Using HTML Canvas from ReasonML using React Hooks

Question

I'm looking for a quick example on how to get started using the following technologies together:

• HTML5 Canvas
• ReasonML
• ReasonReact: "ReasonReact is a safer, simpler way to build React components, in Reason."
• bs-webapi: Web API bindings for Reason
• React Hooks

To get me started a snippet that does the following would be great:

• Manages a reference to the HTML5 Canvas element elegantly and correctly
• Is a simple react component
• Clears the canvas and draws something

I already have the basic ReasonML React project setup.

Answer 1

Here is a sample that shows one way to put everything together:

// Helper type to pass canvas size
type dimensions = {
  width: float,
  height: float,
};

// Actual drawing happens here, canvas context and size as parameters.
let drawOnCanvas =
    (context: Webapi.Canvas.Canvas2d.t, dimensions: dimensions): unit => {
  open Webapi.Canvas.Canvas2d;
  clearRect(context, ~x=0., ~y=0., ~w=dimensions.width, ~h=dimensions.height);

  setFillStyle(context, String, "rgba(0,128,169,0.1)");
  fillRect(context, ~x=10.0, ~y=10.0, ~w=30.0, ~h=30.0);
};

// Extract canvas dimensions from canvas element
let canvasDimensions = (canvasElement: Dom.element): dimensions =>
  Webapi.Canvas.CanvasElement.{
    width: float_of_int(width(canvasElement)),
    height: float_of_int(height(canvasElement)),
  };

// An adapter to give nicer parameters to drawOnCanvas above
let drawOnCanvasElement = (canvasElement: Dom.element): unit =>
  Webapi.Canvas.CanvasElement.(
    drawOnCanvas(
      getContext2d(canvasElement),
      canvasDimensions(canvasElement),
    )
  );

[@react.component]
let make = () => {
  open React;
  let canvasElementRef: Ref.t(option(Dom.element)) = useRef(None);

  useLayoutEffect0(() => {
    Ref.current(canvasElementRef)
    |> Belt.Option.map(_, drawOnCanvasElement)
    |> ignore;
    None;
  });

  <canvas
    width="200"
    height="100"
    ref={ReactDOMRe.Ref.callbackDomRef(elem =>
      React.Ref.setCurrent(canvasElementRef, Js.Nullable.toOption(elem))
    )}
  />;
};

Here are some random links I used when learning how to do this. (Adding them here in case they are useful for others too.):

• The bs-webapi test file to quickly learn the basics
• A specific answer on how to use the setFillStyle (and where I learned the link to the test file above)
• An answer in reason-react project showing how to work with React Refs.

The code has a bit more type declarations than necessary and some open statements could be added, but I like my answers a bit on the verbose side for a bit more instructiveness.

It should be relatively easy to shorten the code.

The intermediate functions canvasDimensions and drawOnCanvasElement add a bit of structure to the code in my opinion, but I'm not sure if they make the sample more or less clear for readers or if there would be a more elegant way to work with the canvas size.