Part 7：对 Text 类进行子分类以生成位图文本

本教程是针对 stackoveflow 问题而创建的，在该问题中，用户需要在 fabricJS 中使用位图而不是字体的文本对象。该代码示例包含启动子类所需了解的所有内容。当您无法通过其他 API（事件，自定义控件）获得所需的功能时，请考虑子类化是一种最后的资源。

var canvas = new fabric.Canvas(id, {
    width: 500,
    height: 500,
});
const charMap = ' !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}'
.split('').reduce((acc, char, index) => {
  acc[char] = index + 31;
  return acc;
}, {});
class BitmapText extends fabric.Textbox {
    static type = 'BitmapText';
    static ownDefaults = {
        charWidth: 6,
        charHeight: 9,
        fontSize: 9,
        bitmap: '/article_assets/font.png',
        readyToRender: false,
    };
    static getDefaults() {
      return { ...super.getDefaults(), ...BitmapText.ownDefaults };
    }
    constructor(text, options) {
      super(text, options);
      var image = fabric.getEnv().document.createElement('img');
      image.onload = (function() {
        var canvas = fabric.getEnv().document.createElement('canvas');
        canvas.width = image.width;
        canvas.height = image.height;
        canvas.getContext('2d').drawImage(image, 0, 0);
        this.bitmapResource = canvas;
        this.readyToRender = true;
        this.dirty = true;
        if (this.canvas) this.canvas.requestRenderAll();
      }).bind(this);
      image.src = this.bitmap;
    }
    // override: make it return a fixed box 6x9
    _measureChar(_char, charStyle, previousChar, prevCharStyle) {
      return { width: 6, kernedWidth: 6 };
    }
    // ovveride, just draw the bitmpa we have.
    _renderChar(method, ctx, lineIndex, charIndex, _char, left, top) {
      if (!this.readyToRender) return;
      var res = this.bitmapResource;
      _char.split('').forEach((singleChar) => {
        ctx.drawImage(res, charMap[singleChar] * 6, 0, 5, 9, left, top - 6, 5, 9);
        ctx.translate(6, 0);
      });
    }
  }
  fabric.classRegistry.setClass(BitmapText);
  // in order for your subclass to work with loadFromJSON you have to define this Static
  // methd `fromObject`.
  var text = new BitmapText('Hello i am a bitmap text', { scaleX: 5, scaleY: 5 });    
  canvas.add(text);