如何使用一个自定义的jQuery事件与角手表类型的接口;

// adds a custom jQuery event which gives the previous and current values of an input on change
(function ($) {
    // new event type tl_change
    jQuery.event.special.tl_change = {
        add: function (handleObj) {
            // use mousedown and touchstart so that if you stay focused on the
            // element and keep changing it, it continues to update the prev val
            $(this)
                .on('mousedown.tl_change touchstart.tl_change', handleObj.selector, focusHandler)
                .on('change.tl_change', handleObj.selector, function (e) {
                // use an anonymous funciton here so we have access to the
                // original handle object to call the handler with our args
                var $el = $(this);
                // call our handle function, passing in the event, the previous and current vals
                // override the change event name to our name
                e.type = "tl_change";
                handleObj.handler.apply($el, [e, $el.data('tl-previous-val'), $el.val()]);
            });
        },
        remove: function (handleObj) {
            $(this)
                .off('mousedown.tl_change touchstart.tl_change', handleObj.selector, focusHandler)
                .off('change.tl_change', handleObj.selector)
                .removeData('tl-previous-val');
        }
    };

    // on focus lets set the previous value of the element to a data attr
    function focusHandler(e) {
        var $el = $(this);
        $el.data('tl-previous-val', $el.val());
    }
})(jQuery);

// usage
$('.some-element').on('tl_change', '.delegate-maybe', function (e, prev, current) {
    console.log(e);         // regular event object
    console.log(prev);      // previous value of input (before change)
    console.log(current);   // current value of input (after change)
    console.log(this);      // element
});

有几种方法可以达到你想要的结果，以下是我的拙见:

让元素保持之前的值，因此添加属性'previousValue'。

<select id="mySelect" previousValue=""></select>

初始化后，'previousValue'现在可以用作属性。在JS中，要访问这个select的previousValue:

$("#mySelect").change(function() {console.log($(this).attr('previousValue'));.....; $(this).attr('previousValue', this.value);}

使用'previousValue'后，将属性更新为当前值。

将焦点事件与更改事件结合起来以实现您想要的结果:

(function () {
    var previous;

    $("select").on('focus', function () {
        // Store the current value on focus and on change
        previous = this.value;
    }).change(function() {
        // Do something with the previous value after the change
        alert(previous);

        // Make sure the previous value is updated
        previous = this.value;
    });
})();

工作示例:http://jsfiddle.net/x5PKf/766

我选择Avi Pinto的解决方案，它使用jquery.data()

使用焦点并不是一个有效的解决方案。它在你第一次改变选项时起作用，但如果你停留在选择元素上，并按“上”或“下”键。它不会再次通过焦点事件。

所以解应该是这样的，

//set the pre data, usually needed after you initialize the select element
$('mySelect').data('pre', $(this).val());

$('mySelect').change(function(e){
    var before_change = $(this).data('pre');//get the pre data
    //Do your work here
    $(this).data('pre', $(this).val());//update the pre data
})

 $("#dropdownId").on('focus', function () {
    var ddl = $(this);
    ddl.data('previous', ddl.val());
}).on('change', function () {
    var ddl = $(this);
    var previous = ddl.data('previous');
    ddl.data('previous', ddl.val());
});

下面是一个简单的解决方案，没有jQuery、DOM遍历、事件绑定、全局变量等开销。它提示用户用包含“before”和“after”值的消息确认更改，并根据用户的选择取消或接受更改。

<select name="test" 
  onfocus="handleOnFocus(this);" 
  onchange="if(handleOnChange(this) == false) { return false; }"
  data-original-selected-index="">
<option value="stack">Stack</option>
<option value="overflow">Overflow</option>
<option value="my">My</option>
<option value="question">Question</option>
</select>


// Prompt user to confirm the change
function handleOnChange(selectObj) {
  var confirmationMessage = 'Change ' +
    selectObj.options[selectObj.dataset.originalSelectedIndex].text + 
    ' to ' + 
    selectObj.options[selectObj.selectedIndex].text + '?';

  if (!confirm(confirmationMessage)) {
    selectObj.selectedIndex = selectObj.dataset.originalSelectedIndex;
    return false;
  } else {
    selectObj.dataset.originalSelectedIndex = selectObj.selectedIndex;
    return true;
  }
}

// Initialize original selected index (one-time)
function handleOnFocus(selectObj) {
  if (selectObj.dataset.originalSelectedIndex == '') {
    selectObj.dataset.originalSelectedIndex = selectObj.selectedIndex;
  }
}

JSFiddle在这里: https://jsfiddle.net/humbads/f3a0v8ys/

注1:onchange处理程序是这样编写的，所以这个解决方案也可以与ASP一起工作。Net下拉列表控件与AutoPostBack=True和OnSelectedIndexChanged处理程序。

注意2:选项不应该包含空值。如果存在，则更改初始值。