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。

在编写下拉“on change”动作函数之前，将当前选中的jquery下拉值保存在全局变量中。 如果你想在函数中设置之前的值，你可以使用全局变量。

//global variable
var previousValue=$("#dropDownList").val();
$("#dropDownList").change(function () {
BootstrapDialog.confirm(' Are you sure you want to continue?',
  function (result) {
  if (result) {
     return true;
  } else {
      $("#dropDownList").val(previousValue).trigger('chosen:updated');  
     return false;
         }
  });
});

我想为解决这个问题贡献另一种选择;因为上面提出的解决方案并没有解决我的情况。

(function()
    {
      // Initialize the previous-attribute
      var selects = $('select');
      selects.data('previous', selects.val());

      // Listen on the body for changes to selects
      $('body').on('change', 'select',
        function()
        {
          $(this).data('previous', $(this).val());
        }
      );
    }
)();

这确实使用了jQuery，所以def.在这里是一个依赖项，但这可以在纯javascript中进行调整。(在body中添加监听器，检查原始目标是否为select, execute函数，…)

通过将更改侦听器附加到主体，您几乎可以确定它将在特定的侦听器之后触发选择，否则'data-previous'的值将在您甚至可以读取它之前被覆盖。

当然，这是假设您更喜欢为set-previous和check-value使用单独的侦听器。它正好符合单一责任模式。

注意:这将把“先前”功能添加到所有选择中，所以如果需要，请确保对选择器进行微调。

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
});

我使用事件“live”，我的解决方案基本上类似于Dimitiar，但不是使用“焦点”，我以前的值被存储时，“点击”被触发。

var previous = "initial prev value";
$("select").live('click', function () {
        //update previous value
        previous = $(this).val();
    }).change(function() {
        alert(previous); //I have previous value 
    });