From f8fe26fb06801d15446cd3e576bdf922588402d6 Mon Sep 17 00:00:00 2001
From: Conner Fromknecht <conner@lightning.engineering>
Date: Wed, 1 Aug 2018 12:20:46 -0700
Subject: [PATCH] ticker/ticker: adds resumable Ticker interface

---
 ticker/ticker.go | 123 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 123 insertions(+)
 create mode 100644 ticker/ticker.go

diff --git a/ticker/ticker.go b/ticker/ticker.go
new file mode 100644
index 00000000..29e0c04b
--- /dev/null
+++ b/ticker/ticker.go
@@ -0,0 +1,123 @@
+package ticker
+
+import "time"
+
+// Ticker defines a resumable ticker interface, whose activity can be toggled to
+// free up resources during periods of inactivity.
+//
+// Example of resuming ticker:
+//
+//   ticker.Resume() // can remove to keep inactive at first
+//   defer ticker.Stop()
+//   for {
+//     select {
+//       case <-ticker.Tick():
+//         if shouldGoInactive {
+//           ticker.Pause()
+//           continue
+//         }
+//         ...
+//
+//       case <-otherEvent:
+//         ...
+//         if shouldGoActive {
+//           ticker.Resume()
+//         }
+//     }
+//
+// NOTE: ONE DOES NOT SIMPLY assume that Tickers are safe for concurrent access.
+type Ticker interface {
+	// Ticks returns a read-only channel delivering ticks according to a
+	// prescribed interval. The value returned does not need to be the same
+	// channel, and may be nil.
+	//
+	// NOTE: Callers should assume that reads from Ticks() are stale after
+	// any invocations of Resume, Pause, or Stop.
+	Ticks() <-chan time.Time
+
+	// Resume starts or resumes the underlying ticker, such that Ticks()
+	// will fire at regular intervals. After calling Resume, Ticks() should
+	// minimally send ticks at the prescribed interval.
+	//
+	// NOTE: It MUST be safe to call Resume at any time, and more than once
+	// successively.
+	Resume()
+
+	// Pause suspends the underlying ticker, such that Ticks() stops
+	// signaling at regular intervals. After calling Pause, the ticker
+	// should not send any ticks scheduled with the chosen interval. Forced
+	// ticks are still permissible, as in the case of the Mock Ticker.
+	//
+	// NOTE: It MUST be safe to call Pause at any time, and more than once
+	// successively.
+	Pause()
+
+	// Stop suspends the underlying ticker, such that Ticks() stops
+	// signaling at regular intervals, and permanently frees up any
+	// remaining resources.
+	//
+	// NOTE: The behavior of a Ticker is undefined after calling Stop.
+	Stop()
+}
+
+// ticker is the production implementation of the resumable Ticker interface.
+// This allows various components to toggle their need for tick events, which
+// may vary depending on system load.
+type ticker struct {
+	// interval is the desired duration between ticks when active.
+	interval time.Duration
+
+	// ticker is the ephemeral, underlying time.Ticker. We keep a reference
+	// to this ticker so that it can be stopped and cleaned up on Pause or
+	// Stop.
+	ticker *time.Ticker
+}
+
+// New returns a new ticker that signals with the given interval when not
+// paused. The ticker starts off inactive.
+func New(interval time.Duration) Ticker {
+	return &ticker{
+		interval: interval,
+	}
+}
+
+// Ticks returns a receive-only channel that delivers times at the ticker's
+// prescribed interval. This method returns nil when the ticker is paused.
+//
+// NOTE: Part of the Ticker interface.
+func (t *ticker) Ticks() <-chan time.Time {
+	if t.ticker == nil {
+		return nil
+	}
+	return t.ticker.C
+}
+
+// Resumes starts underlying time.Ticker and causes the ticker to begin
+// delivering scheduled events.
+//
+// NOTE: Part of the Ticker interface.
+func (t *ticker) Resume() {
+	if t.ticker == nil {
+		t.ticker = time.NewTicker(t.interval)
+	}
+}
+
+// Pause suspends the underlying ticker, such that Ticks() stops signaling at
+// regular intervals.
+//
+// NOTE: Part of the Ticker interface.
+func (t *ticker) Pause() {
+	if t.ticker != nil {
+		t.ticker.Stop()
+		t.ticker = nil
+	}
+}
+
+// Stop suspends the underlying ticker, such that Ticks() stops signaling at
+// regular intervals, and permanently frees up any resources. For this
+// implementation, this is equivalent to Pause.
+//
+// NOTE: Part of the Ticker interface.
+func (t *ticker) Stop() {
+	t.Pause()
+}