select就是用来监听和channel有关的IO操作，当 IO 操作发生时，触发相应的动作

package main   import (     "fmt"     "time" )   func main() {     ch := make(chan int)     o := make(chan bool)       go func() {          for {                  select {                  case <-time.After(3 * time.Second):                          fmt.Println("超时")                          o <- true                  case num := <-ch:                          fmt.Println(num)                  }          }     }()       <-o }