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Go语言：调度间隔用哪个好？time.Sleep v.s. time.After

 5 years ago
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Go编程中循环调度任务的执行间隔我们通常采用 time.Sleep或time.After来实现。

写法1:

go func(ctx context.Context) {
		for {
			select {
			case <-ctx.Done():
				fmt.Println("Cancelled", time.Now())
				wg.Done()
				return
			default:
				time.Sleep(time.Second * 10)
				fmt.Println("Invoked", time.Now())
			}
		}
	}(ctx)

写法2:

go func(ctx context.Context) {
		for {
			select {
			case <-ctx.Done():
				fmt.Println("Cancelled", time.Now())
				wg.Done()
				return
			case <-time.After(time.Second * 10):
				fmt.Println("Invoked", time.Now())
			}
		}
	}(ctx)

这两种方式一样吗？那种方式更好呢？ 以上两种写法，都可以满足需求。那么是否这两种写法是等效的呢？ 当然不是，为了更好的说明它们的差别，请运行下面的这两段测试代码：

var wg sync.WaitGroup
func cancelTaskAfter(interval time.Duration, cancel context.CancelFunc) {
	go func(cancel context.CancelFunc) {
		time.Sleep(interval)
		fmt.Println("Cancell task", time.Now())
		cancel()
		wg.Done()
	}(cancel)
}

func TestTimerTask1(t *testing.T) {
	ctx, cancel := context.WithCancel(context.Background())
	wg.Add(1)
	cancelTaskAfter(time.Second*25, cancel)
	wg.Add(1)
	go func(ctx context.Context) {
		for {
			select {
			case <-ctx.Done():
				fmt.Println("Cancelled", time.Now())
				wg.Done()
				return
			default:
				time.Sleep(time.Second * 10)
				fmt.Println("Invoked", time.Now())
			}
		}
	}(ctx)
	wg.Wait()
}

func TestTimerTask2(t *testing.T) {
	ctx, cancel := context.WithCancel(context.Background())
	wg.Add(1)
	cancelTaskAfter(time.Second*25, cancel)
	wg.Add(1)
	go func(ctx context.Context) {
		for {
			select {
			case <-ctx.Done():
				fmt.Println("Cancelled", time.Now())
				wg.Done()
                               return
			case <-time.After(time.Second * 10):
				fmt.Println("Invoked", time.Now())
			}
		}
	}(ctx)
	wg.Wait()
}

也许看到这，你大概已经知道这两种写法的不同了。

以下是输出结果

=== RUN   TestTimerTask1
 Invoked 2019-09-22 17:12:07.055392 +0800 CST m=+35.007752073
 Invoked 2019-09-22 17:12:17.057073 +0800 CST m=+45.009375450
 Cancell task 2019-09-22 17:12:22.051311 +0800 CST m=+50.003583680
 Invoked 2019-09-22 17:12:27.060766 +0800 CST m=+55.013010382
 Cancelled 2019-09-22 17:12:27.060804 +0800 CST m=+55.013048342
 --- PASS: TestTimerTask1 (30.01s)
=== RUN   TestTimerTask2
Invoked 2019-09-22 17:11:42.049262 +0800 CST m=+10.001766181
Invoked 2019-09-22 17:11:52.053054 +0800 CST m=+20.005500478
Cancell task 2019-09-22 17:11:57.050145 +0800 CST m=+25.002563035
Cancelled 2019-09-22 17:11:57.05041 +0800 CST m=+25.002827648
--- PASS: TestTimerTask2 (25.00s)

采用Sleep来实现间隔的时候，如果cancel调用后任务协程正好处于sleep过程中，这时任务是无法被及时取消的。

小结 如果你需要任务能够被及时的取消，那你应该采用time.After来控制调用的间隔。

更多知识，欢迎订阅学习 《Go 语言入门到实战》

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