MAGETWO-38867: Create mixin for declarative widget description

Author: Ihor Melnychenko

lib/web/mage/apply/main.js

@@ -37,24 +37,25 @@ define([
      *
      * @param {Object} mixinList - object contains mixins list.
      * @param {Object} itemContainer - object with component config and DOM element link.
+     * @return {Promise}
      */
     function initMixins(mixinList, itemContainer){
-        var configStack = {},
-            element = itemContainer.el;
+        var configStack,
+            element = itemContainer.el,
+            result = $.Deferred();
 
         _.each(mixinList, function (mixinArray, componentName) {
             configStack = itemContainer.data[componentName];
             require(mixinArray, function(){
                 for (var i = 0, len = arguments.length; i < len; i++){
-                    configStack = arguments[i](configStack, element);
+                    $.extend(true, configStack, arguments[i](configStack, element));
                 }
 
+                result.resolve(itemContainer);
             });
-            itemContainer.data[componentName] = configStack;
         });
-        delete itemContainer.data.mixins;
 
-        return itemContainer;
+        return result;
     }
     /**
      * Parses elements 'data-mage-init' attribute as a valid JSON data.
@@ -90,9 +91,17 @@ define([
                 .concat(virtuals)
                 .forEach(function (itemContainer) {
                     var mixins = itemContainer.data.mixins;
+                    delete itemContainer.data.mixins;
 
                     if (mixins) {
-                        $.when(itemContainer = initMixins(mixins, itemContainer)).then(_.each(itemContainer.data, init.bind(null, itemContainer.el)));
+                        $.when(
+                            initMixins(mixins, itemContainer)
+                        ).then(function (mergedItemContainer) {
+                            _.each(
+                                mergedItemContainer.data,
+                                init.bind(null, mergedItemContainer.el)
+                            )
+                        });
                     } else {
                         _.each(itemContainer.data, init.bind(null, itemContainer.el));
                     }