Add tracing AutoOpenCloseEvent.

base/trace_event/persistent_async_event.h

Index: base/trace_event/persistent_async_event.h
diff --git a/base/trace_event/persistent_async_event.h b/base/trace_event/persistent_async_event.h
new file mode 100644
index 0000000000000000000000000000000000000000..70f9271e3c7484b75d2d82ad46687b6b0b9ac84e
--- /dev/null
+++ b/base/trace_event/persistent_async_event.h
@@ -0,0 +1,46 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_TRACE_EVENT_PERSISTENT_ASYNC_H_
+#define BASE_TRACE_EVENT_PERSISTENT_ASYNC_H_
+
+#include "base/macros.h"
+#include "base/time/time.h"
+#include "base/trace_event/trace_event.h"
+
+namespace base {
+
+namespace trace_event {
+
+// Note: The categories and event names must have application lifetime
+// (i.e. literals), as documented in trace_event_common.h.
+// Thus, we must make subclasses that have hard-coded categories
+// and event names, rather than passing these into the PersistentAsyncEvent.
+
+class BASE_EXPORT PersistentAsyncEvent
+    : public trace_event::TraceLog::EnabledStateObserver {
+ public:
+  PersistentAsyncEvent();
+  ~PersistentAsyncEvent() override;
+
+  void Start();
+  void Stop();
+
+  void OnTraceLogEnabled() override;
+  void OnTraceLogDisabled() override;
+
+ protected:
+  virtual void RecordStartEvent() = 0;
+  virtual void RecordStartEventWithTimestamp(int64_t timestamp) = 0;
+  virtual void RecordStopEvent() = 0;
+
+  void* id_;

[benjhayden, 2016/08/30 04:17:23]

Fields should be private, with public/protected accessors as necessary.

[alexandermont, 2016/08/30 19:01:22]

I'm not exactly sure what this means. Let's talk about it this afternoon

[benjhayden, 2016/08/31 03:12:53]

https://google.github.io/styleguide/cppguide.html#Access_Control
Data members should be private so that they are encapsulated by the class.
https://en.wikipedia.org/wiki/Encapsulation_(computer_programming)
If the inheritance hierarchy were much broader and deeper than this, then it
would be very difficult to reason about data and control flow if any class in
the chain could modify any field at any time, you would need to read every class
in the hierarchy fully and thoroughly.
Sorry I had to code today. If this is still unclear, then I can make time
tomorrow to talk about it.

+  bool active_;
+  int64_t start_time_;
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_PERSISTENT_ASYNC_H_