Non-Blocking Channel Operations

Basic sends and receives on channels are blocking. However, we can use select with a default clause to implement non-blocking sends, receives, and even non-blocking multi-way selects.

package main

import "fmt"

func main() {
    messages := make(chan string)
    signals := make(chan bool)

    // Here’s a non-blocking receive. 
    // If a value is available on messages then select will 
    // take the <-messages case with that value. 
    // If not it will immediately take the default case.
    select {
    case msg := <-messages:
        fmt.Println("received message", msg)
    default:
        fmt.Println("no message received")
    }

    // A non-blocking send works similarly. 
    // Here msg cannot be sent to the messages channel
    // because the channel has no buffer and there is no receiver. 
    // Therefore the default case is selected.
    msg := "hi"
    select {
    case messages <- msg:
        fmt.Println("sent message", msg)
    default:
        fmt.Println("no message sent")
    }

    // We can use multiple cases above the default clause 
    // to implement a multi-way non-blocking select. 
    // Here we attempt non-blocking receives on both messages and signals.
    select {
    case msg := <-messages:
        fmt.Println("received message", msg)
    case sig := <-signals:
        fmt.Println("received signal", sig)
    default:
        fmt.Println("no activity")
    }
}

$ go run non-blocking-channel-operations.go 
no message received
no message sent
no activity