Module Lwt_js_events

module Lwt_js_events : sig..end

Programming mouse or keyboard events handlers using Lwt


Reminder: Event capturing starts with the outer most element in the DOM and works inwards to the HTML element the event took place on (capture phase) and then out again (bubbling phase).

Examples of use:

Waiting for a click on elt1 before continuing:

let%lwt _ = Lwt_js_events.click elt1 in

Doing some operation for each value change in input element inp:

Lwt_js_events.(async (fun () ->
      clicks inp1 (fun ev _ -> ...)
   ))

Defining a thread that waits for ESC key on an element:

let rec esc elt =
      let%lwt ev = keydown elt in
      if ev##.keyCode = 27
      then Lwt.return ev
      else esc elt

Waiting for a click or escape key before continuing:

let%lwt () =
       Lwt.pick [(let%lwt _ = esc Dom_html.document in Lwt.return ());
                 (let%lwt _ = click Dom_html.document in Lwt.return ())]
     in ...

Create Lwt threads for events

val make_event : 
  (#Dom_html.event as 'a) Js.t Dom_html.Event.typ ->
  ?use_capture:bool -> #Dom_html.eventTarget Js.t -> 'a Js.t Lwt.t

make_event ev target creates a Lwt thread that waits for the event ev to happen on target (once). This thread is cancellable using Lwt.​cancel . If you set the optional parameter ~use_capture:true, the event will be caught during the capture phase, otherwise it is caught during the bubbling phase (default).

val seq_loop : 
  (?use_capture:bool -> 'target -> 'event Lwt.t) ->
  ?cancel_handler:bool ->
  ?use_capture:bool ->
  'target -> ('event -> unit Lwt.t -> unit Lwt.t) -> unit Lwt.t

seq_loop (make_event ev) target handler creates a looping Lwt thread that waits for the event ev to happen on target, then execute handler, and start again waiting for the event. Events happening during the execution of the handler are ignored. See async_loop and buffered_loop for alternative semantics.

For example, the clicks function below is defined by:

let clicks ?use_capture t = seq_loop click ?use_capture t

The thread returned is cancellable using Lwt.​cancel . In order for the loop thread to be canceled from within the handler, the latter receives the former as its second parameter.

By default, cancelling the loop will not cancel the potential currently running handler. This behaviour can be changed by setting the cancel_handler parameter to true.

val async_loop : 
  (?use_capture:bool -> 'target -> 'event Lwt.t) ->
  ?use_capture:bool ->
  'target -> ('event -> unit Lwt.t -> unit Lwt.t) -> unit Lwt.t

async_loop is similar to seq_loop, but each handler runs independently. No event is thus missed, but since several instances of the handler can be run concurrently, it is up to the programmer to ensure that they interact correctly.

Cancelling the loop will not cancel the potential currently running handlers.

val buffered_loop : 
  (?use_capture:bool -> 'target -> 'event Lwt.t) ->
  ?cancel_handler:bool ->
  ?cancel_queue:bool ->
  ?use_capture:bool ->
  'target -> ('event -> unit Lwt.t -> unit Lwt.t) -> unit Lwt.t

buffered_loop is similar to seq_loop, but any event that occurs during an execution of the handler is queued instead of being ingnored.

No event is thus missed, but there can be a non predictible delay between its trigger and its treatment. It is thus a good idea to use this loop with handlers whose running time is short, so the memorized event still makes sense when the handler is eventually executed. It is also up to the programmer to ensure that event handlers terminate so the queue will eventually be emptied.

By default, cancelling the loop will not cancel the (potential) currently running handler, but any other queued event will be dropped. This behaviour can be customized using the two optional parameters cancel_handler and cancel_queue.

val async : (unit -> 'a Lwt.t) -> unit

async t records a thread to be executed later. It is implemented by calling yield, then Lwt.async. This is useful if you want to create a new event listener when you are inside an event handler. This avoids the current event to be catched by the new event handler (if it propagates).

val func_limited_loop : 
  (?use_capture:bool -> 'a -> 'b Lwt.t) ->
  (unit -> 'a Lwt.t) ->
  ?use_capture:bool ->
  'a -> ('b -> unit Lwt.t -> unit Lwt.t) -> unit Lwt.t

func_limited_loop event delay_fun target handler will behave like Lwt_js_events.async_loop event target handler but it will run delay_fun first, and execut handler only when delay_fun is finished and no other event occurred in the meantime.

This allows to limit the number of events catched.

Be careful, it is an asynchrone loop, so if you give too little time, several instances of your handler could be run in same time *

val limited_loop : 
  (?use_capture:bool -> 'a -> 'b Lwt.t) ->
  ?elapsed_time:float ->
  ?use_capture:bool ->
  'a -> ('b -> unit