excalidraw/src/index.js

import React from "react";
import ReactDOM from "react-dom";
import rough from "roughjs/dist/rough.umd.js";

import "./styles.css";

var elements = [];

function newElement(type, x, y) {
  const element = {
    type: type,
    x: x,
    y: y,
    width: 0,
    height: 0,
    isSelected: false
  };
  return element;
}

function rotate(x1, y1, x2, y2, angle) {
  // 𝑎′𝑥=(𝑎𝑥−𝑐𝑥)cos𝜃−(𝑎𝑦−𝑐𝑦)sin𝜃+𝑐𝑥
  // 𝑎′𝑦=(𝑎𝑥−𝑐𝑥)sin𝜃+(𝑎𝑦−𝑐𝑦)cos𝜃+𝑐𝑦.
  // https://math.stackexchange.com/questions/2204520/how-do-i-rotate-a-line-segment-in-a-specific-point-on-the-line
  return [
    (x1 - x2) * Math.cos(angle) - (y1 - y2) * Math.sin(angle) + x2,
    (x1 - x2) * Math.sin(angle) + (y1 - y2) * Math.cos(angle) + y2
  ];
}

var generator = rough.generator();

function generateShape(element) {
  if (element.type === "selection") {
    element.draw = (rc, context) => {
      const fillStyle = context.fillStyle;
      context.fillStyle = "rgba(0, 0, 255, 0.10)";
      context.fillRect(element.x, element.y, element.width, element.height);
      context.fillStyle = fillStyle;
    };
  } else if (element.type === "rectangle") {
    const shape = generator.rectangle(
      element.x,
      element.y,
      element.width,
      element.height
    );
    element.draw = (rc, context) => {
      rc.draw(shape);
    };
  } else if (element.type === "ellipse") {
    const shape = generator.ellipse(
      element.x + element.width / 2,
      element.y + element.height / 2,
      element.width,
      element.height
    );
    element.draw = (rc, context) => {
      rc.draw(shape);
    };
  } else if (element.type === "arrow") {
    const x1 = element.x;
    const y1 = element.y;
    const x2 = element.x + element.width;
    const y2 = element.y + element.height;

    const size = 30; // pixels
    const distance = Math.sqrt(Math.pow(x2 - x1, 2) + Math.pow(y2 - y1, 2));
    // Scale down the arrow until we hit a certain size so that it doesn't look weird
    const minSize = Math.min(size, distance / 2);
    const xs = x2 - ((x2 - x1) / distance) * minSize;
    const ys = y2 - ((y2 - y1) / distance) * minSize;

    const angle = 20; // degrees
    const [x3, y3] = rotate(xs, ys, x2, y2, (-angle * Math.PI) / 180);
    const [x4, y4] = rotate(xs, ys, x2, y2, (angle * Math.PI) / 180);

    const shapes = [
      generator.line(x1, y1, x2, y2),
      generator.line(x3, y3, x2, y2),
      generator.line(x4, y4, x2, y2)
    ];

    element.draw = (rc, context) => {
      shapes.forEach(shape => rc.draw(shape));
    };
    return;
  } else if (element.type === "text") {
    element.draw = (rc, context) => {
      const font = context.font;
      context.font = element.font;
      const height =
        element.measure.actualBoundingBoxAscent +
        element.measure.actualBoundingBoxDescent;
      context.fillText(
        element.text,
        element.x,
        element.y + 2 * element.measure.actualBoundingBoxAscent - height / 2
      );
      context.font = font;
    };
  } else {
    throw new Error("Unimplemented type " + element.type);
  }
}

// If the element is created from right to left, the width is going to be negative
// This set of functions retrieves the absolute position of the 4 points.
// We can't just always normalize it since we need to remember the fact that an arrow
// is pointing left or right.
function getElementAbsoluteX1(element) {
  return element.width >= 0 ? element.x : element.x + element.width;
}
function getElementAbsoluteX2(element) {
  return element.width >= 0 ? element.x + element.width : element.x;
}
function getElementAbsoluteY1(element) {
  return element.height >= 0 ? element.y : element.y + element.height;
}
function getElementAbsoluteY2(element) {
  return element.height >= 0 ? element.y + element.height : element.y;
}

function setSelection(selection) {
  const selectionX1 = getElementAbsoluteX1(selection);
  const selectionX2 = getElementAbsoluteX2(selection);
  const selectionY1 = getElementAbsoluteY1(selection);
  const selectionY2 = getElementAbsoluteY2(selection);
  elements.forEach(element => {
    const elementX1 = getElementAbsoluteX1(element);
    const elementX2 = getElementAbsoluteX2(element);
    const elementY1 = getElementAbsoluteY1(element);
    const elementY2 = getElementAbsoluteY2(element);
    element.isSelected =
      element.type !== "selection" &&
      selectionX1 <= elementX1 &&
      selectionY1 <= elementY1 &&
      selectionX2 >= elementX2 &&
      selectionY2 >= elementY2;
  });
}

function App() {
  const [draggingElement, setDraggingElement] = React.useState(null);
  const [elementType, setElementType] = React.useState("selection");
  const onKeyDown = React.useCallback(event => {
    if (event.key === "Backspace") {
      for (var i = elements.length - 1; i >= 0; --i) {
        if (elements[i].isSelected) {
          elements.splice(i, 1);
        }
      }
      drawScene();
      event.preventDefault();
    } else if (
      event.key === "ArrowLeft" ||
      event.key === "ArrowRight" ||
      event.key === "ArrowUp" ||
      event.key === "ArrowDown"
    ) {
      const step = event.shiftKey ? 5 : 1;
      elements.forEach(element => {
        if (element.isSelected) {
          if (event.key === "ArrowLeft") element.x -= step;
          else if (event.key === "ArrowRight") element.x += step;
          else if (event.key === "ArrowUp") element.y -= step;
          else if (event.key === "ArrowDown") element.y += step;
          generateShape(element);
        }
      });
      drawScene();
      event.preventDefault();
    }
  }, []);
  React.useEffect(() => {
    document.addEventListener("keydown", onKeyDown, false);
    return () => {
      document.removeEventListener("keydown", onKeyDown, false);
    };
  }, [onKeyDown]);

  function ElementOption({ type, children }) {
    return (
      <label>
        <input
          type="radio"
          checked={elementType === type}
          onChange={() => setElementType(type)}
        />
        {children}
      </label>
    );
  }
  return (
    <div>
      {/* Can't use the <ElementOption> form because ElementOption is re-defined
          on every render, which would blow up and re-create the entire DOM tree,
          which in addition to being inneficient, messes up with browser text
          selection */}
      {ElementOption({ type: "rectangle", children: "Rectangle" })}
      {ElementOption({ type: "ellipse", children: "Ellipse" })}
      {ElementOption({ type: "arrow", children: "Arrow" })}
      {ElementOption({ type: "text", children: "Text" })}
      {ElementOption({ type: "selection", children: "Selection" })}
      <canvas
        id="canvas"
        width={window.innerWidth}
        height={window.innerHeight}
        onMouseDown={e => {
          const x = e.clientX - e.target.offsetLeft;
          const y = e.clientY - e.target.offsetTop;
          const element = newElement(elementType, x, y);

          if (elementType === "text") {
            element.text = prompt("What text do you want?");
            element.font = "20px Virgil";
            const font = context.font;
            context.font = element.font;
            element.measure = context.measureText(element.text);
            context.font = font;
            const height =
              element.measure.actualBoundingBoxAscent +
              element.measure.actualBoundingBoxDescent;
            // Center the text
            element.x -= element.measure.width / 2;
            element.y -= element.measure.actualBoundingBoxAscent;
            element.width = element.measure.width;
            element.height = height;
          }

          generateShape(element);
          elements.push(element);
          if (elementType === "text") {
            setDraggingElement(null);
          } else {
            setDraggingElement(element);
          }
          drawScene();
        }}
        onMouseUp={e => {
          setDraggingElement(null);
          if (elementType === "selection") {
            // Remove actual selection element
            elements.pop();
            setSelection(draggingElement);
          }
          drawScene();
        }}
        onMouseMove={e => {
          if (!draggingElement) return;
          let width = e.clientX - e.target.offsetLeft - draggingElement.x;
          let height = e.clientY - e.target.offsetTop - draggingElement.y;
          draggingElement.width = width;
          // Make a perfect square or circle when shift is enabled
          draggingElement.height = e.shiftKey ? width : height;

          generateShape(draggingElement);

          if (elementType === "selection") {
            setSelection(draggingElement);
          }
          drawScene();
        }}
      />
    </div>
  );
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);
const canvas = document.getElementById("canvas");
const rc = rough.canvas(canvas);
const context = canvas.getContext("2d");

// Big hack to ensure that all the 1px lines are drawn at 1px instead of 2px
// https://stackoverflow.com/questions/13879322/drawing-a-1px-thick-line-in-canvas-creates-a-2px-thick-line/13879402#comment90766599_13879402
context.translate(0.5, 0.5);

function drawScene() {
  ReactDOM.render(<App />, rootElement);

  context.clearRect(0, 0, canvas.width, canvas.height);

  elements.forEach(element => {
    element.draw(rc, context);
    if (element.isSelected) {
      const margin = 4;

      const elementX1 = getElementAbsoluteX1(element);
      const elementX2 = getElementAbsoluteX2(element);
      const elementY1 = getElementAbsoluteY1(element);
      const elementY2 = getElementAbsoluteY2(element);
      const lineDash = context.getLineDash();
      context.setLineDash([8, 4]);
      context.strokeRect(
        elementX1 - margin,
        elementY1 - margin,
        elementX2 - elementX1 + margin * 2,
        elementY2 - elementY1 + margin * 2
      );
      context.setLineDash(lineDash);
    }
  });
}

drawScene();