HTML 画布保持在渲染线上，即使在我清除它之后

Question

我正在尝试使用 HTML Canvas 元素在 React 中构建一个绘图应用程序。

import React, { useEffect, useLayoutEffect, useRef, useState } from "react";
///*[EN ROUGH]*/import rough from 'roughjs';

///*[EN ROUGH]*/const generator = rough.generator();

const createElement = (x1, y1, x2, y2, type) => {
  return { x1, y1, x2, y2, type };
};

const drawElement = (context, element) => {
  if (element.type === "connect") {
     context.moveTo(element.x1, element.y1);
     context.lineTo(element.x2, element.y2);
     context.stroke();
    ///*[EN ROUGH]*/const line = generator.line(element.x1, element.y1, element.x2, element.y2);
    ///*[EN ROUGH]*/context.draw(line);

  }
};

function App() {
  const [elements, setElements] = useState([]);
  const [drawing, setDrawing] = useState(false);

  useLayoutEffect(() => {
    const canvas = document.getElementById("canvas");
    const context = canvas.getContext("2d");

    context.clearRect(0, 0, canvas.width, canvas.height);
    ///*[EN ROUGH]*/const context = rough.canvas(canvas);

    //redraw all the components on re-render
    elements.forEach((element) => {
      drawElement(context, element);
    });
  });

  const handleMouseDown = (event) => {
    setDrawing(true);
    const { clientX, clientY } = event;

    const element = createElement(
      clientX,
      clientY,
      clientX,
      clientY,
      "connect"
    );
    setElements((prevState) => [...prevState, element]);
  };

  const handleMouseMove = (event) => {
    if (!drawing) return;
    const { clientX, clientY } = event;
    //index of the last element that was selected
    const lastElemIndex = elements.length - 1;
    const { x1, y1 } = elements[lastElemIndex];
    const updatedElement = createElement(x1, y1, clientX, clientY, "connect");

    //overwrite the previous element with the new x2, y2 coordinates of mouse move
    const elementsCopy = [...elements];
    elementsCopy[lastElemIndex] = updatedElement;
    setElements(elementsCopy);
  };

  const handleMouseUp = () => {
    setDrawing(false);
  };

  return (
    <canvas
      id="canvas"
      width={window.innerWidth}
      height={window.innerHeight}
      onMouseDown={handleMouseDown}
      onMouseUp={handleMouseUp}
      onMouseMove={handleMouseMove}
    >
      Canvas
    </canvas>
  );
};

export default App;

当我运行此代码时，当我单击并拖动鼠标时，它会产生以下输出：

当我启用 RoughJS 来绘制而不是普通画布时，线条工作正常。可以通过删除注释来启用 RoughJS 代码

EN ROUGH

。我完全迷失了为什么会发生这种事。

Answer 1

尝试

context.beginPath()

开始一条新路径，当您使用 Rough 时，它可能正在做的事情：

  if (element.type === "connect") {
     context.beginPath(); // <-- Added

完整示例：

const { useEffect, useLayoutEffect, useRef, useState } = React;
///*[EN ROUGH]*/import rough from 'roughjs';

///*[EN ROUGH]*/const generator = rough.generator();

const createElement = (x1, y1, x2, y2, type) => {
  return { x1, y1, x2, y2, type };
};

const drawElement = (context, element) => {
  if (element.type === "connect") {
     context.beginPath(); // <-- Added
     context.moveTo(element.x1, element.y1);
     context.lineTo(element.x2, element.y2);
     context.stroke();
    ///*[EN ROUGH]*/const line = generator.line(element.x1, element.y1, element.x2, element.y2);
    ///*[EN ROUGH]*/context.draw(line);

  }
};

function App() {
  const [elements, setElements] = useState([]);
  const [drawing, setDrawing] = useState(false);

  useLayoutEffect(() => {
    const canvas = document.getElementById("canvas");
    const context = canvas.getContext("2d");
    context.clearRect(0, 0, canvas.width, canvas.height);
    ///*[EN ROUGH]*/const context = rough.canvas(canvas);

    //redraw all the components on re-render
    elements.forEach((element) => {
      drawElement(context, element);
    });
  });

  const handleMouseDown = (event) => {
    setDrawing(true);
    const { clientX, clientY } = event;

    const element = createElement(
      clientX,
      clientY,
      clientX,
      clientY,
      "connect"
    );
    setElements((prevState) => [...prevState, element]);
  };

  const handleMouseMove = (event) => {
    if (!drawing) return;
    const { clientX, clientY } = event;
    //index of the last element that was selected
    const lastElemIndex = elements.length - 1;
    const { x1, y1 } = elements[lastElemIndex];
    const updatedElement = createElement(x1, y1, clientX, clientY, "connect");

    //overwrite the previous element with the new x2, y2 coordinates of mouse move
    const elementsCopy = [...elements];
    elementsCopy[lastElemIndex] = updatedElement;
    setElements(elementsCopy);
  };

  const handleMouseUp = () => {
    setDrawing(false);
  };

  return (
    <canvas
      id="canvas"
      width={window.innerWidth}
      height={window.innerHeight}
      onMouseDown={handleMouseDown}
      onMouseUp={handleMouseUp}
      onMouseMove={handleMouseMove}
    >
      Canvas
    </canvas>
  );
};

ReactDOM.createRoot(document.querySelector("#app")).render(<App />);

<script crossorigin src="https://unpkg.com/react@18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@18/umd/react-dom.development.js"></script>
<div id="app"></div>