svelte-canvas

Reactive canvas rendering with Svelte.

Installation

npm install svelte-canvas

Usage

<script>
  import { Canvas, Layer, t } from 'svelte-canvas';

  $: render = ({ context, width, height }) => {
    context.fillStyle = `hsl(${$t / 40}, 100%, 50%)`;
    context.beginPath();
    context.arc(($t / 4) % width, ($t / 4) % height, 100, 0, Math.PI * 2);
    context.fill();
  };
</script>

<Canvas width={640} height={640}>
  <Layer {render} />
</Canvas>

If you use TypeScript, add the Render type to your reactive statement:

import { ..., type Render } from "svelte-canvas";

let render: Render;
$: render = ({ context, width, height }) => {
  // ...
}

More examples:

API

Canvas

Canvas is the top-level element. It's a Svelte wrapper around an HTML <canvas> element.

Props

prop	default	description
`width`	640	Canvas width
`height`	640	Canvas height
`pixelRatio`	`window.devicePixelRatio`	Canvas pixel ratio
`style`	`null`	A CSS style string which will be applied to the canvas element
`class`	`null`	A class string which will be applied to the canvas element
`autoclear`	`true`	If `true`, will use `context.clearRect` to clear the canvas at the start of each render cycle
`layerEvents`	`false`	If `true`, enables event handlers on child `Layer` components

Methods

method	description
`getCanvas`	Returns a reference to the canvas DOM element
`getContext`	Returns the canvas's 2D rendering context
`redraw`	Forces a re-render of the canvas

Layer

Layer is a layer to be rendered onto the canvas. It takes two props, setup and render Both take functions with a single argument that receives an object with the properties context, width, and height. context is the 2D rendering context of the parent canvas. width and height are the canvas's dimensions.

setup is optional and is called once at initialization. render is called every time the canvas redraws.

Declaring your render function reactively allows svelte-canvas to re-render anytime the values that the function depends on change.

Event handling

All DOM events on the <canvas> element are forwarded to the Canvas component, so handling an event is as simple as <Canvas on:click={handleClick}>.

Individual Layer instances can also handle events that fall within their bounds when the layerEvents prop on the parent Canvas component is true. Event handlers registered with the on: directive receive a CustomEvent with properties detail.x and detail.y representing the coordinates of the event relative to the parent canvas, as well as detail.originalEvent, which contains the original canvas DOM event.

<script>
  import { Canvas, Layer, type CanvasLayerEvent } from 'svelte-canvas';

  const render = ({ context }) => {
    ...
  };

  const handleClick = (e: CanvasLayerEvent) => console.log(e.detail.x, e.detail.y)
</script>

<Canvas width={640} height={640}>
  <Layer {render} on:click={handleClick} />
</Canvas>

This is an experimental feature that may have a negative performance impact.

t

t is a readable store that provides the time in milliseconds since initialization. Subscribing to t within your render function lets you easily create animations.

Render

Render is a typing for your reactive statements that provides types for the Layer component's setup and render functions: (canvas: CanvasRenderingContext2D, width: number, height: number)

geonya / svelte-canvas