Go Select and Channels

From WikiOD

The select keyword provides an easy method to work with channels and perform more advanced tasks. It is frequently used for a number of purposes: - Handling timeouts. - When there are multiple channels to read from, the select will randomly read from one channel which has data. - Providing an easy way to define what happens if no data is available on a channel.

Syntax[edit | edit source]

  • select {}
  • select { case true: }
  • select { case incomingData := <-someChannel: }
  • select { default: }

Using select with timeouts[edit | edit source]

So here, I have removed the for loops, and made a timeout by adding a second case to the select that returns after 3 seconds. Because the select just waits until ANY case is true, the second case fires, and then our script ends, and chatter() never even gets a chance to finish.

// Use of the select statement with channels, for timeouts, etc.
package main

import (
    "fmt"
    "time"
)

// Function that is "chatty"
//Takes a single parameter a channel to send messages down
func chatter(chatChannel chan<- string) {
    // loop ten times and die
    time.Sleep(5 * time.Second) // sleep for 5 seconds
    chatChannel<- fmt.Sprintf("This is pass number %d of chatter", 1)
}

// out main function
func main() {
    // Create the channel, it will be taking only strings, no need for a buffer on this project
    chatChannel := make(chan string)
    // Clean up our channel when we are done
    defer close(chatChannel)

    // start a go routine with chatter (separate, no blocking)
    go chatter(chatChannel)

    // select statement will block this thread until one of the two conditions below is met
    // because we have a default, we will hit default any time the chatter isn't chatting
    select {
    // anytime the chatter chats, we'll catch it and output it
    case spam := <-chatChannel:
        fmt.Println(spam)
    // if the chatter takes more than 3 seconds to chat, stop waiting
    case <-time.After(3 * time.Second):
        fmt.Println("Ain't no time for that!")
    }
}

Simple Select Working with Channels[edit | edit source]

In this example we create a goroutine (a function running in a separate thread) that accepts a chan parameter, and simply loops, sending information into the channel each time.

In the main we have a for loop and a select. The select will block processing until one of the case statements becomes true. Here we have declared two cases; the first is when information comes through the channel, and the other is if no other case occurs, which is known as default.

// Use of the select statement with channels (no timeouts)
package main

import (
    "fmt"
    "time"
)

// Function that is "chatty"
// Takes a single parameter a channel to send messages down
func chatter(chatChannel chan<- string) {
    // Clean up our channel when we are done.
    // The channel writer should always be the one to close a channel.
    defer close(chatChannel)

    // loop five times and die
    for i := 1; i <= 5; i++ {
        time.Sleep(2 * time.Second) // sleep for 2 seconds
        chatChannel <- fmt.Sprintf("This is pass number %d of chatter", i)
    }
}

// Our main function
func main() {
    // Create the channel
    chatChannel := make(chan string, 1)

    // start a go routine with chatter (separate, non blocking)
    go chatter(chatChannel)

    // This for loop keeps things going while the chatter is sleeping
    for {
        // select statement will block this thread until one of the two conditions below is met
        // because we have a default, we will hit default any time the chatter isn't chatting
        select {
        // anytime the chatter chats, we'll catch it and output it
        case spam, ok := <-chatChannel:
            // Print the string from the channel, unless the channel is closed
            // and we're out of data, in which case exit.
            if ok {
                fmt.Println(spam)
            } else {
                fmt.Println("Channel closed, exiting!")
                return
            }
        default:
            // print a line, then sleep for 1 second.
            fmt.Println("Nothing happened this second.")
            time.Sleep(1 * time.Second)
        }
    }
}

Try it on the Go Playground!

Credit:Stack_Overflow_Documentation