diff --git a/Cargo.toml b/Cargo.toml
index 5b7f8fb..2cbf1e1 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "prongs"
-version = "0.0.3"
+version = "0.0.4"
 authors = ["Ales Katona <almindor@gmail.com>"]
 edition = "2018"
 readme = "README.md"
diff --git a/src/backend_gilrs.rs b/src/backend_gilrs.rs
index f2d32a5..d32765c 100644
--- a/src/backend_gilrs.rs
+++ b/src/backend_gilrs.rs
@@ -33,7 +33,7 @@ where TUserAction: Clone + Serialize,
         self.schema.clear_assignments();
     }
 
-    pub fn process_event(&mut self, event: &Event) -> Option<ProcessingResult<TUserAction>> {
+    pub fn process_event(&self, event: &Event) -> Option<ProcessingResult<TUserAction>> {
         self.schema.process_event(event)
     }
 
diff --git a/src/backend_piston.rs b/src/backend_piston.rs
index e52a816..bc9fd12 100644
--- a/src/backend_piston.rs
+++ b/src/backend_piston.rs
@@ -41,7 +41,7 @@ where TUserAction: Clone + Serialize,
 
     /// Main event processing hook. Will result in ProcessingResult for user's action if
     /// a mapping is withing the schema for the given event.
-    pub fn process_event(&mut self, event: &Event) -> Option<ProcessingResult<TUserAction>> {
+    pub fn process_event(&self, event: &Event) -> Option<ProcessingResult<TUserAction>> {
         self.schema.process_event(event)
     }
 
diff --git a/src/lib.rs b/src/lib.rs
index 2f560ec..ad22d23 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -132,7 +132,7 @@ where TEventType: ToEventType<TControllerID>,
 
     /// Main processing loop. Takes input event and returns an option with processing result
     /// If the mapping was found result will be set, otherwise None
-    fn process_event(&mut self, event: &TEventType) -> Option<ProcessingResult<TUserAction>> {
+    fn process_event(&self, event: &TEventType) -> Option<ProcessingResult<TUserAction>> {
         if let Some(event_type) = event.to_raw() {
             if let Some(action) = self.keymap.get(&event_type) {
                 if self.controller_id.is_some() && self.controller_id != event.controller_id() {