How to draw on a page using react hooks and typescript

This entry was posted on

  • typescript
  • react
  • web development
  • tutorial

Recently, How to Create the Drawing Interaction on DEV’s Offline Page by Ali Spittel showed up in my feed, and it looked pretty cool. This got me wondering if I could create the same thing as a react component using hooks and typescript. Well, the fact that I am writing this post means I was able to recreate it. So let’s see how I did it.

If you are interested in the final product, you can check out the Github repository or the code sandbox.

Creating the Component

The first thing we need to do is to create a Canvas component. The canvas needs to take up some space that we will want any parent component to override, so we will add width and height as props. But, we want sensible defaults so that we don’t have to add these props every time we want to use this component. So, we will add some defaultProps to set these values to window.innerWidth and window.innerHeight respectively.

import React from 'react';

interface CanvasProps {
    width: number;
    height: number;
}

const Canvas = ({ width, height }: CanvasProps) => {
    return <canvas height={height} width={width} />;
};

Canvas.defaultProps = {
    width: window.innerWidth,
    height: window.innerHeight,
};

export default Canvas;

Lets Draw

Since we need to modify the canvas element, we will need to add a ref to it. We can do this using the useRef hook and changing our canvas element to set the ref.

const canvasRef = useRef < HTMLCanvasElement > null;
return <canvas ref={canvasRef} height={height} width={width} />;

Set state

We need to keep track of some variables.

  • the mouse position.
  • whether we are painting or not.

We can do this by adding the useState hook. We will also create a Coordinate type to help with keeping track of mouse positions.

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

const Canvas = ({ width, height }: CanvasProps) => {
const [isPainting, setIsPainting] = useState(false);
const [mousePosition, setMousePosition] = useState<Coordinate | undefined>(undefined);
// ... other stuff here

Start drawing when the mouse is pressed.

We will add the event listener in the useEffect hook. If we have a valid reference to the canvas, we add an event listener to the mouseDown event. We also remove the event listener when we unmount.

useEffect(() => {
    if (!canvasRef.current) {
        return;
    }
    const canvas: HTMLCanvasElement = canvasRef.current;
    canvas.addEventListener('mousedown', startPaint);
    return () => {
        canvas.removeEventListener('mousedown', startPaint);
    };
}, [startPaint]);

startPaint needs to get the current coordinates of the mouse and set isPainting to true. We will also wrap it in a useCallback hook to use it inside the useEffect hook.

 const startPaint = useCallback((event: MouseEvent) => {
        const coordinates = getCoordinates(event);
        if (coordinates) {
            setIsPainting(true);
            setMousePosition(coordinates);
        }
    }, []);

// ...other stuff here

const getCoordinates = (event: MouseEvent): Coordinate | undefined => {
    if (!canvasRef.current) {
        return;
    }

    const canvas: HTMLCanvasElement = canvasRef.current;
    return {event.pageX - canvas.offsetLeft, event.pageY - canvas.offsetTop};
};

Draw the line on mouse move

Like the mouseDown event listener, we will use the useEffect` hook to add the _mousemove event.

useEffect(() => {
    if (!canvasRef.current) {
        return;
    }
    const canvas: HTMLCanvasElement = canvasRef.current;
    canvas.addEventListener('mousemove', paint);
    return () => {
        canvas.removeEventListener('mousemove', paint);
    };
}, [paint]);

paint needs to

  • Check if we are painting.
  • Get the new mouse coordinates.
  • Draw a line from the old coordinates to the new one by getting the rendering context from the canvas.
  • Update the old coordinates.
const paint = useCallback(
    (event: MouseEvent) => {
        if (isPainting) {
            const newMousePosition = getCoordinates(event);
            if (mousePosition && newMousePosition) {
                drawLine(mousePosition, newMousePosition);
                setMousePosition(newMousePosition);
            }
        }
    },
    [isPainting, mousePosition]
);

// ...other stuff here

const drawLine = (originalMousePosition: Coordinate, newMousePosition: Coordinate) => {
    if (!canvasRef.current) {
        return;
    }
    const canvas: HTMLCanvasElement = canvasRef.current;
    const context = canvas.getContext('2d');
    if (context) {
        context.strokeStyle = 'red';
        context.lineJoin = 'round';
        context.lineWidth = 5;

        context.beginPath();
        context.moveTo(originalMousePosition.x, originalMousePosition.y);
        context.lineTo(newMousePosition.x, newMousePosition.y);
        context.closePath();

        context.stroke();
    }
};

Stop drawing on mouse release

We want to stop drawing when either the user releases the mouse or moves the mouse out of the canvas area.

useEffect(() => {
    if (!canvasRef.current) {
        return;
    }
    const canvas: HTMLCanvasElement = canvasRef.current;
    canvas.addEventListener('mouseup', exitPaint);
    canvas.addEventListener('mouseleave', exitPaint);
    return () => {
        canvas.removeEventListener('mouseup', exitPaint);
        canvas.removeEventListener('mouseleave', exitPaint);
    };
}, [exitPaint]);

In exitPaint, we just set the isPainting to false.

const exitPaint = useCallback(() => {
    setIsPainting(false);
}, []);

Conclusion

And we have a React component that we can reuse. You can see the final code in either the Github repository or play with the code sandbox.

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