecondLeftModule. Without this step nothing would be displayed.

File second-left-routing.module.ts.

...
import { ContentComponent } from './content/content.component';

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

@NgModule({
  imports: [RouterModule.forChild(routes)],
  exports: [RouterModule]
})
export class SecondLeftRoutingModule { }

run ng serve to generate files containing the lazy modules.

Figure 9. Second level lazy loading compilation.

Clicking on [Go to left module] triggers the load of SecondLeftModule.

Figure 10. Second level lazy loading network.

Conclusion

Lazy loading is a pattern useful when new features are added, these features are usually identified as modules which can be loaded only if needed as shown in this document, reducing the time spent loading an application.