A pannable, zoomable, html5 canvas in React and Typescript

pannable-zoomable-html5-canvas-react-typescript.tsx

// sandbox here: https://codesandbox.io/s/p3itj?file=/src/Canvas.tsx
import {
  useEffect,
  useCallback,
  useLayoutEffect,
  useRef,
  useState
} from "react";
import * as React from "react";

type CanvasProps = {
  canvasWidth: number;
  canvasHeight: number;
};

type Point = {
  x: number;
  y: number;
};

const ORIGIN = Object.freeze({ x: 0, y: 0 });

// adjust to device to avoid blur
const { devicePixelRatio: ratio = 1 } = window;

function diffPoints(p1: Point, p2: Point) {
  return { x: p1.x - p2.x, y: p1.y - p2.y };
}

function addPoints(p1: Point, p2: Point) {
  return { x: p1.x + p2.x, y: p1.y + p2.y };
}

function scalePoint(p1: Point, scale: number) {
  return { x: p1.x / scale, y: p1.y / scale };
}

const ZOOM_SENSITIVITY = 500; // bigger for lower zoom per scroll

export default function Canvas(props: CanvasProps) {
  const canvasRef = useRef<HTMLCanvasElement>(null);
  const [context, setContext] = useState<CanvasRenderingContext2D | null>(null);
  const [scale, setScale] = useState<number>(1);
  const [offset, setOffset] = useState<Point>(ORIGIN);
  const [mousePos, setMousePos] = useState<Point>(ORIGIN);
  const [viewportTopLeft, setViewportTopLeft] = useState<Point>(ORIGIN);
  const isResetRef = useRef<boolean>(false);
  const lastMousePosRef = useRef<Point>(ORIGIN);
  const lastOffsetRef = useRef<Point>(ORIGIN);

  // update last offset
  useEffect(() => {
    lastOffsetRef.current = offset;
  }, [offset]);

  // reset
  const reset = useCallback(
    (context: CanvasRenderingContext2D) => {
      if (context && !isResetRef.current) {
        // adjust for device pixel density
        context.canvas.width = props.canvasWidth * ratio;
        context.canvas.height = props.canvasHeight * ratio;
        context.scale(ratio, ratio);
        setScale(1);

        // reset state and refs
        setContext(context);
        setOffset(ORIGIN);
        setMousePos(ORIGIN);
        setViewportTopLeft(ORIGIN);
        lastOffsetRef.current = ORIGIN;
        lastMousePosRef.current = ORIGIN;

        // this thing is so multiple resets in a row don't clear canvas
        isResetRef.current = true;
      }
    },
    [props.canvasWidth, props.canvasHeight]
  );

  // functions for panning
  const mouseMove = useCallback(
    (event: MouseEvent) => {
      if (context) {
        const lastMousePos = lastMousePosRef.current;
        const currentMousePos = { x: event.pageX, y: event.pageY }; // use document so can pan off element
        lastMousePosRef.current = currentMousePos;

        const mouseDiff = diffPoints(currentMousePos, lastMousePos);
        setOffset((prevOffset) => addPoints(prevOffset, mouseDiff));
      }
    },
    [context]
  );

  const mouseUp = useCallback(() => {
    document.removeEventListener("mousemove", mouseMove);
    document.removeEventListener("mouseup", mouseUp);
  }, [mouseMove]);

  const startPan = useCallback(
    (event: React.MouseEvent<HTMLCanvasElement, MouseEvent>) => {
      document.addEventListener("mousemove", mouseMove);
      document.addEventListener("mouseup", mouseUp);
      lastMousePosRef.current = { x: event.pageX, y: event.pageY };
    },
    [mouseMove, mouseUp]
  );

  // setup canvas and set context
  useLayoutEffect(() => {
    if (canvasRef.current) {
      // get new drawing context
      const renderCtx = canvasRef.current.getContext("2d");

      if (renderCtx) {
        reset(renderCtx);
      }
    }
  }, [reset, props.canvasHeight, props.canvasWidth]);

  // pan when offset or scale changes
  useLayoutEffect(() => {
    if (context && lastOffsetRef.current) {
      const offsetDiff = scalePoint(
        diffPoints(offset, lastOffsetRef.current),
        scale
      );
      context.translate(offsetDiff.x, offsetDiff.y);
      setViewportTopLeft((prevVal) => diffPoints(prevVal, offsetDiff));
      isResetRef.current = false;
    }
  }, [context, offset, scale]);

  // draw
  useLayoutEffect(() => {
    if (context) {
      const squareSize = 20;

      // clear canvas but maintain transform
      const storedTransform = context.getTransform();
      context.canvas.width = context.canvas.width;
      context.setTransform(storedTransform);

      context.fillRect(
        props.canvasWidth / 2 - squareSize / 2,
        props.canvasHeight / 2 - squareSize / 2,
        squareSize,
        squareSize
      );
      context.arc(viewportTopLeft.x, viewportTopLeft.y, 5, 0, 2 * Math.PI);
      context.fillStyle = "red";
      context.fill();
    }
  }, [
    props.canvasWidth,
    props.canvasHeight,
    context,
    scale,
    offset,
    viewportTopLeft
  ]);

  // add event listener on canvas for mouse position
  useEffect(() => {
    const canvasElem = canvasRef.current;
    if (canvasElem === null) {
      return;
    }

    function handleUpdateMouse(event: MouseEvent) {
      event.preventDefault();
      if (canvasRef.current) {
        const viewportMousePos = { x: event.clientX, y: event.clientY };
        const topLeftCanvasPos = {
          x: canvasRef.current.offsetLeft,
          y: canvasRef.current.offsetTop
        };
        setMousePos(diffPoints(viewportMousePos, topLeftCanvasPos));
      }
    }

    canvasElem.addEventListener("mousemove", handleUpdateMouse);
    canvasElem.addEventListener("wheel", handleUpdateMouse);
    return () => {
      canvasElem.removeEventListener("mousemove", handleUpdateMouse);
      canvasElem.removeEventListener("wheel", handleUpdateMouse);
    };
  }, []);

  // add event listener on canvas for zoom
  useEffect(() => {
    const canvasElem = canvasRef.current;
    if (canvasElem === null) {
      return;
    }

    // this is tricky. Update the viewport's "origin" such that
    // the mouse doesn't move during scale - the 'zoom point' of the mouse
    // before and after zoom is relatively the same position on the viewport
    function handleWheel(event: WheelEvent) {
      event.preventDefault();
      if (context) {
        const zoom = 1 - event.deltaY / ZOOM_SENSITIVITY;
        const viewportTopLeftDelta = {
          x: (mousePos.x / scale) * (1 - 1 / zoom),
          y: (mousePos.y / scale) * (1 - 1 / zoom)
        };
        const newViewportTopLeft = addPoints(
          viewportTopLeft,
          viewportTopLeftDelta
        );

        context.translate(viewportTopLeft.x, viewportTopLeft.y);
        context.scale(zoom, zoom);
        context.translate(-newViewportTopLeft.x, -newViewportTopLeft.y);

        setViewportTopLeft(newViewportTopLeft);
        setScale(scale * zoom);
        isResetRef.current = false;
      }
    }

    canvasElem.addEventListener("wheel", handleWheel);
    return () => canvasElem.removeEventListener("wheel", handleWheel);
  }, [context, mousePos.x, mousePos.y, viewportTopLeft, scale]);

  return (
    <div>
      <button onClick={() => context && reset(context)}>Reset</button>
      <pre>scale: {scale}</pre>
      <pre>offset: {JSON.stringify(offset)}</pre>
      <pre>viewportTopLeft: {JSON.stringify(viewportTopLeft)}</pre>
      <canvas
        onMouseDown={startPan}
        ref={canvasRef}
        width={props.canvasWidth * ratio}
        height={props.canvasHeight * ratio}
        style={{
          border: "2px solid #000",
          width: `${props.canvasWidth}px`,
          height: `${props.canvasHeight}px`
        }}
      ></canvas>
    </div>
  );
}

Do you know how can I add another box into the canvas?

Do you know how can I add another box into the canvas?

The line of code that draws the box is context.fillRect. You should be able to add more stuff like that in that function and have it show up.

Do you know how can I add another box into the canvas?

The line of code that draws the box is context.fillRect. You should be able to add more stuff like that in that function and have it show up.

Thanks for the quick reply. I tried just pasting that line 3 times but I still only have one box. Should I be doing something different?

Do you know how can I add another box into the canvas?

The line of code that draws the box is context.fillRect. You should be able to add more stuff like that in that function and have it show up.

Thanks for the quick reply. I tried just pasting that line 3 times but I still only have one box. Should I be doing something different?

It might be the same box drawn on top of the same place multiple times :) you might be interested in this MDN tutorial, which helped me a lot when learning the fundamentals of the canvas api! https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial

https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial

Thanks!

What's the proper way to serialize and restore a view with this setup?

Hi @gradywetherbee , good question! I implemented this myself at thermalmodel.com

Hitting "Saved View" restores the saved state, and "Overwrite Saved View" updates the stored state.

The TL;DR is you need to store the current zoom and offset in some application state and manage/overwrite it accordingly when the user performs some actions (hits buttons, presses certain keys, whatever)

The way I implemented it in my project is in this hook: https://github.com/robinovitch61/hotstuff/blob/main/src/components/Canvas/Canvas.tsx#L29

The view state is an input to the component. In the parent logic, there is the buttons for storing/reseting that state.

I won't really be able to help much with further implementation details, but hope that sets you off on the right track. Good luck!

@robinovitch61 in my case 'canvasHeight' and 'canvasWidth" are not static ... they change depending on the resize. What I need to do to make it work right ? now it resets to prev state each time I resize parent element

@robinovitch61 in my case 'canvasHeight' and 'canvasWidth" are not static ... they change depending on the resize. What I need to do to make it work right ? now it resets to prev state each time I resize parent element

Hi @YevhenTarashchyk , I can point you to the react component's height and width state that powers the resizability of https://thermalmodel.com/. Hopefully that helps. In general, you'll want to do something like this:

https://github.com/robinovitch61/hotstuff/blob/main/src/components/Canvas/Canvas.tsx#LL98-L100C63

thx, mate 👍 пн, 26 дек. 2022 г. в 21:05, Leo Robinovitch ***@***.***>:

…

***@***.**** commented on this gist. ------------------------------ @robinovitch61 <https://github.com/robinovitch61> in my case 'canvasHeight' and 'canvasWidth" are not static ... they change depending on the resize. What I need to do to make it work right ? now it resets to prev state each time I resize parent element Hi @YevhenTarashchyk <https://github.com/YevhenTarashchyk> , I can point you to the react component <https://github.com/robinovitch61/hotstuff/blob/main/src/components/Canvas/Canvas.tsx#L37-L38>'s height and width state that powers the resizability of https://thermalmodel.com/. Hopefully that helps. In general, you'll want to do something like this: https://github.com/robinovitch61/hotstuff/blob/main/src/components/Canvas/Canvas.tsx#LL98-L100C63 — Reply to this email directly, view it on GitHub <https://gist.github.com/483190546bf8f0617d2cd510f3b4b86d#gistcomment-4413989> or unsubscribe <https://github.com/notifications/unsubscribe-auth/AHBWPIPVJYOSWS36DMMTISDWPHT77BFKMF2HI4TJMJ2XIZLTSKBKK5TBNR2WLJDHNFZXJJDOMFWWLK3UNBZGKYLEL52HS4DFQKSXMYLMOVS2I5DSOVS2I3TBNVS3W5DIOJSWCZC7OBQXE5DJMNUXAYLOORPWCY3UNF3GS5DZVRZXKYTKMVRXIX3UPFYGLK2HNFZXIQ3PNVWWK3TUUZ2G64DJMNZZDAVEOR4XAZNEM5UXG5FFOZQWY5LFVEYTAOJTGA4TGNBSU52HE2LHM5SXFJTDOJSWC5DF> . You are receiving this email because you were mentioned. Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub> .

@robinovitch61 Could u pls help me a little ? https://codesandbox.io/p/github/YevhenTarashchyk/EasyTextureUI_components/draft/patient-cherry
When I resize element the image position in canvas is a little bit off (it is moving)... What should I do ?
Here is a vid for better understanding

zoom.issue.mp4

@YevhenTarashchyk I don't have time to dig in unfortunately, but you've done a good job demonstrating and reproducing your issue! I'm sure someone on stack overflow would be down to help out if you post a question there.