无法读取自定义类的属性'async'错误

Question

我拼命想解决这个问题，我找到的所有信息都没有提供解决方案。我看过StackOverflow上的相关问题，虽然我看到了代码，但我无法解决这个问题。

我创建了一个失败的JSFiddle。这是代码:

var canvas = window._canvas = new fabric.Canvas('c');
fabric.imagePlaceholder = fabric.util.createClass(fabric.Text, {
    type: 'imagePlaceholder',
    initialize: function(text, options) {
        options || (options = { });
        console.log('imagePlaceholder', text, options);
        this.callSuper('initialize', text, options);
        this.set('imageId', options.imageId || '');
        this.set('text', 'Loading ' + text || 'Loading...');
        this.set('type', 'imagePlaceholder');
    },
    toObject: function() {
        return fabric.util.object.extend(this.callSuper('toObject'), {
            imageId: this.get('imageId'),
            type: this.get('type'),
        });
    },
    _render: function(ctx) {
    this.callSuper('_render', ctx);
    ctx.font = '20px Arial';
    ctx.fillStyle = '#333';
    ctx.fillText(this.label, -this.width/2, -this.height/2 + 20);
    },
});
fabric.imagePlaceholder.fromObject = function (object, callback) {
    var _enlivenedObjects;
    fabric.util.enlivenedObjects(object.objects, function (enlivenedObjects) {
        delete object.objects;
        _enlivenedObjects = enlivenedObjects;
    });
    return new fabric.imagePlaceholder(_enlivenedObjects, object);
};
// Added or not it doesn't work :(
fabric.imagePlaceholder.async = true;
newObject = new fabric.imagePlaceholder('cat_image.jpg', {imageId:Date.now()});
canvas.add(newObject);
var json = window._json = canvas.toJSON();
console.log(json);
//Comment out the following to see the 'undefined' object being created..
//canvas.clear();
//canvas.loadFromJSON(json);

本质上，我正在创建一个自定义类，这样我就可以将保存的状态从服务器加载为JSON，将其加载到画布中，然后循环遍历对象，找到占位符并将它们替换为来自安全图像服务的图像。问题是，我似乎无法克服异步问题。什么好主意吗?

提前感谢，

乔纳森

Answer 1

几个小时后，我也遇到了同样的问题，结果发现

type: 'imagePlaceholder'

被转换成CamelCase，以便在读取JSON数据时访问对象，因此

fabric.imagePlaceholder = ...

必须是

fabric.ImagePlaceholder = ...

为了让它工作