当我不再需要这个 `await nextEvent(element, 'mousemove')` 模式时，如何清理它？

Question

我有一个 React 组件，其中包含如下代码：

class MyComponent extends React.Component {
    // ...

    trackStats = false

    componentDidMount() {
        this.monitorActivity()
    }

    componentWillUnmount() {
        this.trackStats = false
    }

    async monitorActivity() {
        this.trackStats = true

        while (this.trackStats && this.elRef.current) {
            // elRef is a React ref to a DOM element rendered in render()
            await Promise.race([
                nextEvent(this.elRef.current, 'keydown'),
                nextEvent(this.elRef.current, 'click'),
                nextEvent(this.elRef.current, 'mousemove'),
                nextEvent(this.elRef.current, 'pointermove'),
            ])

            this.logUserActivity()
        }
    }

    logUserActivity() {
        // ...
    }

    render() { /* ... */ }
}

const nextEvent = (target, eventName) => new Promise(resolve => {
    target.addEventListener(eventName, resolve, { once: true })
})

问题是，如果这个组件被卸载，那么添加到 this.elRef.current 引用的 DOM 元素上的事件处理程序将保留在内存中，因为用户将不再与不再存在的元素交互DOM。

所以 while 循环将一直等待下一个事件，这永远不会发生，并且因为 while 循环仍在等待最后一个事件，我相信这会导致 MyComponent 的实例被泄露记忆。

或者引擎是否足够聪明来清理它？如果我没有对这些东西的任何可访问的引用，并且唯一链接的是while循环的范围，它正在等待一些承诺履行，引擎会丢弃它吗？或者它会让 while 循环范围运行，等待 Promise？

如果 while 循环仍然存在（我猜它确实存在），我应该如何清理它？

Answer 1

啊有趣的用例！这似乎是 AbortController 的一个很好的用例：

function nextEvent(target, type, abortSignal) {
  return new Promise(resolve => {
    target.addEventListener(type, resolve, { once: true });
    abortSignal.addEventListener("abort", () =>
      target.removeEventListener(type, resolve)
    );
  });
}

const abortController = new AbortController();

const event = await Promise.race([
  nextEvent(someButton, "click", abortController.signal),
  nextEvent(someButton, "keydown", abortController.signal)
]);
// Clean up all remaining event handlers
abortController.abort();
// Continue as normal

Answer 2

感谢 Surma 的指导，我能够想出一种在卸载组件时完全清理的方法：

class MyComponent extends React.Component {
    // ...

    trackStats = false
    statsAbort = undefined

    componentDidMount() {
        this.monitorActivity()
    }

    componentWillUnmount() {
        this.trackStats = false
        this.statsAbort.abort()
    }

    async monitorActivity() {
        this.trackStats = true

        while (this.trackStats && this.elRef.current) {
            this.statsAbort = new AbortController

            try {
                // elRef is a React ref to a DOM element rendered in render()
                await Promise.race([
                    nextEvent(this.elRef.current, 'keydown'),
                    nextEvent(this.elRef.current, 'click'),
                    nextEvent(this.elRef.current, 'mousemove'),
                    nextEvent(this.elRef.current, 'pointermove'),
                ])
            } catch(e) {
                if (e.message !== 'abort_stats') throw e
            }

            this.statsAbort.abort()

            this.logUserActivity()
        }
    }

    logUserActivity() {
        // ...
    }

    render() { /* ... */ }
}

const nextEvent = (target, eventName, abortSignal) => new Promise((resolve, reject) => {
    target.addEventListener(eventName, resolve, { once: true })
    abortSignal.addEventListener("abort", () => {
      target.removeEventListener(eventName, resolve)
      reject(new Error('abort_stats'))
    });
})

但是直接使用 addEventListener 会更简单，所以我选择了以下内容，这对于这个用例也更容易理解：

class MyComponent extends React.Component {
    // ...

    componentDidMount() {
        const el = this.elRef.current
        el.addEventListener('keydown', this.logUserActivity)
        el.addEventListener('click', this.logUserActivity)
        el.addEventListener('mousemove', this.logUserActivity)
        el.addEventListener('pointermove', this.logUserActivity)
    }

    componentWillUnmount() {
        const el = this.elRef.current
        el.removeEventListener('keydown', this.logUserActivity)
        el.removeEventListener('click', this.logUserActivity)
        el.removeEventListener('mousemove', this.logUserActivity)
        el.removeEventListener('pointermove', this.logUserActivity)
    }

    logUserActivity() {
        // ...
    }

    render() { /* ... */ }
}