add event holding function

src/ocamlCanvas.ml

@@ -1078,6 +1078,10 @@ module V1 = struct
     external key_of_int : int -> key
       = "ml_canvas_key_of_int"
 
+    let held_events = ref []
+
+    let hold (e : unit React.event) = held_events := e::!held_events
+
   end
 
   module InternalEvent = struct
@@ -1184,6 +1188,7 @@ module V1 = struct
     let run k =
       let open InternalEvent in
       let open Event in
+      let k () = held_events := []; k () in
       let h e =
         (match e with
         | FrameCycle { timestamp } ->

src/ocamlCanvas.mli

@@ -1498,6 +1498,11 @@ module V1 : sig
         {ul
         {- {!Invalid_argument} if [i] < 0 or [i] > 255}} *)
 
+    val hold : unit React.event -> unit
+    (** [hold e] ensures that effectful React event [e] won't be
+        collected early by the GC. In particular, in the case of the
+        Javascript backend where a global reference might not be
+        enough. *)
 
   end