Angular API Reference

Welcome to the comprehensive guide on the Angular API Reference. This article aims to demystify various components of Angular, making it easy for beginners to understand. Each section is packed with definitions, examples, and tables to clarify concepts. Let’s dive in!

Angular Modules

Modules in Angular are essential building blocks. They help organize related code into cohesive blocks. Here are some important modules:

NgModule

The NgModule decorator defines an Angular module. It groups components, directives, pipes, and services. Here’s an example:

@NgModule({
  declarations: [AppComponent],
  imports: [BrowserModule],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

CommonModule

The CommonModule provides common directives (like ngIf and ngFor). It’s often imported in feature modules:

import { CommonModule } from '@angular/common';

@NgModule({
  imports: [CommonModule]
})
export class SharedModule { }

BrowserModule

The BrowserModule is required for applications that run in the browser. It enables core functionalities for browser applications:

import { BrowserModule } from '@angular/platform-browser';

@NgModule({
  imports: [BrowserModule]
})
export class AppModule { }

FormsModule

To build forms, import the FormsModule:

import { FormsModule } from '@angular/forms';

@NgModule({
  imports: [FormsModule]
})
export class AppModule { }

ReactiveFormsModule

For reactive forms, use ReactiveFormsModule:

import { ReactiveFormsModule } from '@angular/forms';

@NgModule({
  imports: [ReactiveFormsModule]
})
export class AppModule { }

HttpClientModule

For making HTTP requests, you need the HttpClientModule:

import { HttpClientModule } from '@angular/common/http';

@NgModule({
  imports: [HttpClientModule]
})
export class AppModule { }

Angular Components

An Angular component controls a patch of screen called a view. Components are the building blocks of Angular applications.

Component

Here’s how to create a basic Angular component:

import { Component } from '@angular/core';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent { }

Directive

Directives are classes that add additional behavior to elements in Angular applications. There are two types: structural and attribute directives.

Pipe

Pipes transform data in templates. Here’s a simple example:

@Pipe({name: 'exponential'})
export class ExponentialPipe implements PipeTransform {
  transform(value: number, exponent: string): number {
    return Math.pow(value, Number(exponent));
  }
}

Angular Services

Services are used to create reusable code for business logic around data management, HTTP requests, etc.

Injectable

The Injectable decorator allows a class to be injected with dependencies:

@Injectable({
  providedIn: 'root',
})
export class DataService { }

HttpClient

The HttpClient is a service for making HTTP requests:

import { HttpClient } from '@angular/common/http';

@Injectable({
  providedIn: 'root',
})
export class ApiService {
  constructor(private http: HttpClient) {}

  getData() {
    return this.http.get('https://api.example.com/data');
  }
}

HttpClientModule

The HttpClientModule is necessary to provide the HttpClient service:

import { HttpClientModule } from '@angular/common/http';

@NgModule({
  imports: [HttpClientModule]
})
export class AppModule { }

Angular Directives

Directives are classes that add specific behavior to elements in Angular applications, enhancing interactivity and functionality.

Attribute Directives

Attribute directives change the appearance or behavior of a DOM element:

@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective {
  constructor(el: ElementRef) {
    el.nativeElement.style.backgroundColor = 'yellow';
  }
}

Structural Directives

Structural directives change the DOM layout by adding/removing HTML elements:

• ngIf: Adds/removes an element based on a condition.
• ngFor: Iterates over a collection.

<div *ngIf="isVisible">Content is visible</div>
<div *ngFor="let item of items">{{item}}</div>

Angular Pipes

Pipes are a powerful feature in Angular that allows you to transform data within Gittemplates.

Pure Pipes

Pure pipes only reprocess the data when the input reference changes:

@Pipe({
  name: 'purePipe',
  pure: true
})
export class PurePipe implements PipeTransform {
  transform(value: string): string {
    return value + ' - transformed';
  }
}

Impure Pipes

Impure pipes are called on every change detection cycle:

@Pipe({
  name: 'impurePipe',
  pure: false
})
export class ImpurePipe implements PipeTransform {
  transform(value: string): string {
    return value + ' - transformed at ' + new Date();
  }
}

Angular Routing

Angular has a powerful router that allows navigation between views or components.

RouterModule

The RouterModule is used to set up the routing:

import { RouterModule, Routes } from '@angular/router';

const routes: Routes = [
  { path: '', component: HomeComponent },
  { path: 'about', component: AboutComponent },
];

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

Router

The Router enables navigation between views or components:

constructor(private router: Router) {}

navigateToHome() {
  this.router.navigate(['/']);
}

ActivatedRoute

ActivatedRoute is used to get information about a route associated with the component:

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

constructor(private route: ActivatedRoute) {
  this.route.params.subscribe(params => {
    console.log(params['id']);
  });
}

Angular Dependency Injection

Dependency Injection is a design pattern in which an object receives its dependencies from an external source rather than creating them itself.

Injector

The Injector is responsible for creating service instances:

import { Injector } from '@angular/core';

constructor(private injector: Injector) {
  const serviceInstance = this.injector.get(SomeService);
}

Provider

A Provider is a way of configuring the dependency injection system to create instantiations of a class:

@NgModule({
  providers: [{ provide: SomeService, useClass: MockService }]
})
export class AppModule { }

Hierarchical Injectors

Angular has a hierarchical injection system. Each injector can retrieve instances from its parent injector:

@Injectable({
  providedIn: 'root'
})
export class ParentService { }

@Injectable()
export class ChildService {
  constructor(private parentService: ParentService) { }
}

Angular Observables

Observables are a built-in part of the reactive programming paradigm and provide a way to organize asynchronous data streams.

Observable

Creating a basic observable:

import { Observable } from 'rxjs';

const myObservable = new Observable(observer => {
  observer.next('Hello');
  observer.complete();
});

Subject

Subjects are special types of observables that allow values to be multicasted to many observers:

import { Subject } from 'rxjs';

const mySubject = new Subject();

mySubject.subscribe(value => {
  console.log(value);
});

mySubject.next('Hello'); // Output: Hello

BehaviorSubject

BehaviorSubject is a special type of Subject that requires an initial value:

import { BehaviorSubject } from 'rxjs';

const myBehaviorSubject = new BehaviorSubject('Initial Value');

myBehaviorSubject.subscribe(value => {
  console.log(value);
});

myBehaviorSubject.next('New Value'); // Output: New Value

Angular Forms

Angular supports two approaches for handling forms: Reactive Forms and Template-driven Forms.

Reactive Forms

Reactive forms are built around observable streams:

import { FormGroup, FormBuilder } from '@angular/forms';

@Component({
  ...
})
export class ExampleComponent {
  myForm: FormGroup;

  constructor(private fb: FormBuilder) {
    this.myForm = this.fb.group({
      name: ['']
    });
  }
}

Template-driven Forms

Template-driven forms rely on Angular’s two-way data binding. Here’s an example:

<form #myForm="ngForm">
  <input name="name" ngModel>
  <button (click)="submit(myForm)">Submit</button>
</form>

Angular Testing

Testing is important in Angular applications, and Angular provides tools such as TestBed and ComponentFixture.

TestBed

TestBed is used for configuring and initializing the environment for testing:

import { TestBed } from '@angular/core/testing';

beforeEach(() => {
  TestBed.configureTestingModule({
    declarations: [AppComponent]
  });
});

ComponentFixture

ComponentFixture creates a test environment for a component:

import { ComponentFixture, TestBed } from '@angular/core/testing';

let component: AppComponent;
let fixture: ComponentFixture;

beforeEach(() => {
  TestBed.configureTestingModule({
    declarations: [AppComponent]
  }).compileComponents(); // compiles template and css

  fixture = TestBed.createComponent(AppComponent);
  component = fixture.componentInstance;
});

Conclusion

This article has provided an in-depth exploration of various components of the Angular API Reference. Understanding these elements is crucial for building efficient and effective Angular applications. Keep practicing with these examples, and soon you’ll be fluent in Angular!

FAQ

• What are Angular modules?
  Angular modules are containers for a cohesive block of code that has a related set of capabilities.
• What is a component in Angular?
  A component is a building block of an Angular application that controls a view.
• How do I create services in Angular?
  You can create services using the @Injectable decorator and then provide them in an NgModule.
• What are directives and pipes?
  Directives are classes that add behavior to elements, whereas pipes transform the displayed data in a template.
• What is dependency injection?
  It’s a design pattern where a class needs dependencies provided from an external source rather than creating them itself.