问如何在onchange事件上获取jQuery下拉值

EN

(function($) {
            $.widget("ui.combobox", {
                _create: function() {
                    var self = this,
                    select = this.element.hide(),
                    selected = select.children(":selected"),
                    value = selected.val() ? selected.text() : "";
                    var input = this.input = $("<input>").insertAfter(select).val(value).autocomplete({
                        delay: 0,
                        minLength: 0,
                        source: function(request, response) {
                            var matcher = new RegExp($.ui.autocomplete.escapeRegex(request.term), "i");
                            response(select.children("option").map(function() {
                                var text = $(this).text();
                                if (this.value && (!request.term || matcher.test(text)))
                                    return {
                                    label: text.replace(new RegExp("(?![^&;]+;)(?!<[^<>]*)(" + $.ui.autocomplete.escapeRegex(request.term) + ")(?![^<>]*>)(?![^&;]+;)", "gi"), "<strong>$1</strong>"),
                                    value: text,
                                    option: this
                                };
                            }));
                        },
                        select: function(event, ui) {
                            ui.item.option.selected = true;
                            self._trigger("selected", event, {
                                item: ui.item.option
                            });
                            select.trigger("change");
                        },
                        change: function(event, ui) {
                            if (!ui.item) {
                                var matcher = new RegExp("^" + $.ui.autocomplete.escapeRegex($(this).val()) + "$", "i"),
                                valid = false;
                                select.children("option").each(function() {
                                    if ($(this).text().match(matcher)) {
                                        this.selected = valid = true;
                                        return false;
                                    }
                                });
                                if (!valid) {
                                    // remove invalid value, as it didn't match anything
                                    $(this).val("");
                                    select.val("");
                                    input.data("autocomplete").term = "";
                                    return false;
                                }
                            }
                        }
                    }).addClass("ui-widget ui-widget-content ui-corner-left");

                    input.data("autocomplete")._renderItem = function(ul, item) {
                        return $("<li></li>").data("item.autocomplete", item).append("<a>" + item.label + "</a>").appendTo(ul);
                    };

                    this.button = $("<button type='button'>&nbsp;</button>").attr("tabIndex", -1).attr("title", "Show All Items").insertAfter(input).button({
                        icons: {
                            primary: "ui-icon-triangle-1-s"
                        },
                        text: false
                    }).removeClass("ui-corner-all").addClass("ui-corner-right ui-button-icon").click(function() {
                        // close if already visible
                        if (input.autocomplete("widget").is(":visible")) {
                            input.autocomplete("close");
                            return;
                        }

                        // pass empty string as value to search for, displaying all results
                        input.autocomplete("search", "");
                        input.focus();
                    });
                },

                destroy: function() {
                    this.input.remove();
                    this.button.remove();
                    this.element.show();
                    $.Widget.prototype.destroy.call(this);
                }
            });
        })(jQuery);

        $(function() {
            /*start point value*/
            $("#pick").combobox({
                change: function() {
                    alert("changed");
                }
            });
            $("#toggle").click(function() {
                $("#pick").toggle();
            });
            $("#pick").change(function() {
                var start = this.value;
            });

            /*end point value*/
            $("#drop").combobox({
                change: function() {
                    alert("changed");
                }
            });
            $("#toggle").click(function() {
                $("#drop").toggle();
            });
            $("#drop").change(function() {
                var end = this.value;
            });
        });

<div class="ui-widget">
        <select id="pick">
            <option value="">Select one...</option>
            <option value="1">1</option>
            <option value="2">2</option>
            <option value="3">3</option>
            <option value="4">4</option>
        </select>

        <select id="drop">
            <option value="">Select one...</option>
            <option value="11">11</option>
            <option value="12">12</option>
            <option value="13">13</option>
            <option value="14">14</option>
        </select>
    </div>