fabricjs TypeError：this.canvas未定义

Question

我已定义var canvas = new fabric.Canvas('tcanvas')，但在创建函数期间，我收到此错误：TypeError: this.canvas is undefined。这是我的代码：

 if (mood === 'hline') {
   canvas.freeDrawingBrush = vLinePatternBrush;
   }

       var vLinePatternBrush = new fabric.PatternBrush(canvas);

        vLinePatternBrush.getPatternSrc = function() {

          var patternCanvas = canvas;
          patternCanvas.width = patternCanvas.height = 10;
          var ctx = patternCanvas.getContext('2d');

          ctx.strokeStyle = $("#drawing-color").val();
          ctx.lineWidth = 5;
          ctx.beginPath();
          ctx.moveTo(0, 5);
          ctx.lineTo(10, 5);
          ctx.closePath();
          ctx.stroke();

          return patternCanvas;
        };

我错在哪里？