详解Golang中Context的三个常见应用场景

    ctx, cancel := context.WithTimeout(context.Background(), time.Millisecond*80)
    defer cancel()
    req = req.WithContext(ctx)

/ The context will be cancelled after 3 seconds
// If it needs to be cancelled earlier, the `cancel` function can
// be used, like before
ctx, cancel := context.WithTimeout(ctx, 3*time.Second)

​​​​​​​// Setting a context deadline is similar to setting a timeout, except
// you specify a time when you want the context to cancel, rather than a duration.
// Here, the context will be cancelled on 2022-11-10 23:00:00
ctx, cancel := context.WithDeadline(ctx, time.Date(2022, time.November, 10, 23, 0, 0, 0, time.UTC))

func main() {
    //定义请求
	req, err := http.NewRequest(http.MethodGet, "https://www.baidu.com", nil)
	if err != nil {
		log.Fatal(err)
	}

    // 定义上下文
	ctx, cancel := context.WithTimeout(context.Background(), time.Millisecond*80)
	defer cancel()
	req = req.WithContext(ctx)

    // 执行请求
	c := &http.Client{}
	res, err := c.Do(req)
	if err != nil {
		log.Fatal(err)
	}
	defer res.Body.Close()

    // 输出日志
	out, err := io.ReadAll(res.Body)
	if err != nil {
		log.Fatal(err)
	}
	log.Println(string(out))
}

func operation1(ctx context.Context) error {
	// 假设该操作因某种原因而失败
	// 下面模拟业务执行一定时间
	time.Sleep(100 * time.Millisecond)
	return errors.New("failed")
}

func operation2(ctx context.Context) {
	// 该方法要么正常执行完成
	// 要么取消，不再继续执行
	select {
	case <-time.After(500 * time.Millisecond):
		fmt.Println("done")
	case <-ctx.Done():
		fmt.Println("halted operation2")
	}
}

func main() {
	// 创建上下文
	ctx := context.Background()

	// 基于上下文创建需求上下文
	ctx, cancel := context.WithCancel(ctx)

	// 在不同协程中执行两个操作
	go func() {
		err := operation1(ctx)
		// 如果该方法返回错误，则取消该上下文中的后续操作
		if err != nil {
			cancel()
		}
	}()

	// 实用相同上下文执行操作2
	operation2(ctx)
}

// 定义key，用于保存上下文值的键
const keyID = "id"

func main() {
    // 定义上下文值
	rand.Seed(time.Now().Unix())
	ctx := context.WithValue(context.Background(), keyID, rand.Int())
	operation1(ctx)
}

func operation1(ctx context.Context) {
	// do some work

	// we can get the value from the context by passing in the key
	log.Println("operation1 for id:", ctx.Value(keyID), " completed")
	operation2(ctx)
}

func operation2(ctx context.Context) {
	// do some work

	// this way, the same ID is passed from one function call to the next
	log.Println("operation2 for id:", ctx.Value(keyID), " completed")
}