Go 關閉 channel 的 close 方法

在 Go 中我們所以 close() 來關閉一個 channel

• 官方的註釋如下

The close built-in function closes a channel, which must be either bidirectional or send-only.
It should be executed only by the sender,never the receiver, and has the effect of shutting down the channel after the last sent value is received.
After the last value has been received from a closed channel c, any receive from c will succeed without blocking, returning the zero value for the channel element.
The form x, ok := <-c will also set ok to false for a closed channel.

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翻譯如下

• close 內置函數關閉一個通道，該通道必須是雙向的或僅發送的。
  • 如下關閉 ch3 就會報錯 invalid operation: close(ch3) (cannot close receive-only channel)

ch1 := make(chan int, 10)
ch2 := make(chan<- int, 10)
ch3 := make(<-chan int, 10)
close(ch1)
close(ch2)
close(ch3)

• channel 應僅由發送方執行，而不應由接收方執行，並且在收到最後發送的值後具有關閉通道的效果。
  • 即 channel 應該由發送的一方執行，由接收 channel 的一方關閉

func send(ch chan int) {
	for i := 0; i < 10; i++ {
		ch <- i
	}
}
func receive(ch chan int) {
	for  {
		fmt.Println(<-ch)
	}
	close(ch)
}
func main() {
	ch := make(chan int, 10)
	go send(ch)
	go receive(ch)
	time.Sleep(1 * time.Millisecond)
}

• 在從閉合通道 c 接收到最後一個值之後，來自 c 的任何接收都將成功而不會阻塞，返回通道元素的零值。
  • 如下，因爲通道元素類型是int，零值是0，所以輸出全是ch 的 this is from ch 0 ，而 ch1由於沒有賦值，所以沒有 IO 操作不能被 select 捕捉

func receive(ch chan int, ch1 chan int) {
	for {
		select {
		case a := <-ch:
			fmt.Println("this is from ch", a)
		case b := <-ch1:
			fmt.Println("this is from ch1", b)
		}
	}
}

func main() {
	ch := make(chan int, 10)
	ch1 := make(chan int, 10)
	close(ch)
	go receive(ch, ch1)
	time.Sleep(1 * time.Millisecond)
}

• 對於封閉的通道，表單x , ok := <- c 也會將 ok 設置爲 false
  • 輸出 0 和 false ，通過這一點我們可以判斷 channel 是否被關閉

	ch := make(chan int, 10)
	close(ch)
	if x, ok := <-ch; !ok {
		fmt.Println(x, ok)
	}