Add runOnExecutor helper for then(executor, callback)

Summary: Helper function to wrap a nested dispatch. Returns a function object that launches the passed function onto the executor and returns a future representing the return value of the function.

Reviewed By: andriigrynenko

Differential Revision: D15300575

fbshipit-source-id: eca4b4139c6405f216a9f1cbc87a8ec2ff437780

folly/futures/Future.h

@@ -1937,6 +1937,19 @@ std::pair<Promise<T>, Future<T>> makePromiseContract(Executor::KeepAlive<> e) {
   return std::make_pair(std::move(p), std::move(f));
 }
 
+template <class F>
+auto makeAsyncTask(folly::Executor::KeepAlive<> ka, F&& func) {
+  return
+      [func = std::forward<F>(func), ka = std::move(ka)](auto&& param) mutable {
+        return via(
+            ka,
+            [func = std::move(func),
+             param = std::forward<decltype(param)>(param)]() mutable {
+              return func(std::forward<decltype(param)>(param));
+            });
+      };
+}
+
 } // namespace folly
 
 #if FOLLY_HAS_COROUTINES

folly/futures/test/ViaTest.cpp

@@ -219,11 +219,12 @@ TEST(Via, priority) {
 
 TEST(Via, then2) {
   ManualExecutor x1, x2;
-  bool a = false, b = false, c = false;
+  bool a = false, b = false, c = false, d = false;
   via(&x1)
       .thenValue([&](auto&&) { a = true; })
       .then(&x2, [&](auto&&) { b = true; })
-      .thenValue([&](auto&&) { c = true; });
+      .thenValue([&](auto&&) { c = true; })
+      .thenValueInline(folly::makeAsyncTask(&x2, [&](auto&&) { d = true; }));
 
   EXPECT_FALSE(a);
   EXPECT_FALSE(b);
@@ -239,6 +240,10 @@ TEST(Via, then2) {
 
   x1.run();
   EXPECT_TRUE(c);
+  EXPECT_FALSE(d);
+
+  x2.run();
+  EXPECT_TRUE(d);
 }
 
 TEST(Via, allowInline) {