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@angular/core module

What Are Components in Angular?

The Concepts of Angular Components  -
Components are the most basic building block of a UI in Angular applications and it controls views (HTML/CSS). They also communicate with other components and services to bring functionality to your applications.

Technically components are basically TypeScript classes that interact with the HTML files of the components, which get displayed on the browsers.

The component is the core functionality of Angular applications but you need to know to pass the data into the components to configure them.

Angular applications must have a root component that contains all other components.
Components are created using @Component decorator that is part of @angular/core module.
You can create your own project using Angular CLI, this command allows you to quickly create an Angular application like - generate components, services, pipes, directive, classes, and modules, and so on as per your requirements.

Create your own component (login) using below command line –
ng g component login


After executing the above Angular CLI command in your project directory, the result looks like –
D:\Angular\DemoApp>ng g component login
  create src/app/login/login.component.html (24 bytes)
  create src/app/login/login.component.spec.ts (621 bytes)
  create src/app/login/login.component.ts (265 bytes)
  create src/app/login/login.component.css (0 bytes)
  update src/app/app.module.ts (394 bytes)


And the application login files are created by default and it looks like –
1.      login.component.html
2.      login.component.spec.ts
3.      login.component.ts
4.      login.component.css
5.      app.module.ts

And Angular CLI commands also import the Login component in the Angular module.
See the example in details -

login.component.ts -
import { Component, OnInit } from '@angular/core';

@Component({
  selector: 'app-login',
  templateUrl: './login.component.html',
  styleUrls: ['./login.component.css']
})
export class LoginComponent implements OnInit {

  constructor() { }

  ngOnInit() {
  }
}

The above component class shows some of the most useful @Component configuration options –
1.      Selector
2.      TemplateUrl
3.      StyleURLs

The selector – It is a CSS selector that tells Angular to create an instance of this component wherever it finds the corresponding tag in template HTML. For example, it is - <app-login></app-login>

The templateUrl – It is the module-relative address of this component's HTML template and you can also provide the inline HTML template.

The styleUrls - It can be used for CSS rules and it will affect the style of the template elements and you can also provide the inline style CSS.

The components provide you some additional metadata configurations–
@Component({
  changeDetection?: ChangeDetectionStrategy
  viewProviders?: Provider[]
  moduleId?: string
  templateUrl?: string
  template?: string
  styleUrls?: string[]
  styles?: string[]
  animations?: any[]
  encapsulation?: ViewEncapsulation
  interpolation?: [string, string]
  entryComponents?: Array<Type<any> | any[]>
  preserveWhitespaces?: boolean
  // inherited from core/Directive
  selector?: string
  inputs?: string[]
  outputs?: string[]
  host?: {...}
  providers?: Provider[]
  exportAs?: string
  queries?: {...}
})

app.module.ts
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { LoginComponent } from './login/login.component';

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

login.component.html
<p>
  Welcome you, Anil!
</p>

login.component.spec.ts
import { TestBed, async } from '@angular/core/testing';
import { AppComponent } from './app.component';
describe('AppComponent', () => {
  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [
        AppComponent
      ],
    }).compileComponents();
  }));

  it('should create the app', async(() => {
    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.debugElement.componentInstance;
    expect(app).toBeTruthy();
  }));

  it(`should have as title 'app'`, async(() => {
    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.debugElement.componentInstance;
    expect(app.title).toEqual('app');
  }));

  it('should render title in a h1 tag', async(() => {
    const fixture = TestBed.createComponent(AppComponent);
    fixture.detectChanges();
    const compiled = fixture.debugElement.nativeElement;
    expect(compiled.querySelector('h1').textContent).toContain('Welcome to app!');
  }));
});


The detail about Component’s Metadata Properties List -
1.        Selector Property – The CSS selector that identifies this component in a template.
2.        StyleUrls Property – The list of URLs to style sheets to be applied to this component's view.
3.        Styles Property – To be applied the inline styles for the component's view.
4.        Template Property – To be applied the inline template for the component's view.
5.        TemplateUrl Property – Used the URLs to an external file containing a template for the view
6.        Animations Property – Applied the list of animations of this component.
7.        ChangeDetection Property – The change detection strategy used by this component.
8.        Encapsulation Property - The style encapsulation strategy used by this component.
9.        EntryComponents Property – Used the list of components that are dynamically inserted into the view of this component.
10.    ExportAs Property – The name under which component instance is exported in a template.
11.    Host Property – Used to map the class property to host element bindings for events, properties, and attributes.
12.    Inputs Property – The list of class property names to data-bind as component inputs.
13.    Interpolation Property – The custom interpolation markers used in this component's template.
14.    ModuleId Property – This is the CommonJS module id of the file in which this component is defined.
15.    Outputs Property – The list of class property names that expose output events that others can subscribe too.
16.    Providers Property – The list of providers available to this component and its children.
17.    Queries Property – To configure queries that can be injected into the components.
18.    ViewProviders Property – The list of providers available to this component and its view children.

Summary
Components are fundamental building blocks of UI in Angular applications and it communicates with other components and services to bring functionality to your applications.
1.   It is a core component of Angular applications.
2.   An angular application must have a root component that contains all other components.
3.   They have well-defined selector.
4.   They have well-defined styles and styleUrls.
5.   They have well-defined template and templateUrl.
6.   They have well-defined inputs and outputs.
7.   They have well-defined encapsulation and animations.
8.   They have a well-defined lifecycle.
9.   They are self-describing property.
10.    And many more...

Stayed InformedWhat Is an EntryComponent in Angular? 
Stayed Informed Why does Angular need entryComponents?

I hope you enjoyed this post. So please write your thoughts in the below comment box. Thank you so much for reading this post.
ANIL SINGH

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