Introduce TraceLog::AsyncEnabledStateObserver

base/trace_event/trace_log.cc

Index: base/trace_event/trace_log.cc
diff --git a/base/trace_event/trace_log.cc b/base/trace_event/trace_log.cc
index 5692e835bab88905786ba53616796c3ff086914c..def404eea83f79953e5d922a4ad1027f3fdc0e1b 100644
--- a/base/trace_event/trace_log.cc
+++ b/base/trace_event/trace_log.cc
@@ -333,6 +333,16 @@ void TraceLog::ThreadLocalEventBuffer::FlushWhileLocked() {
   // find the generation mismatch and delete this buffer soon.
 }
 
+struct TraceLog::RegisteredThreadSafeObserver {
+  RegisteredThreadSafeObserver(WeakPtr<ThreadSafeEnabledStateObserver> observer)
+      : observer_(observer), task_runner_(ThreadTaskRunnerHandle::Get()) {}
+
+  ~RegisteredThreadSafeObserver() {}
+
+  WeakPtr<ThreadSafeEnabledStateObserver> observer_;

[Primiano Tucci (use gerrit), 2016/05/09 17:18:29]

nit since this is an internal struct, and you are directly using its fields
(makes sense) these names should be without final underscore.

From [1] "Data members of structs, both static and non-static, are named like
ordinary nonmember variables. They do not have the trailing underscores that
data members in classes have."

[1] https://google.github.io/styleguide/cppguide.html#Variable_Names

[Xiaocheng, 2016/05/10 09:33:33]

Done.

+  scoped_refptr<TaskRunner> task_runner_;

[Primiano Tucci (use gerrit), 2016/05/09 17:18:29]

Can you s/TaskRunner/SequencedTaskRunner/. It will not make any practical
difference in this CL. It's purely for documentation/reasoning purposes: you
need a sequencing TR if you want to guarantee that you can't race on
destruction.

[Xiaocheng, 2016/05/10 09:33:33]

Done.

+};
+
 TraceLogStatus::TraceLogStatus() : event_capacity(0), event_count(0) {}
 
 TraceLogStatus::~TraceLogStatus() {}
@@ -632,6 +642,12 @@ void TraceLog::SetEnabled(const TraceConfig& trace_config, Mode mode) {
       }
     }
 
+    for (auto it = thread_safe_observers_.begin();

[Primiano Tucci (use gerrit), 2016/05/09 17:18:29]

I think you can more concisely:
for (const auto& it : thread_safe_observers_)
which saves one precious line and improves readability a tiny bit

[Xiaocheng, 2016/05/10 09:33:33]

Done.

+         it != thread_safe_observers_.end(); ++it)
+      it->second.task_runner_->PostTask(
+          FROM_HERE, Bind(&ThreadSafeEnabledStateObserver::OnTraceLogEnabled,
+                          it->second.observer_));
+
     dispatching_to_observer_list_ = true;
     observer_list = enabled_state_observer_list_;
   }
@@ -716,6 +732,12 @@ void TraceLog::SetDisabledWhileLocked() {
   // Remove metadata events so they will not get added to a subsequent trace.
   metadata_events_.clear();
 
+  for (auto it = thread_safe_observers_.begin();
+       it != thread_safe_observers_.end(); ++it)
+    it->second.task_runner_->PostTask(
+        FROM_HERE, Bind(&ThreadSafeEnabledStateObserver::OnTraceLogDisabled,
+                        it->second.observer_));
+
   dispatching_to_observer_list_ = true;
   std::vector<EnabledStateObserver*> observer_list =
       enabled_state_observer_list_;
@@ -1710,6 +1732,33 @@ void ConvertableToTraceFormat::EstimateTraceMemoryOverhead(
   overhead->Add("ConvertableToTraceFormat(Unknown)", sizeof(*this));
 }
 
+int TraceLog::AddThreadSafeEnabledStateObserver(
+    WeakPtr<ThreadSafeEnabledStateObserver> listener) {
+  AutoLock lock(lock_);
+  int id = thread_safe_observers_.empty()

[Primiano Tucci (use gerrit), 2016/05/09 17:18:29]

Yeah I think we don't need this ID logic. Just use listener.get() as a key.

[Xiaocheng, 2016/05/10 09:33:33]

Done.

+               ? 1
+               : thread_safe_observers_.rbegin()->first + 1;
+  thread_safe_observers_.insert(
+      std::make_pair(id, RegisteredThreadSafeObserver(listener)));
+  return id;
+}
+
+void TraceLog::RemoveThreadSafeEnabledStateObserver(int listener_id) {
+  AutoLock lock(lock_);
+  thread_safe_observers_.erase(listener_id);
+}
+
+bool TraceLog::HasThreadSafeEnabledStateObserver(int listener_id) const {
+  AutoLock lock(lock_);
+  return thread_safe_observers_.find(listener_id) !=
+         thread_safe_observers_.end();
+}
+
+size_t TraceLog::GetThreadSafeObserverCountForTest() const {
+  AutoLock lock(lock_);
+  return thread_safe_observers_.size();
+}
+
 }  // namespace trace_event
 }  // namespace base