source: pro-violet-viettel/sourcecode/assets/js/jquery/jquery.alsodrag.js @ 724

$.ui.plugin.add("draggable", "alsoDrag", {
        start: function() {
                var that = $(this).data("ui-draggable"),
                o = that.options,
                _store = function (exp) {
                        $(exp).each(function() {
                                var el = $(this);
                                el.data("ui-draggable-alsoDrag", {
                                        top: parseInt(el.css("top"), 10),
                                        left: parseInt(el.css("left"), 10)
                                });
                        });
                };

                if (typeof(o.alsoDrag) === "object" && !o.alsoDrag.parentNode) {
                        if (o.alsoDrag.length) { o.alsoDrag = o.alsoDrag[0]; _store(o.alsoDrag); }
                        else { $.each(o.alsoDrag, function (exp) { _store(exp); }); }
                }else{
                        _store(o.alsoDrag);
                }
        },
        drag: function () {
                var that = $(this).data("ui-draggable"),
                o = that.options,
                os = that.originalSize,
                op = that.originalPosition,
                delta = {
                        top: (that.position.top - op.top) || 0, 
                        left: (that.position.left - op.left) || 0
                },

                _alsoDrag = function (exp, c) {
                        $(exp).each(function() {
                                var el = $(this), start = $(this).data("ui-draggable-alsoDrag"), style = {},
                                css = ["top", "left"];

                                $.each(css, function (i, prop) {
                                        var sum = (start[prop]||0) + (delta[prop]||0);
                                        style[prop] = sum || null;
                                });

                                el.css(style);
                        });
                };

                if (typeof(o.alsoDrag) === "object" && !o.alsoDrag.nodeType) {
                        $.each(o.alsoDrag, function (exp, c) { _alsoDrag(exp, c); });
                }else{
                        _alsoDrag(o.alsoDrag);
                }
        },
        stop: function() {
                $(this).removeData("draggable-alsoDrag");
        }
});