[Tracing Clock Sync] Add TracingAgent interface in Chrome

base/trace_event/tracing_agent.h

+// Copyright 2015 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_TRACING_AGENT_H_
+#define BASE_TRACE_EVENT_TRACING_AGENT_H_
+
+#include "base/base_export.h"
+#include "base/callback.h"
+#include "base/memory/ref_counted_memory.h"
+#include "base/trace_event/trace_config.h"
+#include "base/values.h"
+
+namespace base {
+namespace trace_event {
+

[oystein (OOO til 10th of July), 2015/11/24 22:52:10]

Can you include some class-level comments here, clarifying what a TracingAgent
is?

[Zhen Wang, 2015/11/25 04:24:55]

Done.

+class BASE_EXPORT TracingAgent {
+ public:
+  typedef base::Callback<void(const scoped_refptr<base::RefCountedString>&)>
+      StopAgentTracingCallback;
+
+  // Get the name of the tracing agent. The name should be one of the possible
+  // tracing agent names declared below.
+  virtual std::string GetTracingAgentName() = 0;
+
+  // Start tracing on the tracing agent.
+  virtual bool StartAgentTracing(const TraceConfig& trace_config) = 0;
+
+  // Stop tracing on the tracing agent.
+  virtual void StopAgentTracing(const StopAgentTracingCallback& callback) = 0;
+
+  // Check if the tracing agent supports explicit clock synchronization.
+  virtual bool SupportsExplicitClockSync() = 0;
+
+  // Record a clock sync marker issued by another tracing agent.
+  virtual void RecordClockSyncMarker(
+      scoped_ptr<base::DictionaryValue> marker) = 0;
+
+  // Issue clock sync markers to other tracing agents if possible.
+  virtual void IssueClockSyncMarker() = 0;
+
+  virtual ~TracingAgent() {}
+
+ protected:
+  // Declare all possible tracing agent names here.
+  static const char kChromeTracingAgentName[];

[oystein (OOO til 10th of July), 2015/11/24 22:52:10]

Hmm why are these hardcoded here? It doesn't seem right that base/ should have
knowledge of this. Can the agents just return the string constants directly?

[Zhen Wang, 2015/11/25 04:24:55]

I originally thought about returning those strings directly. But I think it is
beneficial to have a centralized place to define those strings, so future
tracing agents know that they should use other names. Is there a better way of
doing that?

+  static const char kETWTracingAgentName[];
+  static const char kCrOSTracingAgentName[];
+  static const char kPowerTracingAgentName[];
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_TRACING_AGENT_H_