Angular2中HostListener是否需取消订阅？滚动监听未取消是否有问题

Great question! Let's break down each part of your query to clear up all your doubts:

Do you need to unsubscribe from @HostListener? #

Short answer: No, you don't need to manually unsubscribe from events registered via @HostListener. Angular handles this cleanup automatically for you.

How does @HostListener work? #

When you use @HostListener (like @HostListener('window:scroll', ['$event'])), Angular manages the event binding lifecycle behind the scenes:

• During component initialization, it registers your event handler (onScroll) to the target event (in this case, the scroll event on the window object).
• Angular keeps track of all these event bindings linked to the component instance.
• Unlike manual event subscriptions (e.g., using addEventListener directly or raw RxJS observables), you don't have to handle cleanup yourself because Angular ties the listener's lifecycle directly to the component's lifecycle.

When does it automatically unsubscribe? #

Angular automatically removes the event listener when the component is destroyed—specifically, when the ngOnDestroy lifecycle hook runs for that component. This ensures no lingering listeners are left tied to a component that's no longer in the DOM, preventing memory leaks.

Using @HostListener('window:scroll') in multiple components – will this cause issues? #

Let's split this into two key considerations:

1. Memory leaks: No, you won't run into memory leak issues here. Each component's @HostListener is tracked independently, so when a component is destroyed, its own scroll listener is cleaned up automatically.
2. Performance: This is where you might want to be cautious. Every time the window scrolls, all components with this @HostListener will trigger their onScroll method. If you have many such components, or if the onScroll logic is computationally heavy, this could lead to choppy scrolling performance.

A more efficient approach for shared scroll handling is to create a single global service that listens to the window scroll event once. Then use an RxJS Subject or BehaviorSubject to broadcast scroll events to any components that need to react to them. This way, you only have one active scroll listener, cutting down on unnecessary event handler invocations.

内容的提问来源于stack exchange，提问作者Shilpa J