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0.0.17 • Public • Published

Canvas Render Components (alpha)

Super duper alpha.. Use at your own risk. lol

The basic idea here is a "react-like" API that will create canvas "components" such that it handles:

  1. Events over very specific areas of of the canvas, as defined by components.
  2. Communicating state changes between components.
  3. Ensuring that only what needs to be rendered is actually rendered.

There's a playground link here on Stackblitz.

Known Issues and Missing Features

  • THERE ARE NO TESTS!! (duh, huge red flag!)
  • Missing events:
    • onMouseDown
    • onMouseUp
    • onKeyPress
    • onKeyDown
    • onKeyUp
    • touch events?
  • Have yet to figure out focus management scheme
  • Screen reader updates
  • Components:
    • Ellipse? Circle?
  • Do I want to allow transformations (scale, rotate, etc) on other existing components?

Getting Started

See storybook examples for usage.


  1. Define a component
import { defineComp, rect, text } from 'canvas-render-components';

function MyComp(props: MyCompProps, ctx: CanvasRenderingContext2D) => {
    // There are hooks like React, just prefixed with `crc` instead of use:
    const [count, setCount] = crcState(0);

    // You can return other crc elements, like react, but JSX is annoying to hook up
    // So I don't have that in this example:
    return [
            x: 10,
            y: 10,
            width: 100,
            height: 100,
            fillStyle: 'blue',
            onClick: () => setCount(count + 1)
            x: 10,
            y: 10,
            width: 100,
            text: 'Click Me',
            fillStyle: 'white'

export const myComp = defineComp(MyComp)
  1. Mount the component to an existing HTMLCanvasElement:
import { myComp } from './MyComp';
import { crc } from 'canvas-render-components';

const canvas = document.querySelector('#my-canvas-id');
crc(canvas, myComp);

API List

Sorry, this isn't really documentation, just the basic idea:


  • crc(canvasElement, crcElement) - Mount or update an existing canvas element with a crc element
  • defineComp(compFn) - used to create a more ergonomic means of consuming crc components and returning crc elements when setting up JSX is too annoying (it's always too annoying).


  • path: Renders an arbitrary Path2D
  • rect: Renders a rectangle
  • text: Text rendering (including multiline, singleline, ellipsis overflow, etc)
  • line: Renders a series of coordinates as connected line segments
  • verticalLine/horizontalLine special components for rendering vertical or horizontal line segments, which includes a bit called alignToPixelGrid that allows you to ensure 1px lines are really 1px. (it's a canvas quirk)
  • svgPath: Renders svg path data as a shape
  • img: Loads and renders an image
  • g: A grouping component that allows the group application of transformations such as scale, rotation, etc.
  • clip: A grouping component that applies a clipping path to everything rendered in its children. It ALSO will "clip" events.
  • layer: A component for memoizing another component as a unit of render. Basically, if the props of the CompEl passed to render change, or if the width or height of the layer change (it will default to the canvas width and height), it will re-render itself. Otherwise, it will render a cached image. See the storybook for example. Note that it does a shallow, reference check on the props of the element passed to render.


  • crcRef - basically a simplified version of react's useRef
  • crcState - A simplified version of react's useState
  • crcMemo - Basically react's useMemo. This is VERY useful for memoizing Path2D objects that need to be passed to other hooks. Strongly recommended for that use case.
  • crcWhenChanged - Looks like react's useEffect.. it is NOT. It takes a callback that will execute SYNCHRONOUSLY when dependencies change. It also allows the return of a teardown. This is specifically for use cases where one might need to execute some logic only when some dependencies change. DO NOT USE if you need to synchronously update some _state when dependencies change, use crcMemo instead.
  • crcCursor - A hook to allow the setup of CSS cursor (pointer) changes when hovering a given Path2D.
  • crcEvent - A hook for setting up events related to a particular Path2D (or if no path is provided, the entire canvas)
  • crcRectPath - A simplified hook that returns a memoized Path2D for a rectangle (A common task).
  • crcLinePath - A hook for memoized Path2D objects from coordinates.
  • crcSvgPath - A hook for memoized Path2D objects from svg path data strings.


Events coming through other rendered things

If you're seeing events "bleeding through" things you've rendered over top of them, you need to use the clip component to constrain where the event is allowed to fire. Events are registered separate from rendering anything, they operate on a 2d plain of their own and don't "know" about what pixels are rendered where. The clip component keeps track of clipping paths in a context that it will apply to events registered underneath it. Basically, if your event is registered against a path as part of a clip components children or descendants, for the event to fire, it must match the event path AND the clipping path. Think of the clipping path as a "mask" of where events underneath it are "allowed" to fire. (It also clips what is rendered)

Order matters!

The last thing in a list of children will be rendered "on top". Remember that as you're rendering things.

Perf: More events, more problems.

Events are register and/or unregistered on every render. They're associated with Path2D objects, functions that are handlers, and probably some closure and other things. The more events you have, the slower your render will be. Full stop. So, "event delegation" can be a useful tool for you. Perhaps what you need to do is use the crcEvent hook and register one event against a compound path of some sort. Or maybe register an event against the whole canvas by passing a undefined path to crcEvent, and then do some of your own math to figure out if it's a hit or not. In any case, if you're registering and unregistring 1,000 event handlers on each render, it's going to add up. Don't do that.

Perf: Canvas layering

Another technique is to have more than one canvas, one on top of the other. In this way, you can have elements of your scene that are rarely updated rendered on the "bottom" canvas, while your more frequently updated elements are rendered on the "top" canvas. This means less code being executed on each pass.


npm i canvas-render-components

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