組件： PresentationCore (在 presentationcore.dll 中)
When you call this method, elements with IsMeasureValidfalse or IsArrangeValidfalse will call element-specific MeasureCore and ArrangeCore methods, which forces layout update, and all computed sizes will be validated.
Calling this method has no effect if layout is unchanged, or if neither arrangement nor measurement state of a layout is invalid. However, if layout is invalid in either respect, the UpdateLayout call will redo the entire layout. Therefore, you should avoid calling UpdateLayout after each incremental and minor change in the element tree. The layout system will perform element layout in a deferred manner, using an algorithm that balances performance and currency, and with a weighting strategy to defer changes to roots until all child elements are valid. You should only call UpdateLayout if you absolutely need updated sizes and positions, and only after you are certain that all changes to properties that you control and that may affect layout are completed.