In the
Service in Angular With Examples post we saw an example of creating service where provider metadata was provided with in the
@Injectable() decorator.
@Injectable({
providedIn: 'root',
})
But that is not the only way to register a provider in Angular, there is a complete injector hierarchy and the number of service instances created depends on how you register the provider.
Providing services at different levels
For any service you are going to use in your Angular app you must register at least one provider. The provider can be part of the service's
own metadata, making that service available everywhere, or you can register providers with specific modules or components. There are the
following three ways for registering a provider for the service.
1. In the @Injectable() decorator for the service itself. By default, the Angular CLI command ng generate service registers a provider with the root injector for your service by including provider metadata in the @Injectable() decorator.
@Injectable({
providedIn: 'root',
})
When you provide the service at the root level, Angular creates a single, shared instance of the Service and injects it into any class that
asks for it. As per Angular docs Using the @Injectable() providedIn property is preferable to the @NgModule() providers array because that
allows Angular to optimize an app by removing the service from the compiled app if it isn't used, this process of optimization is knows as
tree-shaking.
2. You can also register a provider with a specific NgModule. In that case same servcie instance is available to all components in that
NgModule.
For example configuring an app-wide provider in the @NgModule() of AppModule. You will have to use the providers property of the @NgModule() decorator.
@NgModule({
providers: [LoggerService],
bootstrap: [AppComponent]
})
export class AppModule { }
3. You can also register a provider at the component level, in that case you get a new instance of the service with each new instance
of that component.
To register a service provider at the component level you can use the providers property of the
@Component() decorator.
@Component({
selector: 'app-user',
templateUrl: './user.component.html',
styleUrls: ['./user.component.css'],
providers: [ LoggerService ]
})
export class UserComponent implements OnInit {
..
..
}
Injector hierarchies in Angular
Based on these types of service providers there are two injector hierarchies in Angular.
- ModuleInjector hierarchy— Configure a ModuleInjector in this hierarchy using an @NgModule() or @Injectable() annotation. A single
service instance will be used at the root level or modulr level based upon the configuration used.
- ElementInjector hierarchy— Angular creates ElementInjectors implicitly for each DOM element. When you provide services in a component,
that service is available via the ElementInjector at that component instance. It will also be available at child components if you are not
providing it again in the child component using the providers property.
Service provider example using @Injectable() decorator
In the post
Service in Angular With Examples there is an example of creating a LoggerService which is decorated with @Injectable() decorator
to provide the metadata that allows Angular to inject it into a component as a dependency.
Service provider example using @NgModule
Let’s see the same example by registering service in providers array of @NgModule.
LoggerService class
Now @Injectable decorator is not used in LoggerService class.
export class LoggerService {
constructor() { }
log(msg: string) {
console.log(msg);
}
error(msg: string) {
console.error(msg);
}
}
Service is registered in the AppModule class now, refer the providers properties of @NgModule decorator.
import { AppRoutingModule } from './app-routing.module';
import { AppComponent } from './app.component';
import { UserComponent } from './user/user.component';
import { LoggerService } from './services/logger.service';
@NgModule({
declarations: [
AppComponent,
UserComponent
],
imports: [
BrowserModule,
AppRoutingModule
],
providers: [LoggerService],
bootstrap: [AppComponent]
})
export class AppModule { }
Service provider example at component level
In an example for registering provider at component level we’ll create an Angular application that shows the existing user and has a UI to add
a new User.
When a new user is added and submit button is clicked that user details should be added to the list of existing users.
After submit button is clicked.
In the example there is a UserService class that has an array of users and a function to add new user.
import { User } from '../user/user.model';
export class UserService {
users = [new User('Jack', 62, new Date('2005-03-25')),
new User('Olivia', 26, new Date('2014-05-09')),
new User('Manish', 34, new Date('2010-10-21'))] ;
addUser(user: User){
this.users.push(user);
}
}
For User objects a User model class is used.
export class User {
name : string;
age : number;
joinDate : Date;
constructor(name: string, age : number, joinDate : Date) {
this.name = name;
this.age = age;
this.joinDate = joinDate;
}
}
AppComponent class
This is the component class where UserService is configured with in the providers array. In the ngOnInit() method all the existing users
are assigned to the users variable, which is an array of type User, by calling the UserService.
import { Component, OnInit } from '@angular/core';
import { User } from './user/user.model';
import { UserService } from './services/user.service';
@Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css'],
providers: [UserService]
})
export class AppComponent implements OnInit{
users: User[] = [];
constructor(private userService: UserService){}
ngOnInit(){
this.users = this.userService.users;
}
}
app.component.html
Template has selector tags for the child components.
<div class="container">
<div class="row">
<div class="col-xs-12 col-md-8">
<app-adduser></app-adduser>
<h3>User Details</h3>
<app-user *ngFor="let u of users" [user]="u"></app-user>
</div>
</div>
</div>
AddUserComponent
In the AddUserComponent which is the child component of the AppComponent, UserService is provided again. There is also a method addUser()
that gets the entered value for the new User, create a User object and call the addUser() method of the userService to add the entered user
to the array of existing users.
import { User } from './user.model';
@Component({
selector: 'app-adduser',
templateUrl: './adduser.component.html',
styleUrls: ['./adduser.component.css'],
providers: [UserService]
})
export class AddUserComponent {
constructor(private userService: UserService){}
addUser(name: HTMLInputElement, age: HTMLInputElement, date: HTMLInputElement){
this.userService.addUser(new User(name.value, Number(age.value), new Date(date.value)));
// resetting input elements
name.value = '';
age.value = '';
date.value = '';
}
}
adduser.component.html
HTML template for providing a UI to add new user. It has three fields for name, age and joining date and a submit button. There is also
click event binding calling the addUser() method.
<div class="row">
<div class="col-xs-12 col-md-8">
<div class="form-group">
<label for="name">Name:</label>
<input class="form-control" placeholder="Enter name" id="name" #enteredname>
</div>
<div class="form-group">
<label for="age">Age:</label>
<input class="form-control" placeholder="Enter age" id="age" #enteredage>
</div>
<div class="form-group">
<label for="joindate">Joining Date:</label>
<input class="form-control" placeholder="Enter joining date" id="joindate" #entereddate>
</div>
<button (click)="addUser(enteredname, enteredage, entereddate)" class="btn btn-primary">Submit</button>
</div>
</div>
UserComponent
In the component class there is a property binding using @Input decorator. In the AppComponent template there is a for loop that binds user to this property in each iteration, that’s how user details are displayed.
import { Component, OnInit, Input } from '@angular/core';
import { User } from './user.model';
@Component({
selector: 'app-user',
templateUrl: './user.component.html',
styleUrls: ['./user.component.css']
})
export class UserComponent implements OnInit {
@Input() user: User;
ngOnInit(): void {
}
}
user.component.html
<div class="container">
<div class="row">
<div class="col-xs-12 col-md-8 px-3">
<p>{{user.name}} {{user.age}} {{user.joinDate | date:'dd/MM/yyyy'}}</p>
</div>
</div>
</div>
So these are all the classes and if you have got everything right then you can try to add new user. It won’t add it to the users array and you
won’t see any new User details. That will happen because of the provider at the child component too. In the AddUserComponent too a provider
is registered, that results in the creation of new UserService instance overriding the instance created by the provider at the AppComponent level.
If you have registered a service provider at parent component level, the created service instance is used by the child components too if they don't register their own provider. In this example since child component registers its own provider so a new service instance is created for the child component.
When you add a new user from AddUserComponent it is added to the array attached to the UserService instance of AddUserComponent. The details
which are showed using the user property of UserComponent gets user from the array which is attached to the UserService instance of AppComponent.
That is why new user details are not added.
You can comment the providers property in the @Component decorator of the AddUserComponent. Now only Single instance of UserService is created
at the AppComponent level (Parent component), child component AddUserComponent shares the same instance.
@Component({
selector: 'app-adduser',
templateUrl: './adduser.component.html',
styleUrls: ['./adduser.component.css'],
//providers: [UserService]
})
export class AddUserComponent {
..
..
}
That's all for this topic Injector Hierarchy and Service Instances in Angular. If you have any doubt or any suggestions to make please drop a comment. Thanks!
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