https://nanxiao.gitbooks.io/golang-101-hacks/content/posts/select-operation.html
---------------------------------------------------------------------------------
Select operation
Go
‘s select
operation looks similar to switch
, but it‘s dedicated to poll send and receive operations channels. Check the following example:
package main
import (
"fmt"
"time"
)
func main() {
ch1 := make(chan int)
ch2 := make(chan int)
go func(ch chan int) { <-ch }(ch1)
go func(ch chan int) { ch <- 2 }(ch2)
time.Sleep(time.Second)
for {
select {
case ch1 <- 1:
fmt.Println("Send operation on ch1 works!")
case <-ch2:
fmt.Println("Receive operation on ch2 works!")
default:
fmt.Println("Exit now!")
return
}
}
}
The running result is like this:
Send operation on ch1 works!
Receive operation on ch2 works!
Exit now!
The select
operation will check which case
branch can be run, that means the send or receive action can be executed successfully. If more than one case
are ready now, the select
will randomly choose one to execute. If no case
is ready, but there is a default
branch, then the default
block will be executed, else the select
operation will block. In the above example, if the main
goroutine doesn‘t sleep (time.Sleep(time.Second)
), the other 2 func
goroutines won‘t obtain the opportunity to run, so only default
block in select
statement will be executed.
The select
statement won‘t process nil
channel, so if a channel used for receive operation is closed, you should mark its value as nil
, then it will be kicked out of the selection list. So a common pattern of selection on multiple receive channels looks like this:
for ch1 != nil && ch2 != nil {
select {
case x, ok := <-ch1:
if !ok {
ch1 = nil
break
}
......
case x, ok := <-ch2:
if !ok {
ch2 = nil
break
}
......
}
}