Introduce TraceLog::AsyncEnabledStateObserver

base/trace_event/trace_log.h

Index: base/trace_event/trace_log.h
diff --git a/base/trace_event/trace_log.h b/base/trace_event/trace_log.h
index 3f09674829381f30825e9ceea2734b0cf7111fc9..945aa45ef268acb72a514d1d366715e82e5dc8b9 100644
--- a/base/trace_event/trace_log.h
+++ b/base/trace_event/trace_log.h
@@ -119,6 +119,26 @@ class BASE_EXPORT TraceLog : public MemoryDumpProvider {
   void RemoveEnabledStateObserver(EnabledStateObserver* listener);
   bool HasEnabledStateObserver(EnabledStateObserver* listener) const;
 
+  // Asynchronous enabled state listeners. When tracing is enabled or disabled,
+  // for each observer, a task for invoking its appropriate callback is posted
+  // to the thread from which AddAsyncEnabledStateObserver() was called.
+  class BASE_EXPORT AsyncEnabledStateObserver {

[Primiano Tucci (use gerrit), 2016/05/10 14:20:26]

Add: this allows the observer to be safely destroyed, provided that it happens
on the same thread that invoked AddAsyncEnabledStateObserver().

[Xiaocheng, 2016/05/11 05:19:20]

Done.

+   public:
+    virtual ~AsyncEnabledStateObserver() = default;
+
+    // Posted just after the tracing system becomes enabled.
+    // TraceLog::IsEnabled() is true at this point.
+    virtual void OnTraceLogEnabled() = 0;
+
+    // Posted just after the tracing system becomes disabled.
+    // TraceLog::IsEnabled() is false at this point.
+    virtual void OnTraceLogDisabled() = 0;
+  };
+  void AddAsyncEnabledStateObserver(
+      WeakPtr<AsyncEnabledStateObserver> listener);
+  void RemoveAsyncEnabledStateObserver(AsyncEnabledStateObserver* listener);
+  bool HasAsyncEnabledStateObserver(AsyncEnabledStateObserver* listener) const;
+
   TraceLogStatus GetStatus() const;
   bool BufferIsFull() const;
 
@@ -365,6 +385,7 @@ class BASE_EXPORT TraceLog : public MemoryDumpProvider {
 
   class ThreadLocalEventBuffer;
   class OptionalAutoLock;
+  struct RegisteredAsyncObserver;
 
   TraceLog();
   ~TraceLog() override;
@@ -443,6 +464,9 @@ class BASE_EXPORT TraceLog : public MemoryDumpProvider {
   bool dispatching_to_observer_list_;
   std::vector<EnabledStateObserver*> enabled_state_observer_list_;
 

[Primiano Tucci (use gerrit), 2016/05/10 14:20:26]

nit: remove the blank line 466 as this logically still belongs to the observer
stuff (not sure why there was a blank line in the first place honestly)

[Xiaocheng, 2016/05/11 05:19:20]

Done. It's just my personal bad habit of inserting blank lines wherever
possible.

+  std::map<AsyncEnabledStateObserver*, RegisteredAsyncObserver>
+      async_observers_;
+
   std::string process_name_;
   base::hash_map<int, std::string> process_labels_;
   int process_sort_index_;