diff --git a/src/KeyHandler.cpp b/src/KeyHandler.cpp
index a95da54..752eb47 100644
--- a/src/KeyHandler.cpp
+++ b/src/KeyHandler.cpp
@@ -102,8 +102,8 @@ KeyHandler::KeyHandler(
 }
 
 bool KeyHandler::handle(Key key, McBopomofo::InputState* state,
-                        const StateCallback& stateCallback,
-                        const ErrorCallback& errorCallback) {
+                        StateCallback stateCallback,
+                        ErrorCallback errorCallback) {
   // From Key's definition, if shiftPressed is true, it can't be a simple key
   // that can be represented by ASCII.
   char simpleAscii = (key.ctrlPressed || key.shiftPressed) ? '\0' : key.ascii;
@@ -431,7 +431,7 @@ bool KeyHandler::handle(Key key, McBopomofo::InputState* state,
 
 void KeyHandler::candidateSelected(
     const InputStates::ChoosingCandidate::Candidate& candidate,
-    const StateCallback& stateCallback) {
+    StateCallback stateCallback) {
   if (inputMode_ == InputMode::PlainBopomofo) {
     reset();
     std::unique_ptr<InputStates::Committing> committingState =
@@ -446,12 +446,12 @@ void KeyHandler::candidateSelected(
 
 void KeyHandler::dictionaryServiceSelected(std::string phrase, size_t index,
                                            InputState* currentState,
-                                           const StateCallback& stateCallback) {
+                                           StateCallback stateCallback) {
   dictionaryServices_->lookup(std::move(phrase), index, currentState,
                               stateCallback);
 }
 
-void KeyHandler::candidatePanelCancelled(const StateCallback& stateCallback) {
+void KeyHandler::candidatePanelCancelled(StateCallback stateCallback) {
   if (inputMode_ == InputMode::PlainBopomofo) {
     reset();
     std::unique_ptr<InputStates::EmptyIgnoringPrevious>
@@ -465,9 +465,8 @@ void KeyHandler::candidatePanelCancelled(const StateCallback& stateCallback) {
 }
 
 bool KeyHandler::handleCandidateKeyForTraditionalBopomofoIfRequired(
-    Key key,
-    const SelectCurrentCandidateCallback& SelectCurrentCandidateCallback,
-    const StateCallback& stateCallback, const ErrorCallback& errorCallback) {
+    Key key, SelectCurrentCandidateCallback SelectCurrentCandidateCallback,
+    StateCallback stateCallback, ErrorCallback errorCallback) {
   if (inputMode_ != McBopomofo::InputMode::PlainBopomofo) {
     return false;
   }
diff --git a/src/KeyHandler.h b/src/KeyHandler.h
index 682fb27..4b43613 100644
--- a/src/KeyHandler.h
+++ b/src/KeyHandler.h
@@ -68,26 +68,24 @@ class KeyHandler {
   // the key should be absorbed, signaling that the key is accepted and handled,
   // or false if the event should be let pass through.
   bool handle(Key key, McBopomofo::InputState* state,
-              const StateCallback& stateCallback,
-              const ErrorCallback& errorCallback);
+              StateCallback stateCallback, ErrorCallback errorCallback);
 
   // Candidate selected. Can assume the context is in a candidate state.
   void candidateSelected(
       const InputStates::ChoosingCandidate::Candidate& candidate,
-      const StateCallback& stateCallback);
+      StateCallback stateCallback);
 
   void dictionaryServiceSelected(std::string phrase, size_t index,
                                  InputState* currentState,
-                                 const StateCallback& stateCallback);
+                                 StateCallback stateCallback);
 
   // Candidate panel canceled. Can assume the context is in a candidate state.
-  void candidatePanelCancelled(const StateCallback& stateCallback);
+  void candidatePanelCancelled(StateCallback stateCallback);
 
   // Workaround for the Traditional Bopomofo mode.
   bool handleCandidateKeyForTraditionalBopomofoIfRequired(
-      Key key,
-      const SelectCurrentCandidateCallback& SelectCurrentCandidateCallback,
-      const StateCallback& stateCallback, const ErrorCallback& errorCallback);
+      Key key, SelectCurrentCandidateCallback SelectCurrentCandidateCallback,
+      StateCallback stateCallback, ErrorCallback errorCallback);
 
   void reset();