Add metronome interface

Change-Id: Iea2f8ddb21a5d5a6880912f63a81cd4be408bb09
Bug: webrtc:13560, chromium:1253787
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/249380
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35844}

api/metronome/BUILD.gn

@@ -0,0 +1,19 @@
+# Copyright (c) 2022 The WebRTC project authors. All Rights Reserved.
+#
+# Use of this source code is governed by a BSD-style license
+# that can be found in the LICENSE file in the root of the source
+# tree. An additional intellectual property rights grant can be found
+# in the file PATENTS.  All contributing project authors may
+# be found in the AUTHORS file in the root of the source tree.
+
+import("../../webrtc.gni")
+
+rtc_source_set("metronome") {
+  visibility = [ "*" ]
+  sources = [ "metronome.h" ]
+  deps = [
+    "../../rtc_base/system:rtc_export",
+    "../task_queue",
+    "../units:time_delta",
+  ]
+}

api/metronome/metronome.h

@@ -0,0 +1,65 @@
+/*
+ *  Copyright (c) 2022 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef API_METRONOME_METRONOME_H_
+#define API_METRONOME_METRONOME_H_
+
+#include "api/task_queue/task_queue_base.h"
+#include "api/units/time_delta.h"
+#include "rtc_base/system/rtc_export.h"
+
+namespace webrtc {
+
+// The Metronome posts OnTick() on task queues provided by its listeners' task
+// queue periodically. The metronome can be used as an alternative to using
+// PostDelayedTask on a thread or task queue for coalescing work and reducing
+// the number of idle-wakeups.
+//
+// Listeners can be added and removed from any sequence, but it is illegal to
+// remove a listener from an OnTick invocation.
+//
+// The metronome concept is still under experimentation, and may not be availble
+// in all platforms or applications. See https://crbug.com/1253787 for more
+// details.
+//
+// Metronome implementations must be thread-safe.
+class RTC_EXPORT Metronome {
+ public:
+  class RTC_EXPORT TickListener {
+   public:
+    virtual ~TickListener() = default;
+
+    // OnTick is run on the task queue provided by OnTickTaskQueue each time the
+    // metronome ticks.
+    virtual void OnTick() = 0;
+
+    // The task queue that OnTick will run on. Must not be null.
+    virtual TaskQueueBase* OnTickTaskQueue() = 0;
+  };
+
+  virtual ~Metronome() = default;
+
+  // Adds a tick listener to the metronome. Once this method has returned
+  // OnTick will be invoked on each metronome tick. A listener may
+  // only be added to the metronome once.
+  virtual void AddListener(TickListener* listener) = 0;
+
+  // Removes the tick listener from the metronome. Once this method has returned
+  // OnTick will never be called again. This method must not be called from
+  // within OnTick.
+  virtual void RemoveListener(TickListener* listener) = 0;
+
+  // Returns the current tick period of the metronome.
+  virtual TimeDelta TickPeriod() const = 0;
+};
+
+}  // namespace webrtc
+
+#endif  // API_METRONOME_METRONOME_H_