Use DelayedInit for SimpleObservable

Summary: The lazy initialization logic can now be simplified.

Reviewed By: andriigrynenko

Differential Revision: D25675031

fbshipit-source-id: 048328220a2b02c13a672bebe47b79521d5f5e7a

folly/experimental/observer/SimpleObservable-inl.h

@@ -74,13 +74,12 @@ struct SimpleObservable<T>::Wrapper {
 
 template <typename T>
 auto SimpleObservable<T>::getObserver() const {
-  folly::call_once(observerInit_, [&]() {
+  return observer_.try_emplace_with([&]() {
     SimpleObservable<T>::Wrapper wrapper;
     wrapper.context = context_;
     ObserverCreator<SimpleObservable<T>::Wrapper> creator(std::move(wrapper));
-    observer_ = unwrap(std::move(creator).getObserver());
+    return unwrap(std::move(creator).getObserver());
   });
-  return *observer_;
 }
 
 } // namespace observer

folly/experimental/observer/SimpleObservable.h

@@ -19,7 +19,7 @@
 #include <folly/Function.h>
 #include <folly/Synchronized.h>
 #include <folly/experimental/observer/Observer.h>
-#include <folly/synchronization/CallOnce.h>
+#include <folly/synchronization/DelayedInit.h>
 
 namespace folly {
 namespace observer {
@@ -51,8 +51,7 @@ class SimpleObservable {
   struct Wrapper;
   std::shared_ptr<Context> context_;
 
-  mutable folly::once_flag observerInit_;
-  mutable folly::Optional<Observer<typename observer_detail::Unwrap<T>::type>>
+  folly::DelayedInit<Observer<typename observer_detail::Unwrap<T>::type>>
       observer_;
 };
 } // namespace observer