为什么 time.After 与选择块中的代码配对时永远不会触发？

Question

我有一个正在监听 2 个频道、一个自动收报机和一个计时器的选择块：

package main

import (

    "fmt"

    "time"

)

func main() {

    ticker := time.NewTicker(5 * time.Second)

    for {

        select {

        case z := <-ticker.C:

            fmt.Printf("tick %d\n", z)

        case <-time.After(12 * time.Second):

            fmt.Println("12 seconds elapsed!")

        }

    }

}

如果我运行代码，time.After案例永远不会运行，但自动收报机工作正常。

如果我删除代码，time.After正确触发：

package main

import (

    "fmt"

    "time"

)

func main() {

    for {

        select {

        case <-time.After(12 * time.Second):

            fmt.Println("12 seconds elapsed!")

        }

    }

}

如果我使用计时器而不是time.After：

package main

import (

    "fmt"

    "time"

)

func main() {

    ticker := time.NewTicker(5 * time.Second)

    timer := time.NewTimer(12 * time.Second)

    for {

        select {

        case z := <-ticker.C:

            fmt.Printf("tick %d\n", z)

        case <-timer.C:

            fmt.Println("12 seconds elapsed!")

        }

    }

}

为什么会这样？

Answer 1

Aselect阻塞，直到它的一个案例准备好，然后它执行那个案例。在您的示例中 time.After() 永远不会被调用。func main() {&nbsp; &nbsp; ticker := time.NewTicker(5 * time.Second)&nbsp; &nbsp; for {&nbsp; &nbsp; &nbsp; &nbsp; select {&nbsp; &nbsp; &nbsp; &nbsp; case z := <-ticker.C:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; fmt.Printf("tick %d\n", z)&nbsp; &nbsp; &nbsp; &nbsp; //This never gets chance to be ready. It'll work if you make it less than 5 seconds.&nbsp; &nbsp; &nbsp; &nbsp; case <-time.After(12 * time.Second):&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; fmt.Println("12 seconds elapsed!")&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }}您可以通过在for循环之前声明计时器来使其工作。func main() {&nbsp; &nbsp; &nbsp; &nbsp; ticker := time.NewTicker(5 * time.Second)&nbsp; &nbsp; &nbsp; &nbsp; timer := time.After(12 * time.Second)&nbsp; &nbsp; &nbsp; &nbsp; for {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; select {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; case z := <-ticker.C:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; fmt.Printf("tick %d\n", z)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; case <-timer:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; fmt.Println("12 seconds elapsed!")&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp;&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp;}&nbsp; &nbsp;

Answer 2

关键是，当调用 select 时，它将在所有 case 子句中重新创建通道。如果您在 中创建计时器case <- newCreateTimerChannel，它将启动一个新计时器。所以把定时器的创建放在for循环之外，让它成为一个全局定时器。