当 go 中计时器完成时如何从函数返回？

问题内容

我是刚开始开发玩具测验 cli 程序。我正在尝试实现一个计时器，以便当计时器结束时，测验就结束了。

这是我的初步实现。

func StartTimer(quizFinished chan bool, timer *time.Timer) {
    // Start timer
    <-timer.C
    fmt.Println("\nQuiz has ended")
    quizFinished <- true
    return
}

func askQuestions(timer *time.Timer, questions []string, answers []string) []string {
    var input string
    var userAnswers []string
    quizFinished := make(chan bool)

    fmt.Println("Starting Quiz...")

    go StartTimer(quizFinished, timer)

    for index, element := range questions {
        select {
        case <-quizFinished:
            fmt.Println("Quiz has ended")
            return userAnswers
        default:
            fmt.Printf("Question %d: %s? ", index+1, element)
            fmt.Scanln(&input)
            userAnswers = append(userAnswers, input)
        }
    }

    return userAnswers
}

编辑实现

package main

import (
    "bufio"
    "flag"
    "fmt"
    "log"
    "os"
    s "strings"
    "time"
)

func readFileData(fileName string) (questions []string, answers []string) {
    file, err := os.Open(fileName)
    if err != nil {
        log.Fatalf("failed reading data from file: %v", err)
    }

    defer file.Close()

    scanner := bufio.NewScanner(file)
    for scanner.Scan() {
        questionAndAnswer := s.Split(scanner.Text(), ",")
        question, answer := questionAndAnswer[0], questionAndAnswer[1]
        questions = append(questions, question)
        answers = append(answers, answer)
    }
    return questions, answers
}

func askQuestions(timer *time.Timer, questions []string) []string {
    var input string
    var userAnswers []string = make([]string, 0, len(questions))
    quizFinished := make(chan bool, 1)

    fmt.Println("Starting Quiz...")

    go func() {
        <-timer.C
        fmt.Println("\nquiz over...")
        quizFinished <- true
    }()

    for index, element := range questions {
        fmt.Printf("Question %d: %s? ", index+1, element)
        answerCh := make(chan string, 1)
        go func() {
            fmt.Scanln(&input)
            answerCh <- input
        }()

        select {
        case <-quizFinished:
            return userAnswers
        case answer := <-answerCh:
            userAnswers = append(userAnswers, answer)
        }
    }

    return userAnswers
}

func main() {
    fileNamePtr := flag.String("filename", "problems.csv", "Specify a file name to read in")
    quizTimePtr := flag.Int("time", 5, "Specify how long the quiz should take")
    flag.Parse()

    timer := time.NewTimer(time.Duration(*quizTimePtr) * time.Second)
    questions, answers := readFileData(*fileNamePtr)

    // var userAnswers = askQuestions(timer, questions, answers)
    var userAnswers = askQuestions(timer, questions)
    correctAnswers := 0
    incorrectAnswers := 0

    for i := 0; i < len(answers); i++ {
        if userAnswers[i] == answers[i] {
            correctAnswers++
        } else {
            incorrectAnswers++
        }
    }

    grade := float32(correctAnswers) / float32(len((questions)))
    fmt.Printf("Total Questions: %d \n", len(questions))
    fmt.Printf("Total Correct Answers: %d \n", correctAnswers)
    fmt.Printf("Total Incorrect Answers: %d \n", incorrectAnswers)
    fmt.Printf("Grade: %.0f%% \n", grade*100)

}

我不断收到以下输出和错误：

Starting Quiz...
Question 1: 5+5? 10
Question 2: 1+1?
Quiz has ended
2
Returning from quiz...
panic: runtime error: index out of range [2] with length 2

goroutine 1 [running]:

我对通道和 goroutine 的理解还不够深入，还无法理解正在发生的事情。预期的行为是当计时器停止时，它会打印“测验已结束”，并将一个值发送到 quizFinished 通道，该值在 select 语句中读取。我遇到的另一个问题是 Scanln 将在测验结束后等待用户输入。如有任何帮助，我们将不胜感激！

正确答案

当你在主goroutine中调用Scanln函数时，它会暂停并等待用户输入。即使你的计时器在另一个 Goroutine 中完成并尝试通知主 Goroutine，主 Goroutine 也不会响应，因为它被“Scanner”阻止了。

这就是为什么即使计时器结束，Scanln 仍在等待用户输入。

package main

import (
    "fmt"
    "time"
)

func askQuestions(timer *time.Timer, questions []string) []string {
    var input string
    var userAnswers []string
    quizFinished := make(chan bool, 1)

    fmt.Println("start quiz...")
    go func() {
        <-timer.C
        fmt.Println("\nquiz over...")
        quizFinished <- true
    }()

    for index, element := range questions {
        fmt.Printf("question %d: %s? ", index+1, element)
        answerCh := make(chan string, 1)

        go func() {
            fmt.Scanln(&input)
            answerCh <- input
        }()

        select {
        case <-quizFinished:
            fmt.Println("exit quiz...")
            return userAnswers
        case answer := <-answerCh:
            userAnswers = append(userAnswers, answer)
        }
    }

    return userAnswers
}

func main() {
    timer := time.NewTimer(10 * time.Second)
    questions := []string{"5+5", "1+1", "8+3"}
    responses := askQuestions(timer, questions)
    fmt.Println("answer:", responses)
}

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