计时器具有一个名为C的chan字段。要返回它，我必须在其后加上括号吗？

Question

I'm a Go rookie.

I'm looking at some Kubernetes source code that looks in part like this:

t := r.clock.NewTimer(r.resyncPeriod)
return t.C(), t.Stop

t is a new Timer, and I can see that the Timer type has a field named C, which is a chan whose messages, should there be any, will be of type Time. I understand that part.

I don't understand why it's t.C() above, and not t.C (no parentheses). To my naïve eyes, this looks like some sort of function invocation on the channel. Could someone kindly explain what's going on here? What do the parentheses indicate, in particular? Thank you.

Answer 1

If you look at Clock

you will find:

type Clock interface {
    Now() time.Time
    Since(time.Time) time.Duration
    After(d time.Duration) <-chan time.Time
    NewTimer(d time.Duration) Timer
    Sleep(d time.Duration)
    Tick(d time.Duration) <-chan time.Time
}

Notice that clock's NewTimer returns a Timer from k8s clock package, which is defined as:

type Timer interface {
    C() <-chan time.Time
    Stop() bool
    Reset(d time.Duration) bool
}

And that's what you are calling to get the channel.

This is so they can swap out the timer, likely for test cases.

So, while it looks like the stdlib's Timer type, it's really an interface to allow swapability.