Virtual Elements

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Revision as of 23:29, 3 April 2017 by 151.229.136.200 (talk) (Accessing virtual elements)
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Virtual elements are created whenever CSS requires an element to be rendered but must not pollute the DOM. Importantly, virtual elements are ordered. For example, the virtual element created by ::after shows up after the 'real' element has been drawn. PowerUI handles this by storing all virtual elements in a SortedDictionary, indexed by a priority value. It's unique and constant for any given type of virtual element.

Accessing virtual elements

There's a web API for doing this which is supported - specifically the getComputedStyle method:

// Get the computed style for the '::after' selector on element:
var after = document.window.getComputedStyle(element,"after");

// (Then use e.g. after.Element)

// Or use the PowerUI convenience version:
var after = element.getComputedStyle("after");

For more advanced uses, you can directly access the set of virtual elements here:

// Get the virtuals set for 'element' (A HtmlElement):
Css.VirtualElements virtuals=element.Style.Computed.Virtuals;

Note that it's null if there aren't any.

The priority value

You'll notice that the virtuals set uses those arbitrary priority values. In order to obtain a particular virtual element, you'll need to know what its priority is - fortunately, they're constant for any given type of element:

Virtual Element Priority (C# const) See Also
Horizontal scrollbar ComputedStyle.HorizontalScrollPriority Styling scrollbars
Vertical scrollbar ComputedStyle.VerticalScrollPriority Styling scrollbars
Textarea/ input field caret HtmlCaretElement.Priority Styling the caret
Select menu dropdown HtmlDropdownElement.Priority It's a virtual <dropdown> element (style it by selecting that).
Select menu down arrow HtmlSelectButtonElement.Priority It's a virtual <selectbutton> element (style it by selecting that).
 ::before BeforeSelector.Priority
 ::after AfterSelector.Priority
 ::marker MarkerSelector.Priority

What can I do with them?

They're completely ordinary elements which act exactly the same as a child of the element they're on. Essentially, you can do whatever you can normally do to elements to them too (The element itself is accessed from ComputedStyle.Element).