Canvas

A canvas is a view you draw yourself with the HTML Canvas 2D API: charts, gauges, rings, signatures, custom shapes. The drawing function only records operations; the platform replays them natively (Core Graphics on iOS and Mac Catalyst, android.graphics.Canvas on Android) at the screen’s resolution.

import { mount } from "nsh";
import { Canvas } from "nsh/canvas";

function Ring(props: { progress: number }) {
  return (
    <Canvas
      style={{ width: 120, height: 120 }}
      draw={(ctx, { width, height }) => {
        const cx = width / 2, cy = height / 2, r = width / 2 - 10;
        ctx.lineWidth = 12;
        ctx.lineCap = "round";
        ctx.strokeStyle = "#e5e7eb";
        ctx.beginPath();
        ctx.arc(cx, cy, r, 0, Math.PI * 2);
        ctx.stroke();
        ctx.strokeStyle = "#6366f1";
        ctx.beginPath();
        ctx.arc(cx, cy, r, -Math.PI / 2, -Math.PI / 2 + props.progress * Math.PI * 2);
        ctx.stroke();
      }}
    />
  );
}

mount(() => <Ring progress={0.7} />);

Canvas comes from nsh/canvas. It creates the native canvas view, measures it, and runs your draw function with a context and the canvas’s size.

#Props

Canvas takes the props of a view (style, class, press and gesture props, onLayout, accessibility, ref, …) plus:

Prop	Type	Description
draw	(ctx: CanvasRenderingContext2D, size: { width: number; height: number }) => void	Draws the canvas. Runs once the size is known, again when the canvas resizes, and again when signals it reads change.

Give the canvas a size through its style (width / height, flex, aspectRatio): it has no content size of its own.

#When it redraws

draw is tracked like an effect: reading a signal inside it subscribes the drawing to that signal.

const values = signal([12, 40, 28, 64]);

<Canvas
  style={{ height: 160 }}
  draw={(ctx, { width, height }) => {
    const data = values(); // a change of values() redraws
    const step = width / data.length;
    ctx.fillStyle = "#0969da";
    data.forEach((v, i) => ctx.fillRect(i * step + 4, height - v, step - 8, v));
  }}
/>;

Numbers and colors may also be native animated values (CanvasNumber, CanvasPaint): the platform then redraws on its own while they animate, without running draw or any JS per frame.

import { Animated, createNativeValue } from "nsh/animated";
import { Canvas } from "nsh/canvas";

const angle = createNativeValue(0);
Animated.timing(angle, { toValue: Math.PI * 2, duration: 1000 }).start();

<Canvas
  style={{ width: 80, height: 80 }}
  draw={(ctx) => {
    ctx.translate(40, 40);
    ctx.rotate(angle); // animates natively
    ctx.fillStyle = "#cf222e";
    ctx.fillRect(-20, -4, 40, 8);
  }}
/>;

#The context

CanvasRenderingContext2D is a subset of the browser’s: the state stack (save, restore), transforms, paths (moveTo, lineTo, arc, arcTo, ellipse, rect, roundRect, Bézier curves), fill / stroke / clip (also of a Path2D, which accepts SVG path data), rectangles, text (fillText, strokeText, measureText), linear / radial / conic gradients, line dashes, shadows, globalAlpha, globalCompositeOperation and drawImage (images by URL or imported asset). Every member is listed in the nsh/canvas reference; the canvas guide walks through drawing charts and animated graphics.

Coordinates are in points with the origin at the top left. measureText is synchronous and returns only width.

Not supported: getImageData / putImageData, filter, isPointInPath and SVG images.

#Events

The view events: onPress, onLayout, gestures and touches. To handle drawing-relative taps, read locationX / locationY in onTouchEnd, or track a drag with onResponderMove.

#Ref methods

None of its own.