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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

            
              /**
 * 等待指定的时间/等待指定表达式成立
 * 如果未指定等待条件则立刻执行
 * 注: 此实现在 nodejs 10- 会存在宏任务与微任务的问题,切记 async-await 本质上还是 Promise 的语法糖,实际上并非真正的同步函数!!!即便在浏览器,也不要依赖于这种特性。
 * @param param 等待时间/等待条件
 * @returns Promise 对象
 */
function wait(param) {
  return new Promise(resolve => {
    if (typeof param === 'number') {
      setTimeout(resolve, param)
    } else if (typeof param === 'function') {
      const timer = setInterval(() => {
        if (param()) {
          clearInterval(timer)
          resolve()
        }
      }, 100)
    } else {
      resolve()
    }
  })
}

/**
 * 将一个异步函数包装为具有时序的异步函数
 * 注: 该函数会丢弃过期的异步操作结果,这样的话性能会稍稍提高(主要是响应比较快的结果会立刻生效而不必等待前面的响应结果)
 * @param fn 一个普通的异步函数
 * @returns 包装后的函数
 */
function switchMap(fn) {
  // 当前执行的异步操作 id
  let id = 0
  // 最后一次异步操作的 id,小于这个的操作结果会被丢弃
  let last = 0
  // 缓存最后一次异步操作的结果
  let cache
  return new Proxy(fn, {
    async apply(_, _this, args) {
      const temp = id
      id++
      const res = await Reflect.apply(_, _this, args)
      if (temp < last) {
        return cache
      }
      cache = res
      last = temp
      return res
    },
  })
}

;(async () => {
  // 模拟一个异步请求,接受参数并返回它,然后等待指定的时间
  async function get(ms) {
    await wait(ms)
    return ms
  }
  const fn = switchMap(get)
  let last = 0
  let sum = 0
  await Promise.all([
    fn(30).then(res => {
      last = res
      sum += res
    }),
    fn(20).then(res => {
      last = res
      sum += res
    }),
    fn(10).then(res => {
      last = res
      sum += res
    }),
  ])
  console.log(last)
  // 实际上确实执行了 3 次,然而结果并不是 3 次调用参数之和,因为前两次的结果均被抛弃,实际上返回了最后一次发送请求的结果
  console.log(sum)
})()

            
          
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