57 KiB

Raw Blame History

NgModules

NgModules help organize an application into cohesive blocks of functionality.

An NgModule is a class adorned with the @NgModule decorator function. @NgModule takes a metadata object that tells Angular how to compile and your code. It identifies the module's own components, directives, and pipes, making some of them public so external components can use them. @NgModule may add service providers to the application dependency injectors. And there are many more options covered here.

{@a bootstrap}

For a quick overview of NgModules, consider reading the Bootstrapping guide, which introduces NgModules and the essentials of creating and maintaining a single root AppModule for the entire application.

This page covers NgModules in greater depth.

Live examples

This page explains NgModules through a progression of improvements to a sample with a "Heroes" theme. Here's an index to live examples at key moments in the evolution of the sample:

The initial app
The first contact module
The revised contact module
Just before adding SharedModule
The final version

Frequently asked questions (FAQs)

This page covers NgModule concepts in a tutorial fashion.

The companion NgModule FAQs guide offers answers to specific design and implementation questions. Read this page before reading those FAQs.

{@a angular-modularity}

Angular modularity

NgModules are a great way to organize an application and extend it with capabilities from external libraries.

Many Angular libraries are NgModules (such as FormsModule, HttpModule, and RouterModule). Many third-party libraries are available as NgModules (such as Material Design, Ionic, AngularFire2).

NgModules consolidate components, directives, and pipes into cohesive blocks of functionality, each focused on a feature area, application business domain, workflow, or common collection of utilities.

NgModules can also add services to the application. Such services might be internally developed, such as the application logger. Services can come from outside sources, such as the Angular router and Http client.

NgModules can be loaded eagerly when the application starts. They can also be lazy-loaded asynchronously by the router.

An NgModule is a class decorated with @NgModule metadata. By setting metadata properties you tell Angular how your application parts fit together. For example, you can do the following:

Declare which components, directives, and pipes belong to the NgModule.
Export some of those classes so that other component templates can use them.
Import other NgModules with the components, directives, and pipes needed by the components in this NgModule.
Provide services at the application level that any application component can use.
Bootstrap the app with one or more top-level, root components.

{@a root-module}

The root AppModule

Every Angular app has at least one NgModule class, the root module. You bootstrap that NgModule to launch the application.

By convention, the root module class is called AppModule and it exists in a file named app.module.ts. The Angular CLI generates the initial AppModule for you when you create a project.

ng new quickstart

The root AppModule is all you need in a simple application with a few components.

As the app grows, you may refactor the root AppModule into feature modules that represent collections of related functionality. For now, stick with the root AppModule created by the CLI.

The initial declarations array identifies the application's only component, AppComponent, the root component at the top of the app's component tree.

Soon you'll declare more components (and directives and pipes too).

The @NgModule metadata imports a single helper module, BrowserModule, which every browser app must import. BrowserModule registers critical application service providers. It also includes common directives like NgIf and NgFor, which become immediately visible and usable in any of this NgModule's component templates.

The providers array registers services with the top-level dependency injector. There are no services to register ... yet.

Lastly, the bootstrap list identifies the AppComponent as the bootstrap component. When Angular launches the app, it renders the AppComponent inside the <app-root> element tag of the index.html.

Learn about that in the bootstrapping guide.

The CLI-generated AppComponent in this guide's sample has been simplified and consolidated into a single app.component.ts file like this:

Run the app and follow along with the steps in this guide:

ng serve

{@a declarations} {@a declare-directive}

Declare directives

{@a declarables}

As the app evolves, you'll add directives, components, and pipes (the declarables). You must declare each of these classes in an NgModule.

As an exercise, begin by adding a highlight.directive.ts to the src/app/ folder by hand.

The HighlightDirective is an attribute directive that sets the background color of its host element. Update the AppComponent template to attach this directive to the <h1> title element:

The screen of the running app has not changed. The <h1> is not highlighted. Angular does not yet recognize the highlight attribute and is ignoring it. You must declare the HighlightDirective in AppModule.

Edit the app.module.ts file, import the HighlightDirective, and add it to the AppModule declarations like this:

The Angular CLI would have done all of this for you if you'd created the HighlightDirective with the CLI command like this:

ng generate directive highlight

But you didn't. You created the file by hand so you must declare the directive by hand.

{@a declare-component}

Declare components

Now add a TitleComponent to the app and this time create it with the CLI.

ng generate component title --flat --no-spec --inline-style

The --flat flag tells the CLI to generate all files to the src/app/ folder.
The --no-spec flag skips the test (.spec) file.
The --inline-style flag prevents generation of the .css file (which you won't need).

To see which files would be created or changed by any ng generate command, append the --dryRun flag (-d for short).

Open the AppModule and look at the declarations where you will see that the CLI added the TitleComponent for you.

Now rewrite the title.component.html like this.

And move the title property from app.component.ts into the title.component.ts, which looks as follows after a little cleanup.

Rewrite AppComponent to display the new TitleComponent in the <app-title> element and get rid of the title property.

Error if component not declared

There was no visible clue when you neglected to declare the HighlightDirective attribute directive. The Angular compiler doesn't recognize highlight as an <h1> attribute but it doesn't complain either. You'd discover it was undeclared only if you were looking for its effect.

Now try removing the declaration of the TitleComponent from AppModule.

The Angular compiler behaves differently when it encounters an unrecognized HTML element. The app ceases to display the page and the browser console logs the following error

Uncaught Error: Template parse errors: 'app-title' is not a known element: 1. If 'app-title' is an Angular component, then verify that it is part of this NgModule. 2. If 'app-title' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.

If you don't get that error, you might get this one:

Uncaught Error: Component TitleComponent is not part of any NgModule or the module has not been imported into your module.

Always declare your components, directives, and pipes.

{@a providers}

Service providers

The Dependency Injection page describes the Angular hierarchical dependency-injection system and how to configure that system with providers.

NgModule providers

An NgModule can provide services. A single instance of each provided service becomes available for injection into every class created with that NgModule's injector (or one of its descendant injectors).

When Angular boots the application, it creates the root AppModule with a root dependency injector. Angular configures the root injector with the providers specified in the module's @NgModule.providers.

Later, when Angular creates a new instance of a class— be it a component, directive, service, or module— that new class can be injected with an instance of a service provided to the root injector by the AppModule.

Angular also configures the root injector with the providers specified by imported NgModules. An NgModule's own providers are registered after imported NgModule providers. When there are multiple providers for the same injection token, the last registration wins.

Compared to Component providers

Providing a service in @Component.providers metadata means that a new service instance will be created for each new instance of that component and will be available for injection into all of that component instance's descendant sub-components.

The service instance won't be injected into any other component instances. Other instances of the same component class cannot see it. Sibling and ancestor component instances cannot see it.

Component providers always supersede NgModule providers. A component provider for injection token X creates a new service instance that "shadows" an NgModule provider for injection token X. When the component or any of its sub-components inject X, they get the component service instance, not the NgModule service instance.

Should you provide a service in an NgModule or a component? The answer depends on how you want to scope the service. If the service should be widely available, provide it in an NgModule. If it should be visible only within a component tree, provide it in the component at the root of that tree.

NgModule provider example

Many applications capture information about the currently logged-in user and make that information accessible through a user service.

Use the CLI to create a UserService and provide it in the root AppModule.

ng generate service user --module=app

This command creates a skeleton UserService in src/app/user.service.ts and a companion test file, src/app/user.service.spec.ts.

The --module=app flag tells the CLI to provide the service class in the NgModule defined in the src/app/app.module.ts file.

If you omit the --module flag, the CLI still creates the service but does not provide it anywhere. You have to do that yourself.

Confirm that the --module=app flag did provide the service in the root AppModule by inspecting the @NgModule.providers array in src/app/app.module.ts

Replace the generated contents of src/app/user.service.ts with the following dummy implementation.

Update the TitleComponent class with a constructor that injects the UserService and sets the component's user property from the service.

Update the TitleComponent template to show the welcome message below the application title.

{@a imports}

NgModule imports

In the revised TitleComponent, an *ngIf directive guards the message. There is no message if there is no user.

Although AppModule doesn't declare the NgIf directive, the application still compiles and runs. How can that be? The Angular compiler should either ignore or complain about unrecognized HTML.

Importing BrowserModule

Angular does recognize NgIf because the AppModule imports it indirectly when it imports BrowserModule.

Importing BrowserModule made all of its public components, directives, and pipes visible to the templates of components declared in AppModule, which include TitleComponent.

{@a reexport}

Re-exported NgModules

The NgIf directive isn't declared in BrowserModule. It's declared in CommonModule from @angular/common.

CommonModule contributes many of the common directives that applications need, including ngIf and ngFor.

AppModule doesn't import CommonModule directly. But it benefits from the fact that BrowserModule imports CommonModule and re-exports it.

The net effect is that an importer of BrowserModule gets CommonModule directives automatically as if it had declared them itself.

Many familiar Angular directives don't belong to CommonModule. For example, NgModel and RouterLink belong to Angular's FormsModule and RouterModule respectively. You must import those NgModules before you can use their directives.

To illustrate this point, you'll extend the sample app with contact editor whose ContactComponent is a form component. You'll have to import form support from the Angular FormsModule.

{@a add-contact-editor}

Add a contact editor

Imagine that you added the following contact editor files to the project by hand without the help of the CLI.

Form components are often complex and this is one is no exception. To make it manageable, all contact-related files are in an src/app/contact folder.

The ContactComponent implementation is spread over three constituent HTML, TypeScript, and css files.

There's a custom pipe (called Awesome), a ContactHighlightDirective, and a ContactService for fetching contacts.

The ContactService was added to the AppModule providers.

Now any class can inject the application-wide instances of the ContactService and UserService.

Import supporting FormsModule

The ContactComponent is written with Angular forms in the template-driven style.

Notice the [(ngModel)] binding in the middle of the component template, contact.component.html.

Two-way data binding [(ngModel)]is typical of the template-driven style. The ngModel is the selector for the NgModel directive. Although NgModel is an Angular directive, the Angular compiler won't recognize it for two reasons:

AppModule doesn't declare NgModel (and shouldn't).
NgModel wasn't imported via BrowserModule.

ContactComponent wouldn't behave like an Angular form anyway because form features such as validation aren't part of the Angular core.

To correct these problems, the AppModule must import both the BrowserModule and the FormsModule from '@angular/forms' like this.

You can write Angular form components in template-driven or reactive style. NgModules with components written in the reactive style import the ReactiveFormsModule.

Now [(ngModel)] binding will work and the user input will be validated by Angular forms, once you declare the new component, pipe, and directive.

Never re-declare

Importing the FormsModule makes the NgModelDirective (and all of the other FORMS_DIRECTIVES) available to components declared in AppModule.

Do not also add these directives to the AppModule metadata's declarations.

Never re-declare classes that belong to another NgModule. Components, directives, and pipes should be declared in exactly one NgModule.

{@a declare-pipe}

Declare pipes

The revised application still won't compile until you declare the contact component, directive, and pipe.

Components and directives are declarables. So are pipes.

You learned earlier to generate and declare both components and directives with the CLI ng generate commands.

There's also a CLI command to generate and declare the AwesomePipe:

ng generate pipe awesome

However, if you write these class files by hand or opt-out of declaration with the --skip-import flag, you'll have to add the declarations yourself.

You were told to add the contact editor files by hand, so you must manually update the declarations in the AppModule:

Display the ContactComponent

Update the AppComponent template to display the ContactComponent by placing an element with its selector (<app-contact>) just below the title.

Run the app

Everything is in place to run the application with its contact editor.

Try the example:

Selector conflicts

Look closely at the screen. Notice that the background of the application title text is blue. It should be gold (see src/app/app.component.html).

Only the contact name should be blue (see src/app/contact/contact.component.html).

What went wrong?

This application defines two highlight directives that set the background color of their host elements with a different color (gold and blue).

One is defined at the root level (src/app/highlight.directive.ts); the other is in the contact editor folder (src/app/contact/contact-highlight.directive.ts).

Their class names are different (HighlightDirective and ContactHighlightDirective) but their selectors both match any HTML element with a highlight attribute.

Both directives are declared in the same AppModule so both directives are active for all components declared in AppModule.

There's nothing intrinsically wrong with multiple directives selecting the same element. Each could modify the element in a different, non-conflicting way.

In this case, both directives compete to set the background color of the same element. The directive that's declared later (ContactHighlightDirective) always wins because its DOM changes overwrite the changes by the earlier HighlightDirective.

The ContactHighlightDirective will make the application title text blue when it should be gold. Only the contact name should be blue (see src/app/contact/contact.component.html).

If you cannot rename the selectors, you can resolve the conflicts by creating feature modules that insulate the declarations in one NgModule from the declarations in another.

While it is legal to declare two directives with the same selector in the same NgModule, the compiler will not let you declare two components with the same selector in the same NgModule because it cannot insert multiple components in the same DOM location.

Nor can you import an NgModule that declares the same selector as another component in this NgModule. The reason is the same: an HTML element may be controlled by at most one Angular component.

Either rename the selectors or use feature modules to eliminate the conflict.

Feature modules

This tiny app is already experiencing structural issues.

The root AppModule grows larger with each new application class.
There are conflicting directives. The ContactHighlightDirective in the contact re-colors the work done by the HighlightDirective declared in AppModule and colors the application title text when it should color only the ContactComponent.
The app lacks clear boundaries between contact functionality and other application features. That lack of clarity makes it harder to assign development responsibilities to different teams.

Feature modules can help resolve these issues.

Architecturally, a feature module is an NgModule class that is dedicated to an application feature or workflow. Technically, it's another class adorned by the @NgModule decorator, just like a root AppModule.

Feature module metadata have the same properties as root module metadata. When loaded together, the root module and the feature module share the same dependency injector, which means the services provided in a feature module are available to all.

These two module types have the following significant technical differences:

You boot the root module to launch the app; you import a feature module to extend the app.
A feature module can expose or hide its declarables from other NgModules.

Otherwise, a feature module is distinguished primarily by its intent.

A feature module delivers a cohesive set of functionality focused on an application business domain, user workflow, facility (forms, http, routing), or collection of related utilities. Feature modules help you partition the app into areas of specific interest and purpose.

A feature module collaborates with the root module and with other NgModules through the services it provides and the components, directives, and pipes that it shares.

{@a contact-module-v1}

Make contact editor a feature

In this section, you refactor the contact editor functionality out of the root AppModule and into a dedicated feature module by following these steps.

Create the ContactModule feature module in its own folder.
Copy the contact editor declarations and providers from AppModule to ContactModule.
Export the ContactComponent.
Import the ContactModule into the AppModule.
Cleanup the AppModule.

You'll create one new ContactModule class and change one existing AppModule class. All other files are untouched.

Create the feature module

Generate the ContactModule and its folder with an Angular CLI command.

ng generate module contact

Here's the generated ContactModule.

After modifying the initial ContactsModule as outlined above, it looks like this.

The following sections discuss the important changes.

Import CommonModule

Notice that ContactModule imports CommonModule, not BrowserModule. The CLI module generation took care of this for you.

Feature module components need the common Angular directives but not the services and bootstrapping logic in BrowserModule. See the NgModule FAQs for more details.

Import FormsModule

The ContactModule imports the FormsModule because its ContactComponent uses NgModel, one of the FormsModule directives.

NgModules don't inherit access to the declarations of the root AppModule or any other NgModule. Each NgModule must import what it needs. Because ContactComponent needs the form directives, its ContactModule must import FormsModule.

Copy declarations

The ContactModule declares the contact editor components, directives and pipes.

The app fails to compile at this point, in part because ContactComponent is currently declared in both the AppModule and the ContactModule. A component may only be declared in one NgModule. You'll fix this problem shortly.

{@a root-scoped-providers}

Providers are root-scoped

The ContactModule provides the ContactService and the AppModule will stop providing it after refactoring.

Architecturally, the ContactService belongs to the contact editor domain. Classes in the rest of the app do not need the ContactService and shouldn't inject it. So it makes sense for the ContactModule to provide the ContactService as it does.

You might expect that the ContactService would only be injectable in classes declared or provided in the ContactModule.

That's not the case. Any class anywhere can inject the ContactService because ContactModule providers are root-scoped.

To be precise, all eagerly loaded modules— modules loaded when the application starts — are root-scoped. This ContactModule is eagerly loaded.

You will learn that services provided in lazy-loaded modules have their own scope.

Angular does not have module-scoping mechanism. Unlike components, NgModule instances do not have their own injectors so they can't have their own provider scopes.

ContactService remains an application-scoped service because Angular registers all NgModule providers with the application's root injector. This is true whether the service is provided directly in the root AppModule or in an imported feature module like ContactModule.

In practice, service scoping is rarely an issue. Components don't accidentally inject a service. To inject the ContactService, you'd have to import its type and explicitly inject the service into a class constructor. Only contact editor components should import the ContactService type.

If it's really important to you to restrict the scope of a service, provide it in the feature's top-level component (ContactComponent in this case).

For more on this topic, see "How do I restrict service scope to a module?" in the NgModule FAQs.

Export public-facing components

The ContactModule makes the ContactComponent public by exporting it.

Declared classes are private by default. Private declarables may only appear in the templates of components declared by the same NgModule. They are invisible to components in other NgModules.

That's a problem for the AppComponent. Both components used to be declared in AppModule so Angular could display the ContactComponent within the AppComponent. Now that the ContactComponent is declared in its own feature module. The AppComponent cannot see it unless it is public.

The first step toward a solution is to export the ContactComponent. The second step is to import the ContactModule in the AppModule, which you'll do when you refactor the AppModule.

The AwesomePipe and ContactHighlightDirective remain private and are hidden from the rest of the application.

The ContactHighlightDirective, being private, no longer overrides the HighlightDirective in the AppComponent. The background of the title text is gold as intended.

{@a refactor-appmodule}

Refactor the AppModule

Return to the AppModule and remove everything specific to the contact editor feature set. Leave only the classes required at the application root level.

Delete the contact editor import statements.
Delete the contact editor declarations and providers.
Delete the FormsModule from the imports list (the AppComponent doesn't need it).
Import the ContactModule so the app can continue to display the exported ContactComponent.

Here's the refactored AppModule, presented side-by-side with the previous version.

Improvements

There's a lot to like in the revised AppModule.

It does not change as the Contact domain grows.
It only changes when you add new NgModules.
It's simpler:
- Fewer import statements.
- No FormsModule import.
- No contact editor declarations.
- No ContactService provider.
- No highlight directive conflicts.

Try this ContactModule version of the sample.

Try the live example.

{@a routing-modules} {@a lazy-loaded-modules}

Routing modules

Navigating the app with the Angular Router reveals new dimensions of the NgModule.

In this segment, you'll learn to write routing modules that configure the router. You'll discover the implications of lazy loading a feature module with the router's loadChildren method.

Imagine that the sample app has evolved substantially along the lines of the Tour of Heroes tutorial.

The app has three feature modules: Contact, Hero (new), and Crisis (new).
The Angular router helps users navigate among these modules.
The ContactComponent is the default destination when the app starts.
The ContactModule continues to be eagerly loaded when the application starts.
HeroModule and the CrisisModule are lazy-loaded.

There's too much code behind this sample app to review every line. Instead, the guide explores just those parts necessary to understand new aspects of NgModules.

You can examine the complete source for this version of the app in the live example.

{@a app-component-template}

The root AppComponent

The revised AppComponent template has a title, three links, and a <router-outlet>.

The <app-contact> element that displayed the ContactComponent is gone; you're routing to the Contact page now.

The root AppModule

The AppModule is slimmer now.

The AppModule is no longer aware of the application domains such as contacts, heroes, and crises. Those concerns are pushed down to ContactModule, HeroesModule, and CrisisModule respectively and only the routing configuration knows about them.

The significant change from version 2 is the addition of the AppRoutingModule to the NgModule imports. The AppRoutingModule is a routing module that handles the app's routing concerns.

AppRoutingModule

The router is the subject of the Routing & Navigation guide, so this section skips many routing details and concentrates on the intersection of NgModules and routing.

You can specify router configuration directly within the root AppModule or within a feature module.

The Router guide recommends instead that you locate router configuration in separate, dedicated NgModules, called routing modules. You then import those routing modules into their corresponding root or feature modules.

The goal is to separate the normal declarative concerns of an NgModule from the often complex router configuration logic.

By convention, a routing module's name ends in ...RoutingModule. The top-level root module is AppModule and it imports its companion routing module called AppRoutingModule.

Here is this app's AppRoutingModule, followed by a discussion.

The AppRoutingModule defines three routes:

The first route redirects the empty URL (such as http://host.com/) to another route whose path is contact (such as http://host.com/contact).

The contact route isn't defined within the AppRoutingModule. It's defined in the Contact feature's own routing module, ContactRoutingModule.

It's standard practice for feature modules with routing components to define their own routes. You'll get to ContactRoutingModule in a moment.

The remaining two routes use lazy loading syntax to tell the router where to find the modules for the hero and crisis features:

A lazy-loaded NgModule location is a string, not a type. In this app, the string identifies both the NgModule file and the NgModule class, the latter separated from the former by a #.

Routing module imports

A routing module typically imports the Angular RouterModule so it can register routes.

It may also import a feature module which registers routes (either directly or through its companion routing module).

This AppRoutingModule does both.

It first imports the ContactModule, which as you'll see, imports its own ContactRoutingModule.

Import order matters! Because "contacts" is the first defined route and the default route for the app, you must import it before all other routing-related modules.

The second import registers the routes defined in this module by calling the RouterModule.forRoot class method.

The forRoot method does two things:

Configures the router with the supplied routes.
Initializes the Angular router itself.

Call RouterModule.forRoot exactly once for the entire app.

Calling it in the AppRoutingModule, the companion to the root AppModule, is a good way to ensure that this method is called exactly once.

Never call RouterModule.forRoot in a feature's routing module.

Re-export RouterModule

All routing modules should re-export the RouterModule.

Re-exporting RouterModule makes the router directives available to the companion module that imports it. This is a considerable convenience for the importing module.

For example, the AppComponent template relies on the routerLink directive to turn the user's clicks into navigations. The Angular compiler only recognizes routerLink because

AppComponentis declared by AppModule,
AppModule imports AppRoutingModule,
AppRoutingModule exports RouterModule, and
RouterModule exports the RouterLink directive.

If AppRoutingModule didn't re-export RouterModule, the AppModule would have to import the RouterModule itself.

{@a contact-routing-module}

Routing to a feature module

The three feature modules (ContactModule, HeroModule, CrisisModule) have corresponding routing modules (ContactRoutingModule, HeroRoutingModule, CrisisRoutingModule).

They follow the same pattern as the AppRoutingModule.

define routes
register the routes with Angular's RouterModule
export the RouterModule.

The ContactRoutingModule is the simplest of the three. It defines and registers a single route to the ContactComponent.

There is one critical difference from AppRoutingModule: you pass the routes to RouterModule.forChild, not forRoot.

Always call RouterModule.forChild in a feature-routing module. Never call RouterModule.forRoot.

ContactModule changes

Because the app navigates to the ContactComponent instead of simply displaying it in the AppComponent template, the ContactModule has changed.

It imports the ContactRoutingModule.
It no longer exports ContactComponent.

The ContactComponent is only displayed by the router, No template references its <app-contact> selector. There's no reason to make it public via the exports array.

Here is the latest version, side-by-side with the previous version.

{@a hero-module}

Lazy-loaded routing

The HeroModule and CrisisModule have corresponding routing modules, HeroRoutingModule and CrisisRoutingModule.

The app lazy loads the HeroModule and the CrisisModule. That means the HeroModule and the CrisisModule are not loaded into the browser until the user navigates to their components.

Do not import the HeroModule or CrisisModule or any of their classes outside of their respective file folders. If you do, you will unintentionally load those modules and all of their code when the application starts, defeating the purpose of lazy loading.

For example, if you import the HeroService in AppModule, the HeroService class and all related hero classes will be loaded when the application starts.

Lazy loading can improve the app's perceived performance because the browser doesn't have to process lazy-loaded code when the app starts. It may never process that code.

You cannot tell that these modules are lazy-loaded by looking at their routing modules. They happen to be a little more complex than ContactRoutingModule. For example, The HeroRoutingModule has child routes. But the added complexity springs from intrinsic hero and crisis functionality, not from lazy loading. Fundamentally, these routing modules are just like ContactRoutingModule and you write them the same way.

{@a lazy-load-DI}

Lazy-loaded NgModule providers

There is a runtime difference that can be significant. Services provided by lazy-loaded NgModules are only available to classes instantiated within the lazy-loaded context. The reason has to do with dependency injection.

When an NgModule is eagerly loaded as the application starts, its providers are added to the application's root injector. Any class in the application can inject a service from the root injector.

When the router lazy loads an NgModule, Angular instantiates the module with a child injector (a descendant of the root injector) and adds the module's providers to this child injector. Classes created with the child injector can inject one of its provided services. Classes created with root injector cannot.

Each of the three feature modules has its own data access service. Because the ContactModule is eagerly loaded when the application starts, its ContactService is provided by the application's root dependency injector. That means the ContactService can be injected into any application class, including hero and crisis components.

Because CrisisModule is lazy-loaded, its CrisisService is provided by the CrisisModule child injector. It can only be injected into one of the crisis components. No other kind of component can inject the CrisisService because no other kind of component can be reached along a route that lazy loads the CrisisModule.

Lazy-loaded NgModule lifetime

Both eager and lazy-loaded NgModules are created once and never destroyed. This means that their provided service instances are created once and never destroyed.

As you navigate among the application components, the router creates and destroys instances of the contact, hero, and crisis components. When these components inject data services provided by their modules, they get the same data service instance each time.

If the HeroService kept a cache of unsaved changes and the user navigated to the ContactComponent or the CrisisListComponent, the pending hero changes would remain in the one HeroService instance, waiting to be saved.

But if you provided the HeroService in the HeroComponent instead of the HeroModule, new HeroService instances would be created each time the user navigated to a hero component. Previously pending hero changes would be lost.

To illustrate this point, the sample app provides the HeroService in the HeroComponent rather than the HeroModule.

Run the app, open the browser development tools, and look at the console as you navigate among the feature pages.

// App starts ContactService instance created. ... // Navigate to Crisis Center CrisisService instance created. ... // Navigate to Heroes HeroService instance created. ... // Navigate to Contact HeroService instance destroyed. ... // Navigate back to Heroes HeroService instance created.

The console log shows the HeroService repeatedly created and destroyed. The ContactService and CrisisService are created but never destroyed, no matter where you navigate.

Run it

Try this routed version of the sample.

Try the live example.

{@a shared-module}

Shared modules

The app is shaping up. But there are a few annoying problems. There are three unnecessarily different highlight directives and the many files cluttering the app folder level could be better organized.

You can eliminate the duplication and tidy-up by writing a SharedModule to hold the common components, directives, and pipes. Then share this NgModule with the other NgModules that need these declarables.

Use the CLI to create the SharedModule class in its src/app/shared folder.

ng generate module shared

Now refactor as follows:

Move the AwesomePipe from src/app/contact to src/app/shared.
Move the HighlightDirective from src/app/hero to src/app/shared.
Delete the highlight directive classes from src/app/ and src/app/contact.
Update the SharedModule as follows:

Note the following:

It declares and exports the shared pipe and directive.
It imports and re-exports the CommonModule and FormsModule
It can re-export FormsModule without importing it.

Re-exporting NgModules

Technically, there is no need for SharedModule to import CommonModule or FormsModule. SharedModule doesn't declare anything that needs material from CommonModule or FormsModule.

But NgModules that would like to import SharedModule for its pipe and highlight directive happen also to declare components that need NgIf and NgFor from CommonModule and do two-way binding with [(ngModel)] from the FormsModule.

Normally, they'd have to import CommonModule and FormsModule as well as SharedModule. Now they can just import SharedModule. By exporting CommonModule and FormsModule, SharedModule makes them available to its importers for free.

A trimmer ContactModule

See how ContactModule became more concise, compared to its previous version:

Notice the following:

The AwesomePipe and ContactHighlightDirective are gone.
The imports include SharedModule instead of CommonModule and FormsModule.
The new version is leaner and cleaner.

Why TitleComponent isn't shared

SharedModule exists to make commonly used components, directives, and pipes available for use in the templates of components in many other NgModules.

The TitleComponent is used only once by the AppComponent. There's no point in sharing it.

{@a no-shared-module-providers}

Why UserService isn't shared

While many components share the same service instances, they rely on Angular dependency injection to do this kind of sharing, not the NgModule system.

Several components of the sample inject the UserService. There should be only one instance of the UserService in the entire application and only one provider of it.

UserService is an application-wide singleton. You don't want each NgModule to have its own separate instance. Yet there is a real danger of that happening if the SharedModule provides the UserService.

Do not specify app-wide singleton providers in a shared module. A lazy-loaded NgModule that imports that shared module makes its own copy of the service.

{@a core-module}

The Core module

At the moment, the root folder is cluttered with the UserService and TitleComponent that only appear in the root AppComponent. You didn't include them in the SharedModule for reasons just explained.

Instead, gather them in a single CoreModule that you import once when the app starts and never import anywhere else.

Perform the following steps:

Create a CoreModule class in an src/app/core folder.
Move the TitleComponent and UserService from src/app/ to src/app/core.
Declare and export the TitleComponent.
Provide the UserService.
Update the root AppModule to import CoreModule.

Most of this work is familiar. The interesting part is the CoreModule.

You're importing some extra symbols from the Angular core library that you're not using yet. They'll become relevant later in this page.

The @NgModule metadata should be familiar. You declare the TitleComponent because this NgModule owns it. You export it because AppComponent (which is in AppModule) displays the title in its template. TitleComponent needs the Angular NgIf directive that you import from CommonModule.

CoreModule provides the UserService. Angular registers that provider with the app root injector, making a singleton instance of the UserService available to any component that needs it, whether that component is eagerly or lazily loaded.

Why bother?

This scenario is clearly contrived. The app is too small to worry about a single service file and a tiny, one-time component.

A TitleComponent sitting in the root folder isn't bothering anyone. The root AppModule can register the UserService itself, as it does currently, even if you decide to relocate the UserService file to the src/app/core folder.

Real-world apps have more to worry about. They can have several single-use components (such as spinners, message toasts, and modal dialogs) that appear only in the AppComponent template. You don't import them elsewhere so they're not shared in that sense. Yet they're too big and messy to leave loose in the root folder.

Apps often have many singleton services like this sample's UserService. Each must be registered exactly once, in the app root injector, when the application starts.

While many components inject such services in their constructors—and therefore require JavaScript import statements to import their symbols—no other component or NgModule should define or re-create the services themselves. Their providers aren't shared.

We recommend collecting such single-use classes and hiding their details inside a CoreModule. A simplified root AppModule imports CoreModule in its capacity as orchestrator of the application as a whole.

A trimmer AppModule

Here is the updated AppModule paired with version 3 for comparison:

AppModule now has the following qualities:

A little smaller because many src/app/root classes have moved to other NgModules.
Stable because you'll add future components and providers to other NgModules, not this one.
Delegated to imported NgModules rather than doing work.
Focused on its main task, orchestrating the app as a whole.

{@a core-for-root}

Configure core services with CoreModule.forRoot

An NgModule that adds providers to the application can offer a facility for configuring those providers as well.

By convention, the forRoot static method both provides and configures services at the same time. It takes a service configuration object and returns a ModuleWithProviders, which is a simple object with the following properties:

ngModule: the CoreModule class
providers: the configured providers

The root AppModule imports the CoreModule and adds the providers to the AppModule providers.

More precisely, Angular accumulates all imported providers before appending the items listed in @NgModule.providers. This sequence ensures that whatever you add explicitly to the AppModule providers takes precedence over the providers of imported NgModules.

Add a CoreModule.forRoot method that configures the core UserService.

You've extended the core UserService with an optional, injected UserServiceConfig. If a UserServiceConfig exists, the UserService sets the user name from that config.

Here's CoreModule.forRoot that takes a UserServiceConfig object:

Lastly, call it within the imports list of the AppModule.

The app displays "Miss Marple" as the user instead of the default "Sherlock Holmes".

Call forRoot only in the root module, AppModule. Calling it in any other NgModule, particularly in a lazy-loaded NgModule, is contrary to the intent and can produce a runtime error.

Remember to import the result; don't add it to any other @NgModule list.

{@a prevent-reimport}

Prevent reimport of the CoreModule

Only the root AppModule should import the CoreModule. Bad things happen if a lazy-loaded NgModule imports it.

You could hope that no developer makes that mistake. Or you can guard against it and fail fast by adding the following CoreModule constructor.

The constructor tells Angular to inject the CoreModule into itself. That seems dangerously circular.

The injection would be circular if Angular looked for CoreModule in the current injector. The @SkipSelf decorator means "look for CoreModule in an ancestor injector, above me in the injector hierarchy."

If the constructor executes as intended in the AppModule, there is no ancestor injector that could provide an instance of CoreModule. The injector should give up.

By default, the injector throws an error when it can't find a requested provider. The @Optional decorator means not finding the service is OK. The injector returns null, the parentModule parameter is null, and the constructor concludes uneventfully.

It's a different story if you improperly import CoreModule into a lazy-loaded NgModule such as HeroModule (try it).

Angular creates a lazy-loaded NgModule with its own injector, a child of the root injector. @SkipSelf causes Angular to look for a CoreModule in the parent injector, which this time is the root injector. Of course it finds the instance imported by the root AppModule. Now parentModule exists and the constructor throws the error.

Conclusion

You made it! You can examine and download the complete source for this final version from the live example.

Frequently asked questions

Now that you understand NgModules, you may be interested in the companion NgModule FAQs page with its ready answers to specific design and implementation questions.

57 KiB Raw Blame History

NgModules

Live examples

Frequently asked questions (FAQs)

Angular modularity

The root AppModule

Declare directives

Declare components

Error if component not declared

Service providers

NgModule providers

Compared to Component providers

NgModule provider example

NgModule imports

Importing BrowserModule

Re-exported NgModules

Add a contact editor

Import supporting FormsModule

Never re-declare

Declare pipes

Display the ContactComponent

Run the app

Selector conflicts

Feature modules

Make contact editor a feature

Create the feature module

Import CommonModule

Import FormsModule

Copy declarations

Providers are root-scoped

Export public-facing components

Refactor the AppModule

Improvements

Routing modules

The root AppComponent

The root AppModule

AppRoutingModule

Routing module imports

Re-export RouterModule

Routing to a feature module

ContactModule changes

Lazy-loaded routing

Lazy-loaded NgModule providers

Lazy-loaded NgModule lifetime

Run it

Shared modules

Re-exporting NgModules

A trimmer ContactModule

Why TitleComponent isn't shared

Why UserService isn't shared

The Core module

Why bother?

A trimmer AppModule

Configure core services with CoreModule.forRoot

Prevent reimport of the CoreModule

Conclusion

Frequently asked questions

57 KiB

Raw Blame History