如何用fabric.js创建自定义画笔？

(function () {
  var customBrush = function () {
    this.width = 5; // 设置画笔宽度
    this.color = "#000000"; // 设置画笔颜色
  };

  customBrush.prototype = new fabric.PencilBrush(); // 继承fabric.PencilBrush，以便继承其功能

  customBrush.prototype._render = function (ctx) {
    this.canvas.contextTop.clearRect(
      this._points[this._lastRenderedIndex].x,
      this._points[this._lastRenderedIndex].y,
      this.canvas.width * this.canvas.scaleX,
      this.canvas.height * this.canvas.scaleY
    ); // 清除之前的绘制内容

    // 在这里添加自定义的绘图代码
    // ...
  };

  fabric.customBrush = fabric.util.createClass(fabric.PencilBrush, customBrush);
})();

var canvas = new fabric.Canvas("canvas");

// 设置画笔
canvas.isDrawingMode = true;
canvas.freeDrawingBrush = new fabric.customBrush();
canvas.freeDrawingBrush.width = 10; // 设置画笔宽度
canvas.freeDrawingBrush.color = "#FF0000"; // 设置画笔颜色

// 添加鼠标事件
canvas.on("mouse:down", function (options) {
  canvas.isDrawingMode = true;
});

canvas.on("mouse:up", function (options) {
  canvas.isDrawingMode = false;
});