go/src/time/tick.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package time

import "unsafe"

// Note: The runtime knows the layout of struct Ticker, since newTimer allocates it.
// Note also that Ticker and Timer have the same layout, so that newTimer can handle both.
// The initTimer and initTicker fields are named differently so that
// users cannot convert between the two without unsafe.

// A Ticker holds a channel that delivers “ticks” of a clock
// at intervals.
type Ticker struct {
	C          <-chan Time // The channel on which the ticks are delivered.
	initTicker bool
}

// NewTicker returns a new [Ticker] containing a channel that will send
// the current time on the channel after each tick. The period of the
// ticks is specified by the duration argument. The ticker will adjust
// the time interval or drop ticks to make up for slow receivers.
// The duration d must be greater than zero; if not, NewTicker will
// panic.
//
// Before Go 1.23, the garbage collector did not recover
// tickers that had not yet expired or been stopped, so code often
// immediately deferred t.Stop after calling NewTicker, to make
// the ticker recoverable when it was no longer needed.
// As of Go 1.23, the garbage collector can recover unreferenced
// tickers, even if they haven't been stopped.
// The Stop method is no longer necessary to help the garbage collector.
// (Code may of course still want to call Stop to stop the ticker for other reasons.)
func NewTicker(d Duration) *Ticker {
	if d <= 0 {
		panic("non-positive interval for NewTicker")
	}
	// Give the channel a 1-element time buffer.
	// If the client falls behind while reading, we drop ticks
	// on the floor until the client catches up.
	c := make(chan Time, 1)
	t := (*Ticker)(unsafe.Pointer(newTimer(when(d), int64(d), sendTime, c, syncTimer(c))))
	t.C = c
	return t
}

// Stop turns off a ticker. After Stop, no more ticks will be sent.
// Stop does not close the channel, to prevent a concurrent goroutine
// reading from the channel from seeing an erroneous "tick".
func (t *Ticker) Stop() {
	if !t.initTicker {
		// This is misuse, and the same for time.Timer would panic,
		// but this didn't always panic, and we keep it not panicking
		// to avoid breaking old programs. See issue 21874.
		return
	}
	stopTimer((*Timer)(unsafe.Pointer(t)))
}

// Reset stops a ticker and resets its period to the specified duration.
// The next tick will arrive after the new period elapses. The duration d
// must be greater than zero; if not, Reset will panic.
func (t *Ticker) Reset(d Duration) {
	if d <= 0 {
		panic("non-positive interval for Ticker.Reset")
	}
	if !t.initTicker {
		panic("time: Reset called on uninitialized Ticker")
	}
	resetTimer((*Timer)(unsafe.Pointer(t)), when(d), int64(d))
}

// Tick is a convenience wrapper for [NewTicker] providing access to the ticking
// channel only. Unlike NewTicker, Tick will return nil if d <= 0.
//
// Before Go 1.23, this documentation warned that the underlying
// [Ticker] would never be recovered by the garbage collector, and that
// if efficiency was a concern, code should use NewTicker instead and
// call [Ticker.Stop] when the ticker is no longer needed.
// As of Go 1.23, the garbage collector can recover unreferenced
// tickers, even if they haven't been stopped.
// The Stop method is no longer necessary to help the garbage collector.
// There is no longer any reason to prefer NewTicker when Tick will do.
func Tick(d Duration) <-chan Time {
	if d <= 0 {
		return nil
	}
	return NewTicker(d).C
}