Angular Signals

Signals are Angular's reactive primitive for state management. They provide synchronous, fine-grained reactivity.

Core Signal APIs

signal() - Writable State

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

// Create writable signal
const count = signal(0);

// Read value
console.log(count()); // 0

// Set new value
count.set(5);

// Update based on current value
count.update(c => c + 1);

// With explicit type
const user = signal<User | null>(null);
user.set({ id: 1, name: 'Alice' });

computed() - Derived State

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

const firstName = signal('John');
const lastName = signal('Doe');

// Derived signal - automatically updates when dependencies change
const fullName = computed(() => `${firstName()} ${lastName()}`);

console.log(fullName()); // "John Doe"
firstName.set('Jane');
console.log(fullName()); // "Jane Doe"

// Computed with complex logic
const items = signal<Item[]>([]);
const filter = signal('');

const filteredItems = computed(() => {
  const query = filter().toLowerCase();
  return items().filter(item => 
    item.name.toLowerCase().includes(query)
  );
});

const totalPrice = computed(() => 
  filteredItems().reduce((sum, item) => sum + item.price, 0)
);

linkedSignal() - Dependent State with Reset

import { signal, linkedSignal } from '@angular/core';

const options = signal(['A', 'B', 'C']);

// Resets to first option when options change
const selected = linkedSignal(() => options()[0]);

console.log(selected()); // "A"
selected.set('B');       // User selects B
console.log(selected()); // "B"
options.set(['X', 'Y']); // Options change
console.log(selected()); // "X" - auto-reset to first

// With previous value access
const items = signal<Item[]>([]);

const selectedItem = linkedSignal<Item[], Item | null>({
  source: () => items(),
  computation: (newItems, previous) => {
    // Try to preserve selection if item still exists
    const prevItem = previous?.value;
    if (prevItem && newItems.some(i => i.id === prevItem.id)) {
      return prevItem;
    }
    return newItems[0] ?? null;
  },
});

effect() - Side Effects

import { signal, effect, inject, DestroyRef } from '@angular/core';

@Component({...})
export class Search {
  query = signal('');
  
  constructor() {
    // Effect runs when query changes
    effect(() => {
      console.log('Search query:', this.query());
    });
    
    // Effect with cleanup
    effect((onCleanup) => {
      const timer = setInterval(() => {
        console.log('Current query:', this.query());
      }, 1000);
      
      onCleanup((