詳解Golang中Context的三個常見應用場景

    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() {
    //定義請求
	req, err := http.NewRequest(http.MethodGet, "https://www.baidu.com", nil)
	if err != nil {
		log.Fatal(err)
	}

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

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

    // 輸出紀錄檔
	out, err := io.ReadAll(res.Body)
	if err != nil {
		log.Fatal(err)
	}
	log.Println(string(out))
}

func operation1(ctx context.Context) error {
	// 假設該操作因某種原因而失敗
	// 下面模擬業務執行一定時間
	time.Sleep(100 * time.Millisecond)
	return errors.New("failed")
}

func operation2(ctx context.Context) {
	// 該方法要麼正常執行完成
	// 要麼取消，不再繼續執行
	select {
	case <-time.After(500 * time.Millisecond):
		fmt.Println("done")
	case <-ctx.Done():
		fmt.Println("halted operation2")
	}
}

func main() {
	// 建立上下文
	ctx := context.Background()

	// 基於上下文建立需求上下文
	ctx, cancel := context.WithCancel(ctx)

	// 在不同協程中執行兩個操作
	go func() {
		err := operation1(ctx)
		// 如果該方法返回錯誤，則取消該上下文中的後續操作
		if err != nil {
			cancel()
		}
	}()

	// 實用相同上下文執行操作2
	operation2(ctx)
}

// 定義key，用於儲存上下文值的鍵
const keyID = "id"

func main() {
    // 定義上下文值
	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")
}