JQuery UI 可排序 - 是否可以动态更改占位符(例如,更改为不同的 class),具体取决于它结束的容器?

JQuery UI Sortable - Can the placeholder be dynamically changed (to a different class, for example), depending on the container it is over?

我想在可排序项目超过特定大陆时更改占位符 class。到目前为止我用过这个:

$(".list").sortable({
    items: ".item",
    over: function (event, ui) {

        // For brevity, the calculation of condition has been omitted as it is irrelevant to the question.

        if( condition === true ){
            $(this).sortable( "option", "placeholder", 'new-placeholder-class' );
        }
    },
    placeholder: ".placeholder-class"
}).disableSelection();

我收到这个错误

Error: this.options.placeholder.update is not a function

知道怎么做吗?

最后我弄清楚了如何进行占位符交换。而不是:

$(this).sortable( "option", "placeholder", 'new-placeholder-class' );

我用过:

$(ui.sortable).removeClass("placeholder-class").addClass("new-placeholder-class");

如果在可排序定义的 'over:' 事件函数中定义,这将在特定可排序容器上更改占位符 class。