Angular Components

What is a component?

A component is a TypeScript class that has a template, associated css, and implementation.

  • Can be composed of other components

  • Can use services, directives, pipes, etc.

  • Can handle user and system events

  • Should have tests to validate functionality

Creating a component

ng generate component login or ng g c login

Generates the following files:

  • login.component.html

  • login.component.css

  • login.component.spec.ts

  • login.component.ts

app.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() {
}
}
  • LoginComponent class decorated with @Component

    • selector <app-login></app-login> to use component

    • template is set to login.component.html

    • CSS is set to login.component.css

Components and modules

To use our new component, we will need to add LoginComponent to app.module.ts. The Angular CLI automatically added our new component to the nearest parent module.

app.module.ts
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { FormsModule } from '@angular/forms';
import { AppComponent } from './app.component';
import { LoginComponent } from './login/login.component';
@NgModule({
imports: [ BrowserModule, FormsModule ],
declarations: [ AppComponent, LoginComponent ],
bootstrap: [ AppComponent ]
})
export class AppModule { }

Login Component HTML

Our login component needs two inputs for username and password, plus a button to start the login.

login.component.ts
<h1>Login</h1>
<p>
<label>User Name: </label>
<input type="text" [(ngModel)]="username" />
</p>
<p>
<label>
Password: </label>
<input type="password" [(ngModel)]="password" />
</p>
<button (click)="submit()">Submit</button>
<p class="message">{{message}}</p>

[(ngModel)] = "username" binds the value of the input to a local variable in the component.

(click)="submit()" maps the click event on the button to the submit function

Login Component

Component Decorator

  • selector - the html name for the component

  • template - inline template

  • templateUrl - url to template

  • styles - inline styles

  • stylesUrl - url to styles file

  • providers - objects to be injected into component

Component Providers - be cautious about declaring providers at the component level. It creates a new instance of the service (which are singleton by design) and makes it more complex to mock the service.

Component Hierarchy

  • All applications have a root component that Angular bootstraps the application with

  • Each page should be a component

  • Pages usually are comprised of child components

  • Create child components to reduce complexity or when functionality appears in multiple pages

Lifecycle Hooks

Angular component instances have a lifecycle as Angular creates, updates, and destroys them. A component can implement them, e.g., export class LoginComponent implements OnInit

  • ngOnInit

    • Initialize the directive/component after Angular first displays the data-bound properties and sets the directive/component's input properties.

      Called once, after the first ngOnChanges().

  • ngOnChanges

    • Respond when Angular (re)sets data-bound input properties. The method receives a SimpleChangesobject of current and previous property values.

      Called before ngOnInit() and whenever one or more data-bound input properties change.

  • ngAfterViewInit

    • Respond after Angular initializes the component's views and child views / the view that a directive is in.

      Called once after the first ngAfterContentChecked().

  • ngOnDestroy

    • Cleanup just before Angular destroys the directive/component. Unsubscribe Observables and detach event handlers to avoid memory leaks.

      Called just before Angular destroys the directive/component.

The lifecycle hook interfaces are optional - since JavaScript doesn't support interfaces, Angular calls any lifecycle methods that are declared. It's considered good practice to implement the interfaces to declare your intent.

Summary

  • What is a component?

  • How to create components

  • HTML templates, data binding, and event handling

  • Component Decorator properties

  • Lifecycle Hooks