Fix makeValueObserver to call creator once on construction

Reviewed By: spalamarchuk

Differential Revision: D29784994

fbshipit-source-id: 0faac9c21f56061d8d787f8bd4e2e61ed5fcaf6c

folly/experimental/observer/Observer-inl.h

@@ -299,11 +299,12 @@ Observer<observer_detail::ResultOfUnwrapObserver<F>> makeValueObserver(
 template <typename F>
 Observer<observer_detail::ResultOfUnwrapSharedPtr<F>> makeValueObserver(
     F&& creator) {
-  auto activeValue = creator();
-  return makeObserver([activeValue = std::move(activeValue),
+  return makeObserver(
+      [activeValue =
+           std::shared_ptr<const observer_detail::ResultOfUnwrapSharedPtr<F>>(),
        creator = std::forward<F>(creator)]() mutable {
         auto newValue = creator();
-    if (!(*activeValue == *newValue)) {
+        if (!activeValue || !(*activeValue == *newValue)) {
           activeValue = newValue;
         }
         return activeValue;

folly/experimental/observer/test/ObserverTest.cpp

@@ -585,6 +585,14 @@ TEST(Observer, MakeValueObserver) {
   EXPECT_EQ(observedIds, std::vector<int>({1, 2, 3, 4, 5}));
   EXPECT_EQ(observedValues, std::vector<int>({1, 2, 3}));
   EXPECT_EQ(observedValues2, std::vector<int>({1, 2, 3}));
+
+  size_t creatorCalls = 0;
+  auto o = makeValueObserver([&] {
+    ++creatorCalls;
+    return 42;
+  });
+  EXPECT_EQ(42, **o);
+  EXPECT_EQ(1, creatorCalls);
 }
 
 TEST(Observer, MakeStaticObserver) {