Link property back to the entity and property description

[andrew-veresov]

This is to allow creating bindings which bound to a property and use entity to which property belongs to and/or property metadata internally.

For example next binding adds isChanged computed to the property and use created computed observable to add/remove 'changed' class to the element:

(function (ko) {

    ko.bindingHandlers['breeze'] = {
        'after': ['value', 'attr'],
        'init': function (element, valueAccessor, allBindings, viewModel, bindingContext) {
            var options = valueAccessor();
            if (options != undefined) {
                if (options.trackChanges != undefined && ko.isObservable(options.trackChanges) && options.trackChanges.parentEntity != undefined) {
                    var entity = options.trackChanges.parentEntity;
                    var propName = options.trackChanges.propertyDescriptor.name;

                    options.trackChanges.isChanged = ko.computed(function () {
                        return entity.entityAspect.originalValues.hasOwnProperty(propName)
                            & entity.entityAspect.originalValues[propName] != options.trackChanges();
                    });

                    options.trackChanges.isChanged.subscribe(function (isChanged) {
                        if (isChanged) {
                            $(element).addClass('changed');
                        } else {
                            $(element).removeClass('changed');
                        }
                    })
                }
                //other bindings...
                if (options.validate != undefined && ko.isObservable(options.validate) && options.validate.parentEntity != undefined) {
                    var propertyName = options.validate.propertyDescriptor.name;
                    ko.bindingHandlers.breezevalidation.init(element, function() { return { properties: [propertyName] }; }, allBindings, viewModel, bindingContext);
                }
            }
        }
    };

})(window.ko || require(['ko']));

Using above binding we can enable per property change tracking only for specific properties which need it and avoid spawning isChanged computed for all properties.