Go by Example: Non-Blocking Channel Operations

原文:    https://gobyexample.com/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.

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

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.

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.

package main

import "fmt"

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

    select {
    case msg := <-messages:
        fmt.Println("received message", msg)
    default:
        fmt.Println("no message received")
    }

    msg := "hi"
    select {
    case messages <- msg:
        fmt.Println("sent message", msg)
    default:
        fmt.Println("no message sent")
    }

    select {
    case msg := <-messages:
        fmt.Println("received message", msg)
    case sig := <-signals:
        fmt.Println("received signal", sig)
    default:
        fmt.Println("no activity")
    }
}