为什么在选择块中将其与股票代码配对后，时间永远不会触发？

Question

I have a select block that is listening on 2 channels, a ticker and a timer:

package main

import (
    "fmt"
    "time"
)

func main() {

    ticker := time.NewTicker(5 * time.Second)
    for {
        select {
        case z := <-ticker.C:
            fmt.Printf("tick %d
", z)

        case <-time.After(12 * time.Second):
            fmt.Println("12 seconds elapsed!")
        }
    }
}

If I run the code, the time.After case never runs, but the ticker works correctly.

If I remove the ticker, time.After fires correctly:

package main

import (
    "fmt"
    "time"
)

func main() {

    for {
        select {
        case <-time.After(12 * time.Second):
            fmt.Println("12 seconds elapsed!")
        }
    }
}

It works correctly if I used a timer rather than time.After:

package main

import (
    "fmt"
    "time"
)

func main() {

    ticker := time.NewTicker(5 * time.Second)
    timer := time.NewTimer(12 * time.Second)
    for {
        select {
        case z := <-ticker.C:
            fmt.Printf("tick %d
", z)

        case <-timer.C:
            fmt.Println("12 seconds elapsed!")
        }
    }
}

Why does this happen?

Answer 1

A select blocks until one of its cases is ready, then it executes that case. In your example time.After() never gets called.

func main() {

    ticker := time.NewTicker(5 * time.Second)
    for {
        select {
        case z := <-ticker.C:
            fmt.Printf("tick %d
", z)

        //This never gets chance to be ready. It'll work if you make it less than 5 seconds.
        case <-time.After(12 * time.Second): 
            fmt.Println("12 seconds elapsed!")
        }
    }
}

You can get it working by having timer declared before for loop.

func main() {

        ticker := time.NewTicker(5 * time.Second)
        timer := time.After(12 * time.Second)
        for {
                select {
                case z := <-ticker.C:
                        fmt.Printf("tick %d
", z)

                case <-timer:
                        fmt.Println("12 seconds elapsed!")
                }   
        }   
}