Angular什麼時候須要Unsubscribe

須要unsubscribe的狀況

Forms —

export class TestComponent {

  ngOnInit() {
    this.form = new FormGroup({...});
    this.valueChanges  = this.form.valueChanges.subscribe(console.log);
    this.statusChanges = this.form.statusChanges.subscribe(console.log);
  }

  ngOnDestroy() {
    this.valueChanges.unsubscribe();
    this.statusChanges.unsubscribe();
  }

}

form control中同理

The Router —

export class TestComponent {
  constructor(private route: ActivatedRoute, private router: Router) { }

  ngOnInit() {
    this.route.params.subscribe(console.log);
    this.route.queryParams.subscribe(console.log);
    this.route.fragment.subscribe(console.log);
    this.route.data.subscribe(console.log);
    this.route.url.subscribe(console.log);
    
    this.router.events.subscribe(console.log);
  }

  ngOnDestroy() {
    // You should unsubscribe from each observable here
  }

}

根據官方文檔，Angular應該自動unsubscribe，但這裏面有個bug。

Renderer Service —

export class TestComponent {
constructor(private renderer: Renderer2, 
            private element : ElementRef) { }

  ngOnInit() {
    this.click = this.renderer.listen(this.element.nativeElement, "click", handler);
  }

  ngOnDestroy() {
    this.click.unsubscribe();
  }

}

Infinite Observables

export class TestComponent {

  constructor(private element : ElementRef) { }

  interval: Subscription;
  click: Subscription;

  ngOnInit() {
    this.interval = Observable.interval(1000).subscribe(console.log);
    this.click = Observable.fromEvent(this.element.nativeElement, 'click').subscribe(console.log);
  }

  ngOnDestroy() {
    this.interval.unsubscribe();
    this.click.unsubscribe();
  }

}

Redux Store —

export class TestComponent {

  constructor(private store: Store) { }

  todos: Subscription;

  ngOnInit() {
     this.todos = this.store.select('todos').subscribe(console.log);  
  }

  ngOnDestroy() {
    this.todos.unsubscribe();
  }

}

不須要 Unsubscribe 的狀況

Async pipe —

@Component({
  selector: 'test',
  template: `<todos [todos]="todos$ | async"></todos>`
})
export class TestComponent {

  constructor(private store: Store) { }

  ngOnInit() {
     this.todos$ = this.store.select('todos');
  }

}

當組件被銷燬時，async管道自動取消訂閱，以免潛在的內存泄漏。

@HostListener —

export class TestDirective {

  @HostListener('click')
  onClick() {
    ....
  }
}

Finite Observable —

當你有一個有限的序列，一般你不須要unsubscribe，例如當使用HTTPservice或timer observable。

export class TestComponent {

  constructor(private http: Http) { }

  ngOnInit() {
    Observable.timer(1000).subscribe(console.log);
    this.http.get('http://api.com').subscribe(console.log);
  }
}

小建議

不要過多的調用unsubscribe方法，RxJS: Don’t Unsubscribe

takeUntil
它發出源 Observable 的值，而後直到第二個 Observable (即 notifier )發出項，它便完成。

export class TestComponent {

  constructor(private store: Store) { }

  private componetDestroyed: Subject = new Subject();
  todos: Subscription;
  posts: Subscription;

  ngOnInit() {
     this.todos = this.store.select('todos').takeUntil(this.componetDestroyed).subscribe(console.log); 

     this.todos = this.store.select('posts').takeUntil(this.componetDestroyed).subscribe(console.log); 
  }

  ngOnDestroy() {
    this.componetDestroyed.next(); // componetDestroyed 發出值後，todos，todos會completed
    this.componetDestroyed.unsubscribe();
  }

}

參考連接

When to Unsubscribe in Angular