https://nanxiao.gitbooks.io/golang-101-hacks/content/posts/select-operation.html

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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
        }
        ......
    }
}  

golang中select case 的用途到底是啥