Tracing V2 prototype [NOT FOR REVIEW]

base/trace_event/v2/trace_event.h

+// 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_V2_TRACE_EVENT_H_
+#define BASE_TRACE_EVENT_V2_TRACE_EVENT_H_
+
+#include "base/base_export.h"
+#include "base/macros.h"
+#include "base/trace_event/common/proto/event.tracing-pb.h"
+
+//TODO add thread checker. The handle should not be moved cross threads.
+
+namespace base {
+namespace trace_event {
+namespace v2 {
+
+class TraceEventHandle;
+
+class BASE_EXPORT TraceEvent : public ::tracing::Event {
+ public:
+  TraceEvent();
+  ~TraceEvent() override;
+
+  void set_event_handle(TraceEventHandle* handle) { handle_ = handle; }
+
+ private:
+  TraceEventHandle* handle_;
+
+  DISALLOW_COPY_AND_ASSIGN(TraceEvent);
+};
+
+// Rationale of this handle: we want to return a (pointer to) Event to clients,
+// so they can add extra properties (fill the proto fields).
+// At the same time, we want to be in charge of its lifetime. More specifically
+// an Event is valid for a given thread only until the next event is added.
+// The functional purposes of this handle are:
+//   1. Finalize the event when it goes out of scope from the caller.
+//   2. Prevent bugs in the client code. If we did just return an Event* ptr,
+//      clients might be tempted to keep it cached. Which means that once the
+//      Event, that we own, is destroyed the client will have an invalid ptr
+//      causing all sort of hard-to-debug memory bugs.
+class BASE_EXPORT TraceEventHandle {
+ public:
+  TraceEventHandle();
+  explicit TraceEventHandle(TraceEvent* event);
+  ~TraceEventHandle();
+
+  // Move-only type.
+  TraceEventHandle(TraceEventHandle&& other);
+  TraceEventHandle& operator=(TraceEventHandle&& other);
+
+  TraceEvent& operator*() const { return *event_; }
+  TraceEvent* operator->() const { return event_; }
+
+ private:
+  void Swap(TraceEventHandle* other);
+  friend class TraceEvent;
+  TraceEvent* event_;
+
+  DISALLOW_COPY_AND_ASSIGN(TraceEventHandle);
+};
+
+}  // namespace v2
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_V2_TRACE_EVENT_H_