Angular Components & Modules

Complete guide to Angular components, modules, and architecture.

Components

Component Basics

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

@Component({
  selector: 'app-user-profile',
  templateUrl: './user-profile.component.html',
  styleUrls: ['./user-profile.component.scss'],
  // Or inline
  // template: `<div>{{user.name}}</div>`,
  // styles: [`h1 { color: blue; }`]
})
export class UserProfileComponent {
  user = { name: 'John', age: 30 };

  greet() {
    console.log(`Hello, ${this.user.name}`);
  }
}

Component Lifecycle Hooks

import { Component, OnInit, OnDestroy, OnChanges, SimpleChanges } from '@angular/core';

@Component({
  selector: 'app-lifecycle',
  template: `<div>{{data}}</div>`
})
export class LifecycleComponent implements OnInit, OnChanges, OnDestroy {
  @Input() data: string;

  // 1. Constructor
  constructor() {
    console.log('Constructor');
  }

  // 2. OnChanges - when input properties change
  ngOnChanges(changes: SimpleChanges) {
    console.log('OnChanges', changes);
  }

  // 3. OnInit - after first ngOnChanges
  ngOnInit() {
    console.log('OnInit - component initialized');
    // Initialize data, subscribe to observables
  }

  // 4. DoCheck - during every change detection
  ngDoCheck() {
    console.log('DoCheck');
  }

  // 5. AfterContentInit - after content projection
  ngAfterContentInit() {
    console.log('AfterContentInit');
  }

  // 6. AfterContentChecked - after projected content checked
  ngAfterContentChecked() {
    console.log('AfterContentChecked');
  }

  // 7. AfterViewInit - after view initialized
  ngAfterViewInit() {
    console.log('AfterViewInit');
  }

  // 8. AfterViewChecked - after view checked
  ngAfterViewChecked() {
    console.log('AfterViewChecked');
  }

  // 9. OnDestroy - cleanup
  ngOnDestroy() {
    console.log('OnDestroy - cleanup subscriptions');
    // Unsubscribe from observables, detach event handlers
  }
}

Component Communication

// Parent to Child: @Input
@Component({
  selector: 'app-child',
  template: `<div>{{title}}</div>`
})
export class ChildComponent {
  @Input() title: string;
  @Input() data: any;
}

// Parent template
`<app-child [title]="parentTitle" [data]="parentData"></app-child>`

// Child to Parent: @Output
@Component({
  selector: 'app-child',
  template: `<button (click)="sendData()">Send</button>`
})
export class ChildComponent {
  @Output() dataEmitted = new EventEmitter<string>();

  sendData() {
    this.dataEmitted.emit('Hello from child');
  }
}

// Parent template
`<app-child (dataEmitted)="onDataReceived($event)"></app-child>`

// Parent class
onDataReceived(data: string) {
  console.log('Received:', data);
}

ViewChild & ContentChild

@Component({
  selector: 'app-parent',
  template: `
    <app-child #childComponent></app-child>
    <div #myDiv>Content</div>
  `
})
export class ParentComponent implements AfterViewInit {
  @ViewChild('childComponent') child: ChildComponent;
  @ViewChild('myDiv') divElement: ElementRef;
  @ViewChildren(ChildComponent) children: QueryList<ChildComponent>;

  ngAfterViewInit() {
    // Access child component
    this.child.someMethod();

    // Access DOM element
    this.divElement.nativeElement.style.color = 'red';

    // Access all children
    this.children.forEach(child => console.log(child));
  }
}

// Content projection
@Component({
  selector: 'app-container',
  template: `
    <div class="header">
      <ng-content select="[header]"></ng-content>
    </div>
    <div class="body">
      <ng-content></ng-content>
    </div>
  `
})
export class ContainerComponent implements AfterContentInit {
  @ContentChild('headerContent') header: ElementRef;

  ngAfterContentInit() {
    console.log('Projected content:', this.header);
  }
}

// Usage
`<app-container>
  <div header>Header Content</div>
  <div>Body Content</div>
</app-container>`

Standalone Components (Angular 14+)

import { Component } from '@angular/core';
import { CommonModule } from '@angular/common';
import { FormsModule } from '@angular/forms';

@Component({
  selector: 'app-standalone',
  standalone: true,
  imports: [CommonModule, FormsModule],
  template: `
    <div *ngIf="show">
      <input [(ngModel)]="name">
      <p>{{name}}</p>
    </div>
  `
})
export class StandaloneComponent {
  show = true;
  name = '';
}

// Bootstrap standalone component
import { bootstrapApplication } from '@angular/platform-browser';

bootstrapApplication(StandaloneComponent);

Modules

NgModule Basics

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { FormsModule } from '@angular/forms';
import { HttpClientModule } from '@angular/common/http';

import { AppComponent } from './app.component';
import { UserComponent } from './user/user.component';
import { SharedModule } from './shared/shared.module';

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

Feature Modules

// Feature module
import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { RouterModule, Routes } from '@angular/router';

import { UsersComponent } from './users.component';
import { UserDetailComponent } from './user-detail.component';

const routes: Routes = [
  { path: '', component: UsersComponent },
  { path: ':id', component: UserDetailComponent }
];

@NgModule({
  declarations: [
    UsersComponent,
    UserDetailComponent
  ],
  imports: [
    CommonModule,
    RouterModule.forChild(routes)
  ]
})
export class UsersModule { }

Shared Module

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { FormsModule, ReactiveFormsModule } from '@angular/forms';

import { HighlightDirective } from './directives/highlight.directive';
import { PhonePipe } from './pipes/phone.pipe';
import { CardComponent } from './components/card.component';

@NgModule({
  declarations: [
    HighlightDirective,
    PhonePipe,
    CardComponent
  ],
  imports: [
    CommonModule,
    FormsModule,
    ReactiveFormsModule
  ],
  exports: [
    CommonModule,
    FormsModule,
    ReactiveFormsModule,
    HighlightDirective,
    PhonePipe,
    CardComponent
  ]
})
export class SharedModule { }

Lazy Loading Modules

// app-routing.module.ts
const routes: Routes = [
  {
    path: 'users',
    loadChildren: () => import('./users/users.module').then(m => m.UsersModule)
  },
  {
    path: 'admin',
    loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule),
    canLoad: [AuthGuard]
  }
];

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

Change Detection

Default Change Detection

@Component({
  selector: 'app-default',
  template: `<div>{{data}}</div>`
})
export class DefaultComponent {
  data = 'Initial';

  // Angular checks this component every time change detection runs
}

OnPush Change Detection

import { Component, ChangeDetectionStrategy, Input } from '@angular/core';

@Component({
  selector: 'app-onpush',
  template: `<div>{{data.value}}</div>`,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class OnPushComponent {
  @Input() data: { value: string };

  // Only checks when:
  // 1. Input reference changes
  // 2. Event handler fires
  // 3. Observable emits (with async pipe)
  // 4. Manual detection (ChangeDetectorRef)
}

Manual Change Detection

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

@Component({
  selector: 'app-manual',
  template: `<div>{{data}}</div>`,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class ManualComponent {
  data: any;

  constructor(private cdr: ChangeDetectorRef) {}

  updateData() {
    this.data = { value: 'New data' };
    this.cdr.markForCheck(); // Mark for check
    // or
    this.cdr.detectChanges(); // Run change detection immediately
  }

  ngOnDestroy() {
    this.cdr.detach(); // Detach from change detection
  }
}

Dynamic Components

import { Component, ViewChild, ViewContainerRef, ComponentFactoryResolver } from '@angular/core';

@Component({
  selector: 'app-dynamic',
  template: `<ng-template #container></ng-template>`
})
export class DynamicComponent {
  @ViewChild('container', { read: ViewContainerRef }) container: ViewContainerRef;

  constructor(private resolver: ComponentFactoryResolver) {}

  loadComponent() {
    // Clear existing
    this.container.clear();

    // Create component
    const factory = this.resolver.resolveComponentFactory(DynamicChildComponent);
    const componentRef = this.container.createComponent(factory);

    // Set inputs
    componentRef.instance.data = 'Some data';

    // Subscribe to outputs
    componentRef.instance.dataEmitted.subscribe(data => {
      console.log('Received:', data);
    });
  }

  // Angular 13+ - simplified
  loadComponentSimple() {
    const componentRef = this.container.createComponent(DynamicChildComponent);
    componentRef.instance.data = 'Some data';
  }
}

Questions & Answers

• Component relies only on inputs
• Inputs are immutable
• Component uses observables with async pipe
• You want to manually control change detection

• Unsubscribe from observables
• Detach event handlers
• Clear intervals/timeouts
• Cancel pending HTTP requests

• Services with Subject/BehaviorSubject
• State management (NgRx, Akita)
• Route parameters
• Local storage
• Query parameters

• Use OnPush change detection
• Lazy load modules
• TrackBy with ngFor
• Unsubscribe from observables
• Avoid function calls in templates
• Use pure pipes
• Virtual scrolling for large lists

• declarations: Components, directives, pipes that belong to this module
• imports: Other modules needed by this module
• exports: Make declarations available to other modules