memory-infra: Start disentangling tracing from memory-infra

base/trace_event/memory_tracing_frontend.h

+// Copyright 2017 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_MEMORY_TRACING_FRONTEND_H_
+#define BASE_TRACE_EVENT_MEMORY_TRACING_FRONTEND_H_
+
+#include "base/trace_event/memory_dump_manager.h"
+#include "base/trace_event/trace_event.h"
+
+namespace base {
+
+namespace trace_event {
+
+// This is the interface exposed to the rest of the codebase to deal with
+// memory tracing. The main entry point for clients is represented by
+// RequestDumpPoint(). The extension by Un(RegisterDumpProvider).

[ssid, 2017/04/14 23:33:09]

What is RequestDumpPoint()

[hjd, 2017/04/19 09:54:56]

...I'm not sure, I think I must have pasted some random text with vim.

+class BASE_EXPORT MemoryTracingFrontend

[ssid, 2017/04/14 23:33:09]

Why is this called frontend? Why not MemoryTracingObserver?
This class just observes tracing.
What do you mean by "exposed to the rest of the codebase to deal with memory
tracing"? Why would rest of the codebase use this class?

[hjd, 2017/04/19 09:54:56]

Done.

+    : public TraceLog::EnabledStateObserver {
+ public:
+  static const char* const kTraceCategory;
+
+  static void Initialize(MemoryDumpManager*);

[ssid, 2017/04/14 23:33:09]

Why do you need this argument?

[hjd, 2017/04/19 09:54:56]

Gone in the new version.

+
+  MemoryTracingFrontend(TraceLog*, MemoryDumpManager*);

[ssid, 2017/04/14 23:33:09]

Why are you passing these pointers to singleton?

[hjd, 2017/04/19 09:54:56]

Code that uses singletons directly is difficult to test well since it is
invisibly and forever coupled to the singleton code. This makes tests hard to
write but it also means any tests that are written are not isolated since they
implicitly test large amounts of behaviour (via a global). Isolation makes tests
fast, targeted and stable so this is a bad property to give. Because of this
where reasonable I try to pass pointers to singletons.

+  ~MemoryTracingFrontend() override;
+
+  // TraceLog::EnabledStateObserver implementation.
+  void OnTraceLogEnabled() override;
+  void OnTraceLogDisabled() override;
+
+ private:
+  bool IsMemoryInfraTracingEnabled();
+  bool enabled_;
+  MemoryDumpManager* memory_dump_manager_;
+  TraceLog* trace_log_;
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_MEMORY_TRACING_FRONTEND_H_