主要的技术点：

        1、利用Canvas的lineTo方法和stroke方式绘制连续填充的线；

        2、利用canvasContext的strokeStyle更改画笔颜色；

        3、利用canvasContext的lineWidth更改画笔宽度；

一、支持的功能：

        1、调整画笔的颜色；

        2、调整画笔的宽度；

二、效果图：

 

三、React组件代码(注意我用的是tsx的文件格式，如果用jsx请移除数据类型声明即可)

import React from "react";
import styles from './index.less';

export default class CanvasComponent extends React.Component {
  canvasRef: any;
  paintColors = ['#000000', '#999999', '#CC66FF', '#FF0000', '#FF9900', '#FFFF00'];
  paintWidths = [1, 2, 3, 4, 5, 6];
  canvasContext: any;
  // 颜色面板
  paintColorPanel = this.paintColors.map((element, index) => {
    return <span className={styles.bk} style={{backgroundColor: element}}
                 onClick={() => {
                   this.onColorSpanClick(index)
                 }
                 }/>
  });
  // 画笔宽度面板
  paintWidthPanel = this.paintWidths.map((element, index) => {
    return <span className={styles.bk}
                 onClick={() => {
                   this.onWidthSpanClick(index)
                 }
                 }>{element}</span>
  });

  constructor(props: {} | Readonly<{}>) {
    super(props);
    this.canvasRef = React.createRef();
  }

  /**
   * 选择画笔颜色
   * @param index
   */
  onColorSpanClick = (index: number) => {
    this.canvasContext.strokeStyle = this.paintColors[index];
  }

  /**
   * 选择画笔宽度
   * @param index
   */
  onWidthSpanClick = (index: number) => {
    this.canvasContext.lineWidth = this.paintWidths[index];
  }


  /**
   * 初始化画笔和事件监听
   */
  initPainter = () => {
    let canvas = this.canvasRef.current;
    this.canvasContext = canvas.getContext('2d');
    let x = 0;
    let y = 0;
    this.canvasContext.lineWidth = 2;
    this.canvasContext.strokeStyle = '#000000';

    //鼠标按下事件
    canvas.addEventListener('mousedown', (e: MouseEvent) => {
      e.preventDefault();
      x = e.offsetX;
      y = e.offsetY;
      this.canvasContext.beginPath();
      this.canvasContext.moveTo(x, y);
      //鼠标移动事件
      let onm ouseMoveEvent = (e: MouseEvent) => {
        let newX = e.offsetX;
        let newY = e.offsetY;
        this.canvasContext.lineTo(newX, newY);
        this.canvasContext.stroke();
        x = newX;
        y = newY;
      }

      canvas.addEventListener('mousemove', onm ouseMoveEvent);
      canvas.addEventListener('mouseup', () => {
        canvas.removeEventListener('mousemove', onm ouseMoveEvent);
      })
    })
  }

  componentDidMount() {
    this.initPainter();
  }

  render() {
    return <>
      画笔颜色:
      <div id={styles.bk}>{this.paintColorPanel}</div>
      画笔宽度:
      <div id={styles.bk}>{this.paintWidthPanel}</div>
      <canvas ref={this.canvasRef} id={'cav'} width={800} height={600} style={{background: '#1677ff'}}/>
    </>;
  }
}

四、less样式代码

body div {
  margin: 0;
  padding: 0;
  text-align: center;
}

#cav {
  border: 2px solid black;
  border-radius: 4px;
  box-shadow: 0 0 10px black;
  -webkit-box-shadow: 0 0 10px black;
  -moz-box-shadow: 0 0 10px black;
}
#bk{
  margin: 10px auto;
  width: 800px;
  height: 36px;
}

.bk{
  text-align: center;
  width: 20px;
  height: 20px;
  margin: 12px;
  display: inline-block;
  border: 1px solid grey;
}