golang 进程数据同步sync.WaitGroup

加sync.WaitGroup 会等待WaitGroup内的线程执行完毕后再执行后面的代码，例如

var fmtMap sync.Map
func producer(ch chan<- int)  {
	defer waitGroup.Done()
	for i := 0; i < 10; i++ {
		ch <- i
	}
	close(ch)
	fmtMap.Store("A","A")
}

func consumer(ch <-chan int){
	defer waitGroup.Done()
	for {
		select {
		case _, ok:=<-ch:
			if ok == false {
				goto done
			}
		}
	}
	done:
		fmtMap.Store("B","B")
}

var waitGroup = new(sync.WaitGroup)
func waitTest()  {
	ch := make(chan int, 5)
	waitGroup.Add(2)
	go producer(ch)
	go consumer(ch)
	waitGroup.Wait()
	fmt.Println(fmtMap)
}
func main() {
	go waitTest()
	router := gin.Default()
	router.Run(":8031")
}

会输出

如果没加sync.WaitGroup则会打印nil,因为没等待开启的协程执行完毕就已经执行打印语句了