fix(onbeforeunload): an unsafe, cross-compatible hack to workaround #467

This hack is needed to address <#467>
because [Dom.handler] <https://ocsigen.org/js_of_ocaml/latest/api/js_of_ocaml/Js_of_ocaml/Dom/index.html#val-handler>
appears to be incompatible with "onbeforeunload" and Firefox.

* Namely, the semantics of [Dom.handler] requires to "return true" to tell [Dom.handler] that we want to unload normally,
* but with some browsers such as Firefox we should actually "return undefined" (better than "return null" BTW),
* but then, "undefined" is seen as "false", implying [Dom.handler] will call "preventDefault",
* end_of_the_story.

Credits: the working code from this commit reuses a patch by @leunam217,
pfitaxel@15780b5

See also: https://developer.mozilla.org/en-US/docs/Web/API/WindowEventHandlers/onbeforeunload#example

src/app/learnocaml_exercise_main.ml

@@ -288,7 +288,56 @@ let () =
         Ace.focus ace ;
         typecheck true
   end ;
-  Window.onbeforeunload (fun _ -> (not (Ace.is_synchronized ace), false));
+  (* BEGIN HACK, to simplify one day with a later version of js_of_ocaml?
+
+   This is needed to address <https://github.com/ocaml-sf/learn-ocaml/issues/467>
+   because [Dom.handler] <https://ocsigen.org/js_of_ocaml/latest/api/js_of_ocaml/Js_of_ocaml/Dom/index.html#val-handler>
+   appears to be incompatible with "onbeforeunload" and Firefox.
+
+   * Namely, the semantics of [Dom.handler] requires to "return true" to tell [Dom.handler] that we want to unload normally,
+   * but with some browsers such as Firefox we should actually "return undefined" (better than "return null" BTW),
+   * but then, "undefined" is seen as "false", implying [Dom.handler] will call "preventDefault",
+   * end_of_the_story.
+   *)
+  (* Copy of private [Dom.window_event] *)
+  (*
+  let window_event' () : 'a #Dom.event Js.t = Js.Unsafe.pure_js_expr "event" in
+  (* Fixed copy of [Dom.handler], working for onbeforeunload
+     even for Firefox *)
+  let handler_onbeforeunload :
+        ((#Js_of_ocaml.Dom_html.event as 'b) Js_of_ocaml__.Js.t ->
+         bool Js_of_ocaml__.Js.t) ->
+        (* ('a, 'b Js_of_ocaml__.Js.t) Js_of_ocaml.Dom_html.event_listener = *)
+        ('a, 'b Js_of_ocaml__.Js.t -> bool Js.t) Js.meth_callback Js.opt =
+    fun f ->
+  Js.some
+    (Js.Unsafe.callback (fun e ->
+         (* depending on the internet explorer version, e can be null or undefined. *)
+         if not (Js.Opt.test (Js.some e))
+         then (
+           let e = window_event' () in
+           let res = f e in
+           if not (Js.to_bool res) then
+             (e##.returnValue := res; res)
+           else
+             Js.Unsafe.pure_js_expr "undefined"
+         ) else (
+           let res = f e in
+           if not (Js.to_bool res) then
+             ((Js.Unsafe.coerce e)##preventDefault (* ; res *))
+           else
+             (Js.Unsafe.delete (Js.Unsafe.coerce e) "returnValue";
+              Js.Unsafe.pure_js_expr "undefined")))) in
+   *)
+  let prompt_before_unload () : unit =
+    Js.Unsafe.js_expr "window.onbeforeunload = function(e) {e.preventDefault(); return false;}" in
+  let resume_before_unload () : unit =
+    Js.Unsafe.js_expr "window.onbeforeunload = null" in
+  let () =
+    Ace.register_sync_observer ace (fun sync ->
+        if not sync then prompt_before_unload ()
+        else resume_before_unload ()) in
+  (* END HACK *)
   (* ---- return -------------------------------------------------------- *)
   toplevel_launch >>= fun _ ->
   typecheck false >>= fun () ->

src/utils/js_utils.ml

@@ -1158,16 +1158,6 @@ module Window = struct
   let head win = Tyxml_js.Of_dom.of_head win##.document##.head
   let onunload ?(win = Dom_html.window) f =
     win##.onunload := Dom_html.handler (fun ev -> Js.bool (f ev))
-  let onbeforeunload ?(win = Dom_html.window) f =
-    win##.onbeforeunload := Dom_html.handler (fun ev ->
-                                let (status, propagate) = f ev in
-                                if status then (
-                                   Js.bool propagate
-                                ) else
-                                  (
-                                    Js.(Unsafe.eval_string "undefined");
-                                  )
-                             )
   let onresize ?(win = Dom_html.window) f =
     win##.onresize := Dom_html.handler (fun ev -> Js.bool (f ev))
   let prompt ?(win = Dom_html.window) ?(value = "") msg =

src/utils/js_utils.mli

@@ -384,9 +384,6 @@ module Window : sig
   val onunload:
     ?win:Dom_html.window Js.t ->
     (Dom_html.event Js.t -> bool) -> unit
-  val onbeforeunload:
-    ?win:Dom_html.window Js.t ->
-    (Dom_html.event Js.t -> bool * bool) -> unit
   val onhashchange:
     ?win:Dom_html.window Js.t ->
     (Dom_html.hashChangeEvent Js.t -> bool) -> unit