Why does the jquery change event not trigger when

问题:

The logic in the change() event handler is not being run when the value is set by val(), but it does run when user selects a value with their mouse. Why is this?

<select id=\"single\">
    <option>Single</option>
    <option>Single2</option>
</select>

<script>
    $(function() {
        $(\":input#single\").change(function() {
           /* Logic here does not execute when val() is used */
        });
    });

    $(\"#single\").val(\"Single2\");
</script>

回答1:

Because the change event requires an actual browser event initiated by the user instead of via javascript code.

Do this instead:

$(\"#single\").val(\"Single2\").trigger(\'change\');

or

$(\"#single\").val(\"Single2\").change();

回答2:

I believe you can manually trigger the change event with trigger():

$(\"#single\").val(\"Single2\").trigger(\'change\');

Though why it doesn\'t fire automatically, I have no idea.

回答3:

As far as I can read in API\'s. The event is only fired when the user clicks on an option.

http://api.jquery.com/change/

For select boxes, checkboxes, and radio buttons, the event is fired immediately when the user makes a selection with the mouse, but for the other element types the event is deferred until the element loses focus.

回答4:

Adding this piece of code after the val() seems to work:

$(\":input#single\").trigger(\'change\');

回答5:

To make it easier add a custom function and call it when ever you want that changing the value also trigger change

$.fn.valAndTrigger = function (element) {
    return $(this).val(element).trigger(\'change\');
}

and

$(\"#sample\").valAndTirgger(\"NewValue\");

Or you can override the val function to always call the change when the val is called

(function ($) {
    var originalVal = $.fn.val;
    $.fn.val = function (value) {
        this.trigger(\"change\");
        return originalVal.call(this, value);
    };
})(jQuery);

Sample at http://jsfiddle.net/r60bfkub/

回答6:

In case you don\'t want to mix up with default change event you can provide your custom event

$(\'input.test\').on(\'value_changed\', function(e){
    console.log(\'value changed to \'+$(this).val());
});

to trigger the event on value set, you can do

$(\'input.test\').val(\'I am a new value\').trigger(\'value_changed\');

回答7:

I ran into the same issue while using CMB2 with Wordpress and wanted to hook into the change event of a file upload metabox.

So in case you\'re not able to modify the code that invokes the change (in this case the CMB2 script), use the code below. The trigger is being invoked AFTER the value is set, otherwise your change eventHandler will work, but the value will be the previous one, not the one being set.

Here\'s the code i use:

(function ($) {
    var originalVal = $.fn.val;
    $.fn.val = function (value) {
        if (arguments.length >= 1) {
            // setter invoked, do processing
            return originalVal.call(this, value).trigger(\'change\');
        }
        //getter invoked do processing
        return originalVal.call(this);
    };
})(jQuery);

回答8:

$(\":input#single\").trigger(\'change\');

This worked for my script. I have 3 combos & bind with chainSelect event, I need to pass 3 values by url & default select all drop down. I used this

$(\'#machineMake\').val(\'<?php echo $_GET[\'headMake\']; ?>\').trigger(\'change\');

And the first event worked.

回答9:

If you\'ve just added the select option to a form and you wish to trigger the change event, I\'ve found a setTimeout is required otherwise jQuery doesn\'t pick up the newly added select box:

window.setTimeout(function() { jQuery(\'.languagedisplay\').change();}, 1);