Compute a design-time value. This value will be displayed when editing the element in the Pentaho Report Designer.
If there is not enough data available to produce a sensible output, return a mock-object so that the user can
interact with the element in a sensible fashion.
When the element is called, all style and attribute expressions have been resolved and can be accessed via the
normal static style and attribute accessor methods. If the content of your element depends on style information,
the fully computed style must be retrieved via the
.Element#getComputedStyle()
method. The local style-sheet for the
element may not have all information.