你可以使用fabric.js中的`canvas`对象来实现这个功能。具体步骤如下：
1. 定义网格线的宽度和高度，以及网格线的颜色。
```javascript
var gridWidth = 20;
var gridHeight = 20;
var gridColor = '#ccc';
```
2. 获取画布对象的宽度和高度。
```javascript
var canvasWidth = canvas.getWidth();
var canvasHeight = canvas.getHeight();
```
3. 计算出可以画网格线的区域的宽度和高度。
```javascript
var availableWidth = canvasWidth % gridWidth;
var availableHeight = canvasHeight % gridHeight;
```
4. 如果剩余区域的宽度不足以画一条完整的网格线，则将网格线的宽度和高度互换。
```javascript
if (availableWidth < gridWidth) {
var temp = gridWidth;
gridWidth = gridHeight;
gridHeight = temp;
}
```
5. 如果剩余区域的高度仍然不足以画一条完整的网格线，则停止划线。
```javascript
if (availableHeight < gridHeight) {
return;
}
```
6. 在画布对象上画出网格线。
```javascript
for (var i = gridWidth; i < canvasWidth; i += gridWidth) {
var line = new fabric.Line([i, 0, i, canvasHeight], {
stroke: gridColor,
selectable: false
});
canvas.add(line);
}
for (var j = gridHeight; j < canvasHeight; j += gridHeight) {
var line = new fabric.Line([0, j, canvasWidth, j], {
stroke: gridColor,
selectable: false
});
canvas.add(line);
}
```
完整的代码如下：
```javascript
var gridWidth = 20;
var gridHeight = 20;
var gridColor = '#ccc';
var canvasWidth = canvas.getWidth();
var canvasHeight = canvas.getHeight();
var availableWidth = canvasWidth % gridWidth;
var availableHeight = canvasHeight % gridHeight;
if (availableWidth < gridWidth) {
var temp = grid
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