ui.helper vs ui.item in jQuery UI

I was studying the jQuery UI Sortable widget and it said that all callback functions receive a ui object. Two of its properties are ui.helper nad ui.item. ui.helper is mentioned to be mostly a clone of ui.item, but I can't get the significance of calling it 'helper'. Also, the offset and position properties are defined in terms of the helper 开发者_JAVA技巧and not the item. What's the difference between the two ?

One of the answers mention that the helper fills the void, but the I read in the docs that the helper element is "used for dragging display". What exactly is this helper element and why is it named so?

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From what I understand ui.helper is a "clone" of the ui.item. It is cloned so you can edit it with css and make it look like whatever you want it to look like while you are dragging it and it won't effect the original ui.item. So if you had a box and while it was being dragged you wanted it to turn into a blue box with curved edges but only while it was dragging you can by modifying ui.helper.

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The helper is the element that fills the void (with visibility: hidden) when the item is dragged.