如果已经运行，则不应执行代码

Question

我有一些功能必须定期运行。我为此使用了自动收报机。但是如果ticker已经是运行ning了，时间间隔又过了，应该不会再执行了。

package main

import (
    "fmt"
    "time"
)

func main() {
    ticker := time.NewTicker(3*time.Second)
    flag := 0
    defer ticker.Stop()
    for {
        select {
        case t := <-ticker.C:
            flag = flag + 1
            if (flag % 2 ==0 ) {
                time.Sleep(time.Second*4)
            }   
            fmt.Println("Current time: ", t)
        }
    }
}

https://play.golang.org/p/2xV2MYInn4I

在操场上，自动收报机每 3 秒打印一次，但自动收报机每均匀转动一次，作业花费的时间比间隔时间长。我希望它不会运行然后放弃那些滴答声。

我该怎么做？

Answer 1

行情通道已缓冲，这就是为什么您可能会一个接一个地看到多个触发器。您可以通过简单地将代码的值传输到无缓冲通道来防止这种情况（另请注意，从代码接收到的 time.Time 值不是当前时间，而是最后一个报价的时间）：

package main

import (
    "fmt"
    "time"
)

func main() {
    c := make(chan time.Time) // unbuffered
    ticker := time.NewTicker(3 * time.Second)
    defer ticker.Stop()

    go func() {
        for t := range ticker.C {
            select {
            case c <- t:
            default:
            }
        }
    }()

    for flag := 0; flag < 8; flag++ {
        <-c

        if flag%2 == 0 {
            time.Sleep(time.Second * 4)
        }
        fmt.Println("Current time: ", time.Now())
    }
}

// Output:
// Current time:  2020-02-19 12:21:57.095433032 +0100 CET m=+3.000213350
// Current time:  2020-02-19 12:22:04.095585208 +0100 CET m=+10.000365520
// Current time:  2020-02-19 12:22:06.095363327 +0100 CET m=+12.000143680
// Current time:  2020-02-19 12:22:13.095605268 +0100 CET m=+19.000385598
// Current time:  2020-02-19 12:22:15.095371885 +0100 CET m=+21.000152174
// Current time:  2020-02-19 12:22:22.095537562 +0100 CET m=+28.000317857
// Current time:  2020-02-19 12:22:24.095431317 +0100 CET m=+30.000211625
// Current time:  2020-02-19 12:22:31.095524308 +0100 CET m=+37.000304595

在操场上试试：https://play.golang.org/p/jDe5uJiRVe2

Answer 2

在同一个 goroutine 中休眠只会延迟执行。 ticker 同时在一个单独的 goroutine 中运行。因此，即使您使用全局变量来维护执行状态 - 它也不会在睡眠时为您提供所需的结果。然而，将整个 "sleeping" 迁移到一个单独的 goroutine 中会产生：

package main

import (
    "fmt"
    "time"
)

type Tick struct {
    ticker *time.Ticker
    executing bool
}

func somethingYouWantToDo(tick *Tick, flag *int, t time.Time) {
    if tick.executing {
        return
    }

    tick.executing = true

    *flag = *flag + 1

    if (*flag % 2 ==0 ) {
                time.Sleep(time.Second*4)
    }   
        fmt.Println("Current time: ", t)
    tick.executing = false
}

func main() {
    tick := &Tick{
        ticker: time.NewTicker(3*time.Second),
    }
    flag := 0
    defer tick.ticker.Stop()
    for {
        select {
        case t := <-tick.ticker.C:
            go somethingYouWantToDo(tick, &flag, t)
        }
    }
}
// output
// Current time:  2009-11-10 23:00:03 +0000 UTC m=+3.000000001
// Current time:  2009-11-10 23:00:06 +0000 UTC m=+6.000000001
// Current time:  2009-11-10 23:00:12 +0000 UTC m=+12.000000001
// Current time:  2009-11-10 23:00:15 +0000 UTC m=+15.000000001
// Current time:  2009-11-10 23:00:21 +0000 UTC m=+21.000000001
// Current time:  2009-11-10 23:00:24 +0000 UTC m=+24.000000001
// Current time:  2009-11-10 23:00:30 +0000 UTC m=+30.000000001
// Current time:  2009-11-10 23:00:33 +0000 UTC m=+33.000000001
// Current time:  2009-11-10 23:00:39 +0000 UTC m=+39.000000001
// Current time:  2009-11-10 23:00:42 +0000 UTC m=+42.000000001
// Current time:  2009-11-10 23:00:48 +0000 UTC m=+48.000000001
// Current time:  2009-11-10 23:00:51 +0000 UTC m=+51.000000001
// Current time:  2009-11-10 23:00:57 +0000 UTC m=+57.000000001
// Current time:  2009-11-10 23:01:00 +0000 UTC m=+60.000000001
// Current time:  2009-11-10 23:01:06 +0000 UTC m=+66.000000001
// Current time:  2009-11-10 23:01:09 +0000 UTC m=+69.000000001

Try it on the playground

如果已经 运行，则不应执行代码

Ticker should not execute if already running

go

ticker

如果已经运行，则不应执行代码