Select

The select statement lets a goroutine wait on multiple communication operations.

A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.

Example

package main

import "fmt"

func fibonacci(c, quit chan int) {
	x, y := 0, 1
	for {
		select {
		case c <- x:
			x, y = y, x+y
		case <-quit:
			fmt.Println("quit")
			return
		}
	}
}

func main() {
	c := make(chan int)
	quit := make(chan int)
	go func() {
		for i := 0; i < 8; i++ {
			fmt.Println(<-c)
		}
		quit <- 0
	}()
	fibonacci(c, quit)
}

0
1
1
2
3
5
8
13
quit

Default Selection

The default case in a select is run if no other case is ready.

Example

package main

import (
	"fmt"
	"time"
)

func main() {
	tick := time.Tick(100 * time.Millisecond)
	boom := time.After(500 * time.Millisecond)
	for {
		select {
		case <-tick:
			fmt.Println("tick")
		case <-boom:
			fmt.Println("BOOM!")
			return
		default:
			fmt.Println("...")
			time.Sleep(50 * time.Millisecond)
		}
	}
}

...
...
tick
...
...
tick
...
...
tick
...
...
tick
...
...
BOOM!