package main

import (
	"context"
	"fmt"
	"sync"
	"time"
)

var WG sync.WaitGroup

func f1(ctx context.Context) {
	defer WG.Done()
	go f2(ctx)
loop:
	for {
		fmt.Println("hello world")
		time.Sleep(time.Millisecond * 500)
		select {
		case <-ctx.Done():
			break loop
		default:

		}
	}

}
func f2(ctx context.Context) {
	defer WG.Done()
loop:
	for {
		fmt.Println("hello wangao")
		time.Sleep(time.Millisecond * 500)
		select {
		case <-ctx.Done():
			break loop
		default:

		}
	}
}

func main() {
	ctx, cancel := context.WithCancel(context.Background())
	WG.Add(1)
	go f1(ctx)
	time.Sleep(time.Second * 3)
	//如何通知goroutine退出？
	cancel()
	WG.Wait()

}