From 995137aeb3f465861fd6c34dab0d9b0d53e6c833 Mon Sep 17 00:00:00 2001
From: Paul Davis <paul@linuxaudiosystems.com>
Date: Sat, 26 Jun 2021 11:03:33 -0600
Subject: [PATCH] add PBD::WaitTimerRAII with reverse semantics from
 PBD::TimerRAII

---
 libs/pbd/pbd/timing.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/libs/pbd/pbd/timing.h b/libs/pbd/pbd/timing.h
index 42bc90649f..14e4fe9a0d 100644
--- a/libs/pbd/pbd/timing.h
+++ b/libs/pbd/pbd/timing.h
@@ -103,6 +103,8 @@ public:
 		return elapsed;
 	}
 
+	bool started() const { return m_start_val != 0; }
+
 	/// @return Elapsed time in microseconds
 	uint64_t elapsed () const {
 		return m_last_val - m_start_val;
@@ -219,6 +221,10 @@ private:
 	int      _queue_reset;
 };
 
+/** Provides an exception (and return path)-safe method to measure a timer
+ * interval. The timer is started at scope entry, and updated at scope exit
+ * (however that occurs)
+ */
 class LIBPBD_API TimerRAII
 {
   public:
@@ -227,6 +233,20 @@ class LIBPBD_API TimerRAII
 	TimingStats& stats;
 };
 
+/** Reverse semantics from TimerRAII. This starts the timer at scope exit,
+ *  and then updates it (computes interval) at scope entry. This is designed
+ *  for use with a callback API like CoreAudio, where we want to time the
+ *  interval between us being done with our work, and when our callback is
+ *  next executed.
+ */
+class LIBPBD_API WaitTimerRAII
+{
+  public:
+	WaitTimerRAII (TimingStats& ts) : stats (ts) { if (stats.started()) { stats.update(); } }
+	~WaitTimerRAII() { stats.start(); }
+	TimingStats& stats;
+};
+
 class LIBPBD_API TimingData
 {
 public: