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ocaml-vdom: Elm architecture and (V)DOM for OCaml
This package contains:
An implementation of the Elm architecture, where the
UI is specified as a functional "view" on the current state.
Installation (with OPAM)
opam install ocaml-vdom
git clone https://github.com/LexiFi/ocaml-vdom.git cd ocaml-vdom make all make doc make examples # Optional (browse index.html files in _build/default/examples to try out) make install
Js_browser exposes (partial) OCaml bindings of the browser's DOM and
It is implemented with
gen_js_api, making it
realistic to have it working with Bucklescript in the future. This
would open the door to writing client-side web applications in OCaml
The Elm architecture is a
functional way to describe UI applications. In this architecture, the
current state of the UI is represented with a single data type and a
"view" function projects this state to a concrete rendering. In our
case, this rendering is done to a tree-like abstraction of the browser
DOM, called a VDOM (Virtual DOM). This VDOM can itself be rendered to
a concrete DOM. Whenever the state changes, the view function produces
a new VDOM tree, which is then diffed with the previous one to update
the concrete DOM accordingly. The VDOM also specifies how DOM events
are wrapped into "messages" that are processed by an "update" function
to modify the current state. This function can also spawn "commands"
(such as AJAX calls) whose outcome is also notified by messages.
The implementation of this architecture relies on two modules:
Vdom: definition of the VDOM tree and of "virtual
applications". This is a "pure" module, which does not depend on
e.g. for automated testing).
Vdom_blit: rendering of virtual applications into the actual
DOM. This modules implements the initial "blit" operation
(rendering a VDOM tree to the DOM) and the "diff/synchronization"
algorithm. It also manages the state of a running application.
Vdom_blitis implemented on top of
This implementation of VDOM has some specificities:
Each node in the VDOM tree has a "key" string field. By default,
the key corresponds to the tag name for elements but it can be
overriden. The key is used by the synchronization algorithm
as follows: when synchronizing the old and new children of an
element, the children are first grouped by key. Two children with
different keys are never synchronized, and the sequence of old and
new children with a given key are synchronized in a pairwise way
(first old child with key K against first new child with key K;
etc...), adding or removing extra/missing children if needed.
Children are also reordered in the DOM, if needed, to match the
Event handlers are not attached on DOM nodes created when a VDOM
tree is rendered. Instead, we attach fixed event handlers on the
root container, and rely on event delegation. The handler
corresponding to a given element and responsible for a given kind
of event is searched directly in the VDOM. The rationale for this
design choice is that comparing functional values is not
well-defined in OCaml, so it would not be clear, when the "old"
and "new" VDOMs are diffed, if the event handler on the DOM node
should be refreshed.
A "bridge" structure in created in
Vdom_blitto represent the
correspondence between VDOM and DOM nodes. This structure mimics
the shape of both trees and avoids having to query the concrete
DOM to navigate in the tree.
No data structure is created to represent the "diff" between old
and new VDOMs. Instead, the synchronization algorithm detects
VDOM changes and apply them on the fly to the corresponding DOM
There is some special support for the "value" property. When this
property is explicitly bound in the VDOM (typically on an input
field), the value is forced on the element: whenever the DOM value
changes, the event is potentially dispatched to an event handler,
and the new VDOM property is forced on the DOM element. In
particular, if the internal state is not updated by the event
handler, the field becomes in practice read-only.
Some special VDOM node attributes are provided to present
"superficial state changes" that are not reflected in the proper
functional state (currently: giving focus to an element, or
ensuring an element is visible by y-scrolling its parent). These
attributes produce the corresponding DOM action when they are
first put on an element (which is not completely well-defined,
since this depends on the synchronization algorithm).
The "view" function is not applied synchronously when the
state ("model") changes. Instead, a rendering (applying the
"view" function and updating the actual DOM accordingly) is
scheduled. This means that multiple changes can be grouped
without triggering a redraw. The current strategy is to delay
redrawing with window.requestAnimationFrame, which is supposed to be available (natively,
or through a polyfill).
A simple one-module application would look like:
open Vdom (* Definition of the vdom application *) type model = .... (* the state of the application *) let view model = ... (* the state->vdom rendering function *) let init = return ... (* the initial state *) let update model = function .... (* the state-updating function *) let my_app = app ~init ~update ~view () (* Driver *) open Js_browser let run () = Vdom_blit.run my_app (* run the application *) |> Vdom_blit.dom (* get its root DOM container *) |> Element.append_child (Document.body document) (* insert the DOM in the document *) let () = Window.set_onload window run
ocamlfind ocamlc -package ocaml-vdom -no-check-prims -linkpkg -o myprog.exe myprog.ml js_of_ocaml +gen_js_api/ojs_runtime.js -o myprog.js myprog.exe
<html> <head> <script src="myprog.js"></script> </head> <body> </body> </html>
With Eliom service
This project has been created by LexiFi initially for its internal
use. It is already used in production but it is still relatively new
and no commitment is made on the stability of its interface. So
please let us know if you consider using it!
This ocaml-vdom package is licensed by LexiFi under the terms of the