Go 通道未接收/打印发送到通道的最后一个值

Question

这是一段代码，它输出推送到通道的整数列表。否则，选择检查并打印必要的超时消息。

package main

import (

    "fmt"

    "sync"

    "time"

)

var wg sync.WaitGroup

func main() {

    wg.Add(1)

    c := make(chan int)

    go readFromChannel(c, time.After(time.Duration(2)*time.Second))

//  time.Sleep(time.Duration(5) * time.Second) //Talking about uncommenting this line

    c <- 10

    c <- 20

    c <- 30

    c <- 40

    c <- 50

    wg.Wait()

}

func readFromChannel(c chan int, ti <-chan time.Time) {

    defer wg.Done()

    go func() {

        for {

            select {

            case x := <-c:

                fmt.Println("Read", x)

            case t :=<-ti:

                fmt.Println("TIMED OUT with "+t.String())

            }

        }

    }()

}

另外，这里还有相同的游乐场链接： https: //play.golang.org/p/4hNWze4Pfwr 此代码输出整数列表，例如

Read 10

Read 20

Read 30

Read 40

Read 50

但是，当我取消注释使主例程进入睡眠状态 5 秒的行（在代码中称为注释/行号 16）时，输出更改为：

TIMED OUT with 2009-11-10 23:00:02 +0000 UTC m=+2.000000001

Read 10

Read 20

Read 30

Read 40

我想了解为什么Read 50在第二种情况下没有打印最后一个。

Answer 1

问题是你的wg.Done()位置不对。它必须在你的 goroutine 中，但是你在 goroutine 启动之前执行它，所以你的程序很可能在做任何工作之前退出。改变这个：defer wg.Done()go func() {对此：go func() {&nbsp; &nbsp; defer wg.Done()当然，你将拥有一个无限运行的 goroutine，因为你的for循环没有退出条件。您需要添加一个，可能是通过检查通道关闭来添加：&nbsp; &nbsp; &nbsp; &nbsp; select {&nbsp; &nbsp; &nbsp; &nbsp; case x, ok := <-c:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if !ok { // channel was closed&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; fmt.Println("Read", x)&nbsp; &nbsp; &nbsp; &nbsp; case t :=<-ti:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; fmt.Println("TIMED OUT with "+t.String())&nbsp; &nbsp; &nbsp; &nbsp; }然后告诉你的主 go 例程在完成后关闭通道：c <- 40c <- 50close(c)&nbsp; // We're done, so tell the goroutine to finish upwg.Wait() // But wait until it's done