Fabric.js - how to save canvas on server with custom attributes

Wow. Am I missing something here?

I've done this plenty of times and it doesn't need any fancy subclassing.

The docs cover it: http://fabricjs.com/docs/fabric.Canvas.html#toJSON

Just include an array of property names as strings in your call to toJSON().

Eg

canvas.toJSON(['wizard','hobbit']);

Hopefully.... for bonus points you can add a reviver function which will rehydrate your custom attributes.

Again this is covered in the docs and has an example.

Good question.

If you're adding custom properties to objects, those objects are likely "special" in some way. It seems like subclassing them would be a reasonable solution.

For example, here's how we would subclass a fabric.Image into a named image. Those image objects could then have names like "Gandalf" or "Samwise".

fabric.NamedImage = fabric.util.createClass(fabric.Image, {

  type: 'named-image',

  initialize: function(element, options) {
    this.callSuper('initialize', element, options);
    options && this.set('name', options.name);
  },

  toObject: function() {
    return fabric.util.object.extend(this.callSuper('toObject'), { name: this.name });
  }
});

First, we give these objects a type. This type is used by loadFromJSON to automatically invoke fabric.<type>.fromObject method. In this case it would be fabric.NamedImage.fromObject.

Then we overwrite initialize (constructor) instance method, to also set "name" property when initializing an object (if that property is given).

Then we overwrite toObject instance method to include "name" in returned object (this is a cornerstone of object serialization in fabric).

Finally, we'll also need to implement that fabric.NamedImage.fromObject that I mentioned earlier, so that loadFromJSON would know which method to invoke during JSON parsing:

fabric.NamedImage.fromObject = function(object, callback) {
  fabric.util.loadImage(object.src, function(img) {
    callback && callback(new fabric.NamedImage(img, object));
  });
};

We're loading an image here (from "object.src"), then creating an instance of fabric.NamedImage out of it. Note how at that point, constructor will already take care of "name" setting, since we overwrote "initialize" method earlier.

And we'll also need to specify that fabric.NamedImage is an asynchronous "class", meanining that its fromObject does not return an instance, but passes it to a callback:

fabric.NamedImage.async = true;

And now we can try this all out:

// create image element
var img = document.createElement('img');
img.src = 'https://www.google.com/images/srpr/logo3w.png';

// create an instance of named image
var namedImg = new fabric.NamedImage(img, { name: 'foobar' });

// add it to canvas
canvas.add(namedImg);

// save json
var json = JSON.stringify(canvas);

// clear canvas
canvas.clear();

// and load everything from the same json
canvas.loadFromJSON(json, function() {

  // making sure to render canvas at the end
  canvas.renderAll();

  // and checking if object's "name" is preserved
  console.log(canvas.item(0).name);
});