angular-signals

Angular Signals

Signals 是 Angular 用於狀態管理的反應式基礎元件，提供同步且細粒度的反應性。

核心 Signal API

signal() - 可寫入狀態

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

// 建立可寫入 signal
const count = signal(0);

// 讀取值
console.log(count()); // 0

// 設定新值
count.set(5);

// 根據目前值更新
count.update(c => c + 1);

// 明確指定型別
const user = signal<User | null>(null);
user.set({ id: 1, name: 'Alice' });

computed() - 衍生狀態

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

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

// 衍生 signal - 當相依項目變更時自動更新
const fullName = computed(() => `${firstName()} ${lastName()}`);

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

// 複雜邏輯的 computed
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() - 可重置的相依狀態

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

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

// 當 options 變更時，重置為第一個選項
const selected = linkedSignal(() => options()[0]);

console.log(selected()); // "A"
selected.set('B');       // 使用者選取 B
console.log(selected()); // "B"
options.set(['X', 'Y']); // 選項變更
console.log(selected()); // "X" - 自動重置為第一個

// 搭配前一個值存取
const items = signal<Item[]>([]);

const selectedItem = linkedSignal<Item[], Item | null>({
  source: () => items(),
  computation: (newItems, previous) => {
    // 嘗試保留選取項目（若仍存在）
    const prevItem = previous?.value;
    if (prevItem && newItems.some(i => i.id === prevItem.id)) {
      return prevItem;
    }
    return newItems[0] ?? null;
  },
});

effect() - 副作用

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

@Component({...})
export class Search {
  query = signal('');
  
  constructor() {
    // 當 query 變更時執行 effect
    effect(() => {
      console.log('搜尋查詢:', this.query());
    });
    
    // 含清理的 effect
    effect((onCleanup) => {
      const timer = setInterval(() => {
        console.log('目前查詢:', this.query());
      }, 1000);
      
      onCleanup(() => clearInterval(timer));
    });
  }
}

Effect 規則：

• 必須在注入上下文（建構子或使用 runInInjectionContext）中執行
• 元件銷毀時自動清理

元件狀態模式

@Component({
  selector: 'app-todo-list',
  template: `
    <input [value]="newTodo()" (input)="newTodo.set($any($event.target).value)" />
    <button (click)="addTodo()" [disabled]="!canAdd()">新增</button>
    
    <ul>
      @for (todo of filteredTodos(); track todo.id) {
        <li [class.done]="todo.done">
          {{ todo.text }}
          <button (click)="toggleTodo(todo.id)">切換</button>
        </li>
      }
    </ul>
    
    <p>{{ remaining() }} 項待辦</p>
  `,
})
export class TodoList {
  // 狀態
  todos = signal<Todo[]>([]);
  newTodo = signal('');
  filter = signal<'all' | 'active' | 'done'>('all');
  
  // 衍生狀態
  canAdd = computed(() => this.newTodo().trim().length > 0);
  
  filteredTodos = computed(() => {
    const todos = this.todos();
    switch (this.filter()) {
      case 'active': return todos.filter(t => !t.done);
      case 'done': return todos.filter(t => t.done);
      default: return todos;
    }
  });
  
  remaining = computed(() => 
    this.todos().filter(t => !t.done).length
  );
  
  // 動作
  addTodo() {
    const text = this.newTodo().trim();
    if (text) {
      this.todos.update(todos => [
        ...todos,
        { id: crypto.randomUUID(), text, done: false }
      ]);
      this.newTodo.set('');
    }
  }
  
  toggleTodo(id: string) {
    this.todos.update(todos =>
      todos.map(t => t.id === id ? { ...t, done: !t.done } : t)
    );
  }
}

RxJS 互通

toSignal() - Observable 轉 Signal

import { toSignal } from '@angular/core/rxjs-interop';
import { interval } from 'rxjs';

@Component({...})
export class Timer {
  private http = inject(HttpClient);
  
  // 從 observable 轉換 - 需要初始值或 allowUndefined
  counter = toSignal(interval(1000), { initialValue: 0 });
  
  // 從 HTTP 轉換 - 載入完成前為 undefined
  users = toSignal(this.http.get<User[]>('/api/users'));
  
  // 使用 requireSync 處理同步 observable（BehaviorSubject）
  private user$ = new BehaviorSubject<User | null>(null);
  currentUser = toSignal(this.user$, { requireSync: true });
}

toObservable() - Signal 轉 Observable

import { toObservable } from '@angular/core/rxjs-interop';
import { switchMap, debounceTime } from 'rxjs';

@Component({...})
export class Search {
  query = signal('');
  
  private http = inject(HttpClient);
  
  // 將 signal 轉為 observable 以使用 RxJS 運算子
  results = toSignal(
    toObservable(this.query).pipe(
      debounceTime(300),
      switchMap(q => this.http.get<Result[]>(`/api/search?q=${q}`))
    ),
    { initialValue: [] }
  );
}

Signal 相等性

// 自訂相等函式
const user = signal<User>(
  { id: 1, name: 'Alice' },
  { equal: (a, b) => a.id === b.id }
);

// 僅在 ID 變更時觸發更新
user.set({ id: 1, name: 'Alice Updated' }); // 不觸發更新
user.set({ id: 2, name: 'Bob' }); // 觸發更新

非追蹤讀取

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

const a = signal(1);
const b = signal(2);

// 僅相依於 'a'，而非 'b'
const result = computed(() => {
  const aVal = a();
  const bVal = untracked(() => b());
  return aVal + bVal;
});

服務狀態模式

@Injectable({ providedIn: 'root' })
export class Auth {
  // 私有可寫入狀態
  private _user = signal<User | null>(null);
  private _loading = signal(false);
  
  // 公開唯讀 signal
  readonly user = this._user.asReadonly();
  readonly loading = this._loading.asReadonly();
  readonly isAuthenticated = computed(() => this._user() !== null);
  
  private http = inject(HttpClient);
  
  async login(credentials: Credentials): Promise<void> {
    this._loading.set(true);
    try {
      const user = await firstValueFrom(
        this.http.post<User>('/api/login', credentials)
      );
      this._user.set(user);
    } finally {
      this._loading.set(false);
    }
  }
  
  logout(): void {
    this._user.set(null);
  }
}

如需進階模式（包含 resource()），請參閱 references/signal-patterns.md。