javascript - 使用 fabric.js 在 Canvas 中加载 JSON

Question

我正在尝试将数据保存在 JSON 中并使用 fabric.js 将其加载回 Canvas 中我不断收到以下简单代码的错误:

canvas.add(new fabric.Rect({ width: 50, height: 50, fill: 'red', top: 100, left: 100 }));
var c = canvas.toJSON();
canvas.clear();
canvas.loadFromJSON(c);

我得到:

SyntaxError: JSON.parse: unexpected character
[Break On This Error] var Cufon=(function(){var k=function()....Image.fromElement.async=true})(this);

当我使用我自己的 JSON 时，它验证得很好，但是当我使用 fabric 的方法 canvas.toJSON() 输出的任何内容时，我仍然会遇到错误。任何人都会有从 fabric.js 中以前的 Canvas 状态保存的加载回空 Canvas 数据的工作示例？将不胜感激!

Answer 1

fabric.Canvas#toJSON 实际上返回一个对象，而不是 JSON 字符串。

抱歉，如果这令人困惑。

之所以这样工作是因为 JSON.stringify。 JSON.stringify 的特点是它支持自定义序列化；您需要做的就是传递一个具有 toJSON 方法的对象。这正是我在 fabric 中所做的 — fabric.Canvas 有 toJSON 方法，它本质上是 toObject 方法的别名。

这允许我们通过做一些简单的事情来序列化 Canvas :

JSON.stringify(canvas);

.. 其中 canvas 是对 fabric.Canvas 实例的引用。

toObject 和toDatalessObject(以及toJSON 和toDatalessJSON)之间的区别是 toDatalessObject 返回“url”而不是实际路径数据。这样做是为了节省具有大尺寸形状的 Canvas 表示的尺寸。如果加载大型 SVG 形状，其路径数据可能会产生数百万个字符串。如果您稍后需要序列化此数据(例如，为了保存目的)，将路径数据替换为 svg 形状的 URL 更有意义。想象一下必须来回上传/下载如此巨大的字符串到服务器。

所以，而不是这个:

{ "angle" : 3,
  "fill" : "#00274D",
  "flipX" : false,
  "flipY" : false,
  "height" : 115,
  "left" : 353,
  "opacity" : 1,
  "overlayFill" : null,
  "path" : [ [ "M",
        67.390000000000001,
        22.390000000000001
      ],
      [ "c",
        2.5899999999999999,
        -0.42999999999999999,
        5.1100000000000003,
        1.4399999999999999,
        5.54,
        4.1799999999999997
      ],
      [ "c",
        0.42999999999999999,
        2.6600000000000001,
        -1.3,
        5.2599999999999998,
        -3.8900000000000001,
        5.6900000000000004
      ],
      [ "c",
        -1.8,
        0.28999999999999998,
        -3.6000000000000001,
        -0.57999999999999996,
        -4.6100000000000003,
        -2.02
      ],
      [ "L",
        44.5,
        34.560000000000002
      ],
      [ "l",
        10.869999999999999,
        59.619999999999997
      ],
      [ "c",
        17.420000000000002,
        -4.46,
        26.059999999999999,
        -14.18,
        27.5,
        -29.02
      ],
      [ "l",
        -10.01,
        -0.71999999999999997
      ],
      [ "L",
        88.700000000000003,
        51.909999999999997
      ],
      [ "l",
        9.4299999999999997,
        21.239999999999998
      ],
      [ "c",
        -3.3799999999999999,
        -1.95,
        -5.9000000000000004,
        -5.1100000000000003,
        -9.2899999999999991,
        -7.0599999999999996
      ],
      [ "c",
        -0.28999999999999998,
        25.059999999999999,
        -27.140000000000001,
        32.759999999999998,
        -33.770000000000003,
        47.950000000000003
      ],
      [ "C",
        44.420000000000002,
        100.08,
        26.5,
        114.77,
        6.9100000000000001,
        82.799999999999997
      ],
      [ "L",
        0,
        92.450000000000003
      ],
      [ "l",
        1.51,
        -21.600000000000001
      ],
      [ "l",
        19.66,
        4.6799999999999997
      ],
      [ "l",
        -9.4299999999999997,
        3.6699999999999999
      ],
      [ "c",
        7.4900000000000002,
        11.59,
        17.57,
        19.870000000000001,
        36.43,
        16.420000000000002
      ],
      [ "L",
        36.219999999999999,
        36.57
      ],
      [ "l",
        -18.649999999999999,
        2.3799999999999999
      ],
      [ "c",
        -0.14000000000000001,
        2.1600000000000001,
        -1.73,
        4.0300000000000002,
        -3.8900000000000001,
        4.3899999999999997
      ],
      [ "c",
        -2.5899999999999999,
        0.42999999999999999,
        -5.04,
        -1.4399999999999999,
        -5.54,
        -4.0999999999999996
      ],
      [ "c",
        -0.42999999999999999,
        -2.7400000000000002,
        1.3,
        -5.2599999999999998,
        3.8900000000000001,
        -5.6900000000000004
      ],
      [ "c",
        1.9399999999999999,
        -0.35999999999999999,
        3.8900000000000001,
        0.65000000000000002,
        4.9000000000000004,
        2.2999999999999998
      ],
      [ "l",
        17.93,
        -4.8200000000000003
      ],
      [ "l",
        -1.3700000000000001,
        -6.8399999999999999
      ],
      [ "c",
        -4.8200000000000003,
        -0.79000000000000004,
        -8.9299999999999997,
        -4.75,
        -9.7899999999999991,
        -10.08
      ],
      [ "c",
        -1.1499999999999999,
        -6.6200000000000001,
        3.1000000000000001,
        -12.890000000000001,
        9.4299999999999997,
        -13.970000000000001
      ],
      [ "c",
        6.4100000000000001,
        -1.01,
        12.460000000000001,
        3.46,
        13.539999999999999,
        10.08
      ],
      [ "c",
        0.85999999999999999,
        5.1799999999999997,
        -1.5800000000000001,
        10.15,
        -5.6900000000000004,
        12.6
      ],
      [ "l",
        1.8700000000000001,
        6.1200000000000001
      ],
      [ "l",
        20.739999999999998,
        -2.8799999999999999
      ],
      [ "C",
        64.010000000000005,
        24.260000000000002,
        65.519999999999996,
        22.75,
        67.390000000000001,
        22.390000000000001
      ],
      [ "L",
        67.390000000000001,
        22.390000000000001
      ],
      [ "z" ],
      [ "M",
        33.909999999999997,
        5.1799999999999997
      ],
      [ "c",
        -3.46,
        0.57999999999999996,
        -5.7599999999999998,
        3.96,
        -5.1100000000000003,
        7.5599999999999996
      ],
      [ "c",
        0.57999999999999996,
        3.6000000000000001,
        3.8900000000000001,
        6.0499999999999998,
        7.2699999999999996,
        5.4699999999999998
      ],
      [ "c",
        3.46,
        -0.57999999999999996,
        5.7599999999999998,
        -3.96,
        5.1799999999999997,
        -7.5599999999999996
      ],
      [ "C",
        40.609999999999999,
        7.0499999999999998,
        37.369999999999997,
        4.6100000000000003,
        33.909999999999997,
        5.1799999999999997
      ],
      [ "z" ]
    ],
  "scaleX" : 3.0299999999999998,
  "scaleY" : 3.0299999999999998,
  "selectable" : true,
  "stroke" : null,
  "strokeWidth" : 1,
  "top" : 220,
  "type" : "path",
  "width" : 99
}

你会得到这个:

{ "angle" : 3,
  "fill" : "#00274D",
  "flipX" : false,
  "flipY" : false,
  "height" : 115,
  "left" : 353,
  "opacity" : 1,
  "overlayFill" : null,
  "path" : "http://example.com",
  "scaleX" : 3.0299999999999998,
  "scaleY" : 3.0299999999999998,
  "selectable" : true,
  "stroke" : null,
  "strokeWidth" : 1,
  "top" : 220,
  "type" : "path",
  "width" : 99
}

注意该数据有多大，以及通过将路径 block 替换为 url 而变得相对较小。

这是一个非常简单的形状的表示。

这里唯一的要求是在调用 toDatalessObject/toDatalessJSON 之前使用 setSourcePath 方法设置对象的“sourcePath”(“sourcePath”是内部复制到“路径”)。

希望这能让事情变得更清楚一些。