Signals & Reactivity

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Signals in Components (Angular 18+)

Signals are the reactive primitive in modern Angular, replacing many uses of RxJS for component-level state:

import { Component, signal, computed, effect } from '@angular/core';

@Component({
  selector: 'app-counter',
  template: `
    <p>Count: {{ count() }}</p>
    <p>Double: {{ double() }}</p>
    <button (click)="increment()">+1</button>
    <button (click)="reset()">Reset</button>
  `
})
export class CounterComponent {
  count = signal(0);

  double = computed(() => this.count() * 2);

  constructor() {
    effect(() => {
      console.log(`Count changed to: ${this.count()}`);
    });
  }

  increment(): void {
    this.count.update(c => c + 1);
  }

  reset(): void {
    this.count.set(0);
  }
}

Why Signals Matter

Reactivity without complexity — no need to manually subscribe/unsubscribe like with Observables in simple cases.
Fine-grained updates — only the parts of the UI depending on the signal re-render.
Better performance — avoids unnecessary change detection cycles.
Predictable state changes — easy to debug and reason about.

Signal API Summary

API	Purpose
`signal(value)`	Create a writable signal with initial value
`signal.set(value)`	Replace the current value
`signal.update(fn)`	Update based on current value
`computed(() => expr)`	Derived read-only signal (auto-tracks dependencies)
`effect(() => { ... })`	Side effect that re-runs when dependencies change

View Queries (Angular 18+)

`viewChild` and `viewChildren`

Access child elements or components from the template using signal-based queries:

import { Component, viewChild, viewChildren, ElementRef, AfterViewInit } from '@angular/core';

@Component({
  selector: 'app-gallery',
  template: `
    <input #searchInput placeholder="Search..." />
    @for (item of items(); track item.id) {
      <div #card class="card">{{ item.name }}</div>
    }
  `
})
export class GalleryComponent implements AfterViewInit {
  searchInput = viewChild.required<ElementRef>('searchInput');
  cards = viewChildren<ElementRef>('card');

  items = signal([
    { id: 1, name: 'Item A' },
    { id: 2, name: 'Item B' }
  ]);

  ngAfterViewInit(): void {
    this.searchInput().nativeElement.focus();
    console.log(`Found ${this.cards().length} cards`);
  }
}

`contentChild` and `contentChildren`

Query projected content from a parent:

import { Component, contentChildren } from '@angular/core';
import { TabComponent } from './tab.component';

@Component({
  selector: 'app-tabs',
  template: `
    <div class="tab-headers">
      @for (tab of tabs(); track tab.label()) {
        <button (click)="selectTab(tab)">{{ tab.label() }}</button>
      }
    </div>
    <ng-content />
  `
})
export class TabsComponent {
  tabs = contentChildren(TabComponent);

  selectTab(tab: TabComponent): void {}
}

Query API Comparison

Legacy	Modern (Angular 18+)
`@ViewChild('ref')`	`viewChild('ref')`
`@ViewChild(Component)`	`viewChild(Component)`
`@ViewChildren('ref')`	`viewChildren('ref')`
`@ContentChild(Component)`	`contentChild(Component)`
`@ContentChildren(Component)`	`contentChildren(Component)`

ChangeDetection Strategy

For performance optimization, use OnPush change detection:

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

@Component({
  selector: 'app-user-card',
  changeDetection: ChangeDetectionStrategy.OnPush,
  template: `
    <div class="card">
      <h3>{{ name() }}</h3>
      <p>{{ email() }}</p>
    </div>
  `
})
export class UserCardComponent {
  name = input.required<string>();
  email = input<string>('');
}

When to use OnPush:

Always — it’s the recommended default for all components.
Signal-based components work seamlessly with OnPush.
The component only re-renders when its inputs change or a signal it reads is updated.