How to check a channel is closed or not without reading it?

Question

This is a good example of workers & controller mode in Go written by @Jimt, in answer to \"Is there some elegant way to pause & resume any other goroutine in golang?

Answer 1

From the documentation:

A channel may be closed with the built-in function close. The multi-valued assignment form of the receive operator reports whether a received value was sent before the channel was closed.

https://golang.org/ref/spec#Receive_operator

Example by Golang in Action shows this case:

// This sample program demonstrates how to use an unbuffered
// channel to simulate a game of tennis between two goroutines.
package main

import (
    "fmt"
    "math/rand"
    "sync"
    "time"
)

// wg is used to wait for the program to finish.
var wg sync.WaitGroup

func init() {
    rand.Seed(time.Now().UnixNano())
}

// main is the entry point for all Go programs.
func main() {
    // Create an unbuffered channel.
    court := make(chan int)
    // Add a count of two, one for each goroutine.
    wg.Add(2)
    // Launch two players.
    go player("Nadal", court)
    go player("Djokovic", court)
    // Start the set.
    court <- 1
    // Wait for the game to finish.
    wg.Wait()
}

// player simulates a person playing the game of tennis.
func player(name string, court chan int) {
    // Schedule the call to Done to tell main we are done.
    defer wg.Done()
    for {
        // Wait for the ball to be hit back to us.
        ball, ok := <-court
        fmt.Printf("ok %t\n", ok)
        if !ok {
            // If the channel was closed we won.
            fmt.Printf("Player %s Won\n", name)
            return
        }
        // Pick a random number and see if we miss the ball.
        n := rand.Intn(100)
        if n%13 == 0 {
            fmt.Printf("Player %s Missed\n", name)
            // Close the channel to signal we lost.
            close(court)
            return
        }

        // Display and then increment the hit count by one.
        fmt.Printf("Player %s Hit %d\n", name, ball)
        ball++
        // Hit the ball back to the opposing player.
        court <- ball
    }
}

Answer 2

If you listen this channel you always can findout that channel was closed.

case state, opened := <-ws:
    if !opened {
         // channel was closed 
         // return or made some final work
    }
    switch state {
        case Stopped:

But remember, you can not close one channel two times. This will raise panic.