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..5aac1a3bbabcccd1206bfa40a8557245e3de77d9 100644
--- a/base/trace_event/trace_log.cc
+++ b/base/trace_event/trace_log.cc
@@ -333,6 +333,15 @@ void TraceLog::ThreadLocalEventBuffer::FlushWhileLocked() {
   // find the generation mismatch and delete this buffer soon.
 }
 
+struct TraceLog::RegisteredAsyncObserver {
+  RegisteredAsyncObserver(WeakPtr<AsyncEnabledStateObserver> observer)
+      : observer(observer), task_runner(ThreadTaskRunnerHandle::Get()) {}
+  ~RegisteredAsyncObserver() {}
+
+  WeakPtr<AsyncEnabledStateObserver> observer;
+  scoped_refptr<SequencedTaskRunner> task_runner;
+};
+
 TraceLogStatus::TraceLogStatus() : event_capacity(0), event_count(0) {}
 
 TraceLogStatus::~TraceLogStatus() {}
@@ -632,6 +641,11 @@ void TraceLog::SetEnabled(const TraceConfig& trace_config, Mode mode) {
       }
     }
 
+    for (auto& it : async_observers_)

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

I think this could even be +const, i.e. const auto& (not a big deal if that's
not the case)

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

Done.

+      it.second.task_runner->PostTask(

[Sami, 2016/05/10 14:42:16]

nit: Please add braces around this loop.

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

Done.

+          FROM_HERE, Bind(&AsyncEnabledStateObserver::OnTraceLogEnabled,
+                          it.second.observer));
+
     dispatching_to_observer_list_ = true;
     observer_list = enabled_state_observer_list_;
   }
@@ -716,6 +730,11 @@ void TraceLog::SetDisabledWhileLocked() {
   // Remove metadata events so they will not get added to a subsequent trace.
   metadata_events_.clear();
 
+  for (auto& it : async_observers_)
+    it.second.task_runner->PostTask(

[Sami, 2016/05/10 14:42:16]

nit: Please add braces around this loop.

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

Done.

+        FROM_HERE, Bind(&AsyncEnabledStateObserver::OnTraceLogDisabled,
+                        it.second.observer));
+
   dispatching_to_observer_list_ = true;
   std::vector<EnabledStateObserver*> observer_list =
       enabled_state_observer_list_;
@@ -1710,6 +1729,25 @@ void ConvertableToTraceFormat::EstimateTraceMemoryOverhead(
   overhead->Add("ConvertableToTraceFormat(Unknown)", sizeof(*this));
 }
 
+void TraceLog::AddAsyncEnabledStateObserver(
+    WeakPtr<AsyncEnabledStateObserver> listener) {
+  AutoLock lock(lock_);
+  async_observers_.insert(
+      std::make_pair(listener.get(), RegisteredAsyncObserver(listener)));
+}
+
+void TraceLog::RemoveAsyncEnabledStateObserver(
+    AsyncEnabledStateObserver* listener) {
+  AutoLock lock(lock_);
+  async_observers_.erase(listener);
+}
+
+bool TraceLog::HasAsyncEnabledStateObserver(
+    AsyncEnabledStateObserver* listener) const {
+  AutoLock lock(lock_);
+  return async_observers_.find(listener) != async_observers_.end();

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

I think that .count(listener) != 0 is, in some STL implementations, a bit more
efficient, as you don't need to populate the iterator

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

Let's use ContainsKey() which looks less hacky (and makes it someone else's
responsibility to care about efficiency).

+}
+
 }  // namespace trace_event
 }  // namespace base