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https://github.com/LexiFi/ocaml-vdom

Elm architecture and (V)DOM for OCaml
https://github.com/LexiFi/ocaml-vdom

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Elm architecture and (V)DOM for OCaml

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README 

        
vdom: Elm architecture and (V)DOM for OCaml

=================================================



[![Build Status](https://travis-ci.com/LexiFi/ocaml-vdom.svg?branch=master)](https://travis-ci.com/LexiFi/ocaml-vdom)



Overview

--------



This package contains:



  - OCaml bindings to DOM and other client-side Javascript APIs

    (using [gen_js_api](https://github.com/LexiFi/gen_js_api)).



  - An implementation of the [Elm architecture](https://guide.elm-lang.org/architecture/), where the

    UI is specified as a functional "view" on the current state.



Dependencies

------------



  - OCaml



  - js_of_ocaml



  - gen_js_api



  - odoc



Installation (with OPAM)

------------------------



```

opam install vdom

```



Install dependencies

------------------------

```

opam install vdom --deps-only

```



Manual installation

-------------------



```bash

git clone https://github.com/LexiFi/ocaml-vdom.git

cd ocaml-vdom

make all

```



DOM bindings

------------



[`Js_browser`](lib/js_browser.mli) exposes (partial) OCaml bindings of the browser's DOM and

other common client-side Javascript APIs.



It is implemented with

[gen_js_api](https://github.com/LexiFi/gen_js_api), making it

realistic to have it working with ReScript in the future.  This

would open the door to writing client-side web applications in OCaml

that could be compiled to Javascript either with js_of_ocaml or

ReScript.



VDOM

----



The [Elm architecture](https://guide.elm-lang.org/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`](lib/vdom.mli): definition of the VDOM tree and of "virtual

    applications".  This is a "pure" module, which does not depend on

    any Javascript bindings (it could be executed on the server-side,

    e.g. for automated testing).



  - [`Vdom_blit`](lib/vdom_blit.mli): 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_blit` is implemented on top of `Js_browser`.



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

    overridden.  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

    new ordering.



  - 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 is created in `Vdom_blit` to 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

    node.



  - 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](https://developer.mozilla.org/fr/docs/Web/API/Window/requestAnimationFrame), which is supposed to be available (natively,

    or through a polyfill).



Usage

-----



A simple one-module application would look like:



```ocaml

open Vdom



(* Definition of the vdom 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

```



Use the following Dune file to compile this to JavaScript:



```

(executable

 (name myprog)

 (libraries vdom)

 (modes js))

```



The resulting JavaScript file `myprog.bc.js` can be found in the `_build` directory and can then be used from a simple HTML file such as:



```html



  

    

  

  

  



```



Examples: [`Demo`](examples/demo/demo.ml), [`Counters`](examples/counters/counters.ml).



Third-party examples:

  `TodoMVC`

    ([source](https://github.com/slegrand45/examples_ocaml_vdom/blob/master/todomvc/todomvc.ml),

     [demo](https://slegrand45.github.io/examples_ocaml_vdom.site/todomvc/)),

  [`With Eliom service`](https://github.com/slegrand45/examples_ocsigen/blob/master/eliom/with-ocaml-vdom/simple/mixvdomandeliom.eliom).



About

-----



This project has been created by LexiFi initially for its internal

use. It has been used in production since 2016 but 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

[MIT license](LICENSE).



Contact: [email protected]