Add folly::observer::makeStaticObserver

Summary: `makeStaticObserver` is a helper method that returns an observer that never changes in value.

Reviewed By: andriigrynenko

Differential Revision: D25254488

fbshipit-source-id: 1336d0088aff1e5abfa8160bd76e4f61b68eaed8

folly/experimental/observer/Observer-inl.h

@@ -98,6 +98,16 @@ Observer<observer_detail::ResultOfUnwrapObserver<F>> makeObserver(F&& creator) {
   return unwrap(observer_detail::makeObserver(std::forward<F>(creator)));
 }
 
+template <typename T>
+Observer<T> makeStaticObserver(T value) {
+  return makeStaticObserver<T>(std::make_shared<T>(std::move(value)));
+}
+
+template <typename T>
+Observer<T> makeStaticObserver(std::shared_ptr<T> value) {
+  return makeObserver([value = std::move(value)] { return value; });
+}
+
 template <typename T>
 TLObserver<T>::TLObserver(Observer<T> observer)
     : observer_(observer),

folly/experimental/observer/Observer.h

@@ -213,6 +213,15 @@ template <typename F>
 Observer<observer_detail::ResultOfUnwrapSharedPtr<F>> makeValueObserver(
     F&& creator);
 
+/**
+ * The returned Observer will never update and always return the passed value.
+ */
+template <typename T>
+Observer<T> makeStaticObserver(T value);
+
+template <typename T>
+Observer<T> makeStaticObserver(std::shared_ptr<T> value);
+
 template <typename T>
 class TLObserver {
  public:

folly/experimental/observer/test/ObserverTest.cpp

@@ -563,6 +563,21 @@ TEST(Observer, MakeValueObserver) {
   EXPECT_EQ(observedValues2, std::vector<int>({1, 2, 3}));
 }
 
+TEST(Observer, MakeStaticObserver) {
+  auto explicitStringObserver = makeStaticObserver<std::string>("hello");
+  EXPECT_EQ(**explicitStringObserver, "hello");
+
+  auto implicitIntObserver = makeStaticObserver(5);
+  EXPECT_EQ(**implicitIntObserver, 5);
+
+  auto explicitSharedPtrObserver =
+      makeStaticObserver<std::shared_ptr<int>>(std::make_shared<int>(5));
+  EXPECT_EQ(***explicitSharedPtrObserver, 5);
+
+  auto implicitSharedPtrObserver = makeStaticObserver(std::make_shared<int>(5));
+  EXPECT_EQ(**implicitSharedPtrObserver, 5);
+}
+
 TEST(Observer, Unwrap) {
   SimpleObservable<bool> selectorObservable{true};
   SimpleObservable<int> trueObservable{1};