Add ability to return moved value from FunctorMessageHandler, Optional.

This functionality is desired for this CL:
https://codereview.webrtc.org/2675173003/

BUG=None

Review-Url: https://codereview.webrtc.org/2681283002
Cr-Commit-Position: refs/heads/master@{#16546}

webrtc/base/messagehandler.h

@@ -45,25 +45,15 @@ class FunctorMessageHandler : public MessageHandler {
   }
   const ReturnT& result() const { return result_; }
 
+  // Returns moved result. Should not call result() or MoveResult() again
+  // after this.
+  ReturnT MoveResult() { return std::move(result_); }
+
  private:
   FunctorT functor_;
   ReturnT result_;
 };
 
-// Specialization for std::unique_ptr<ReturnT>.
-template <class ReturnT, class FunctorT>
-class FunctorMessageHandler<class std::unique_ptr<ReturnT>, FunctorT>
-    : public MessageHandler {
- public:
-  explicit FunctorMessageHandler(const FunctorT& functor) : functor_(functor) {}
-  virtual void OnMessage(Message* msg) { result_ = std::move(functor_()); }
-  std::unique_ptr<ReturnT> result() { return std::move(result_); }
-
- private:
-  FunctorT functor_;
-  std::unique_ptr<ReturnT> result_;
-};
-
 // Specialization for ReturnT of void.
 template <class FunctorT>
 class FunctorMessageHandler<void, FunctorT> : public MessageHandler {
@@ -74,6 +64,7 @@ class FunctorMessageHandler<void, FunctorT> : public MessageHandler {
     functor_();
   }
   void result() const {}
+  void MoveResult() {}
 
  private:
   FunctorT functor_;

webrtc/base/optional.h

@@ -239,6 +239,12 @@ class Optional final {
                       : default_val;
   }
 
+  // Dereference and move value.
+  T MoveValue() {
+    RTC_DCHECK(has_value_);
+    return std::move(value_);
+  }
+
   // Equality tests. Two Optionals are equal if they contain equivalent values,
   // or if they're both empty.
   friend bool operator==(const Optional& m1, const Optional& m2) {

webrtc/base/optional_unittest.cc

@@ -725,4 +725,19 @@ TEST(OptionalTest, TestSwap) {
             *log);
 }
 
+TEST(OptionalTest, TestMoveValue) {
+  auto log = Logger::Setup();
+  {
+    Optional<Logger> x(Logger(42));
+    log->push_back("---");
+    Logger moved = x.MoveValue();
+    log->push_back("---");
+  }
+  EXPECT_EQ(
+      V("0:42. explicit constructor", "1:42. move constructor (from 0:42)",
+        "0:42. destructor", "---", "2:42. move constructor (from 1:42)", "---",
+        "2:42. destructor", "1:42. destructor"),
+      *log);
+}
+
 }  // namespace rtc

webrtc/base/thread.h

@@ -168,7 +168,7 @@ class LOCKABLE Thread : public MessageQueue {
   ReturnT Invoke(const Location& posted_from, const FunctorT& functor) {
     FunctorMessageHandler<ReturnT, FunctorT> handler(functor);
     InvokeInternal(posted_from, &handler);
-    return handler.result();
+    return handler.MoveResult();
   }
 
   // From MessageQueue