Fabric.js是一个可以简化Canvas程序编写的库，通过对象的方式提供画布能力，支持创建、填充图形（包括图片、文字、规则图形和复杂路径组成图形），组合图形，设置动画及用户交互，生成JSON、SVG数据等。
Fabric.js使用指南
Fabric.js 是一个功能强大且易于使用的 JavaScript HTML5 Canvas 库，它提供了一组丰富的 API，帮助开发者创建、操作和管理 Canvas 元素，使图形处理更加简单和直观，以下是关于如何使用 Fabric.js 的详细指南，包括基本用法、进阶功能、自定义对象与扩展功能，以及实例项目和实战经验。
一、基本用法
1. 安装与引入
Fabric.js 可以通过多种方式引入项目中：
通过 npm 安装：
npm install fabric --save
通过 CDN 引入：
<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/4.6.0/fabric.min.js"></script>
2. 创建 Canvas
在 HTML 文件中创建一个<canvas> 元素，然后通过 Fabric.js 对其进行初始化：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Fabric.js Example</title>
<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/4.6.0/fabric.min.js"></script>
</head>
<body>
<canvas id="canvas" width="800" height="600"></canvas>
<script src="app.js"></script>
</body>
</html>在app.js 中初始化 Canvas：
const canvas = new fabric.Canvas('canvas');3. 绘制基本图形
Fabric.js 支持多种基本图形的绘制，如矩形、圆形、线条和文本等。
矩形：
const rect = new fabric.Rect({
left: 100,
top: 100,
fill: 'red',
width: 200,
height: 100
});
canvas.add(rect);圆形：
const circle = new fabric.Circle({
left: 300,
top: 100,
fill: 'blue',
radius: 50
});
canvas.add(circle);线条：
const line = new fabric.Line([50, 50, 200, 200], {
left: 100,
top: 200,
stroke: 'green'
});
canvas.add(line);文本：
const text = new fabric.Text('Hello Fabric.js', {
left: 100,
top: 300,
fontSize: 30,
fill: 'black'
});
canvas.add(text);4. 图像加载与操作
Fabric.js 还支持加载和操作图像：
fabric.Image.fromURL('path/to/image.jpg', function(img) {
img.set({
left: 400,
top: 200,
scaleX: 0.5,
scaleY: 0.5
});
canvas.add(img);
});5. 事件处理
Fabric.js 提供了丰富的事件处理机制，可以监听对象的各种事件，如拖动、缩放、旋转等：
rect.on('selected', function() {
console.log('矩形被选中');
});
canvas.on('mouse:down', function(options) {
if (options.target) {
console.log('点击了对象:', options.target.type);
}
});二、进阶功能
1. 组合（Grouping）与层次管理
Fabric.js 支持将多个对象进行分组，以便于一起操作：
const group = new fabric.Group([rect, circle, text], {
left: 150,
top: 150
});
canvas.add(group);2. 动画与滤镜效果
Fabric.js 提供了动画和滤镜效果的支持，可以让图形对象更具交互性和视觉效果：
rect.animate('left', '+=100', {
onChange: canvas.renderAll.bind(canvas),
duration: 500
});3. 序列化与反序列化
Canvas 的内容可以轻松导出为图像文件或 JSON 数据：
导出为图像：
const dataURL = canvas.toDataURL({ format: 'png', quality: 0.8 });
// 将 dataURL 用于图像下载或显示导出为 JSON：
const json = canvas.toJSON(); // 将 JSON 数据保存或传递到服务器
三、自定义对象与扩展功能
Fabric.js 允许你创建自定义对象，并且可以通过子类化来扩展其功能，如果你需要在项目中实现特定的图形或交互效果，这将非常有用，你可以创建一个自定义的多边形类：
fabric.Polygon = fabric.util.createClass(fabric.Shape, {
type: 'polygon',
initialize: function(points, options) {
options || (options = {});
this.callSuper('initialize', points, options);
},
toObject: function() {
return fabric.util.object.extend(this.callSuper('toObject'), {});
},
_render: function(ctx) {
this._renderPaintCall(ctx, this.calculateBoundaryPoints());
}
});四、实例项目与实战经验
实践是学习的重要部分，通过实际项目来应用所学知识，可以帮助你更好地理解和掌握 Fabric.js，以下是一个简单的实例项目，展示如何使用 Fabric.js 创建一个交互式的绘图板：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Fabric.js Interactive Drawing Board</title>
<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/4.6.0/fabric.min.js"></script>
<style>
canvas { border: 1px solid #ccc; }
</style>
</head>
<body>
<canvas id="drawingBoard" width="800" height="600"></canvas>
<script>
const canvas = new fabric.Canvas('drawingBoard');
const isDown = false;
let origX, origY;
const pointer = canvas.getPointer((pointerEvent) => {
origX = pointerEvent.x;
origY = pointerEvent.y;
});
const pointerMove = (pointerEvent) => {
if (!isDown) return;
const pointerActive = canvas.getPointer(pointerEvent);
if (pointerActive === null) return;
if (!canvas.containsPoint(pointerActive)) return;
canvas.fxClear(); // Clear the drawing area when moving the mouse with the button down.
canvas.freeDrawingBrush.color = getComputedColor(canvas.freeDrawingBrush.color, pointerEvent); // Change color dynamically.
canvas.freeDrawingBrush.lineCap = canvas.freeDrawingBrush.lineCap || 'round'; // Round off the stroke endpoints.
canvas.freeDrawingBrush.lineJoin = canvas.freeDrawingBrush.lineJoin || 'round'; // Round off the stroke corners and miters.
canvas.freeDrawingBrush.shadow = new fabric.Shadow({ // Drop shadow using HTML5 canvas dynamically.
blur: 5, offsetX: -5, offsetY: 5, affectStroke: true, color: 'rgba(0,0,0,0.4)'
});
canvas.freeDrawingBrush.width = canvas.freeDrawingBrush.width || pointerEvent.x origX; // Set the width of the brush (useful for creating circles and lines). The default width is always set at the start point of free draw mode. It can be changed later by dragging outwards from original position which will increase its size accordingly as well as inward dragging that decreases its thickness too !important note about this feature: If you want more control over how big/small your strokes are then consider implementing something like an "eraser" tool instead since there isn't much room left within these few lines of code here anyways due to space constraints... etcetera ad nauseam :-)))))); // End parentheses!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!