Assembly: PresentationCore (in presentationcore.dll)
'Declaration Public Sub Measure ( _ availableSize As Size _ ) 'Usage Dim instance As UIElement Dim availableSize As Size instance.Measure(availableSize)
The available space that a parent element can allocate a child element. A child element can request a larger space than what is available; the provided size might be accommodated if scrolling is possible in the content model for the current element.
Computation of layout positioning in Windows Presentation Foundation (WPF) is comprised of a Measure call and an Arrange call. During the Measure call, an element determines its size requirements by using an availableSize input. During the Arrange call, the element size is finalized.
availableSize can be any number from zero to infinite. Elements to be laid out should return the minimum Size they require for a given availableSize.
When a layout is first instantiated, it always receives a Measure call before Arrange. However, after the first layout pass, it may receive an Arrange call without a Measure; this can happen when a property that affects only Arrange is changed (such as alignment), or when the parent receives an Arrange without a Measure. A Measure call will automatically invalidate an Arrange call.
Layout updates happen asynchronously, such that the main thread is not waiting for every possible layout change. Qeruying an element via code-behind checking of property values may not immediately reflect changes to properties that interact with the element's sizing or layout characteristics (the Width property, for example).
Layout updates can be forced by using the UpdateLayout method. However, calling this method is usually unnecessary and can cause poor performance.
The layout system keeps two separate queues of invalid layouts, one for Measure and one for Arrange. The layout queue is sorted based upon the order of elements in the element's visual tree; elements higher in the tree are at the top of the queue, to avoid redundant layouts caused by repeated changes in parents. Duplicate entries are automatically removed from the queue, and elements are automatically removed from the queue if they are already layout-validated.
When updating layout, the Measure queue is emptied first, followed by the Arrange queue. An element in the Arrange queue will never be arranged if there is an element in the Measure queue.
This example shows how to override the default layout behavior of the Panel element and create custom layout elements that are derived from Panel.
The example defines a simple custom Panel element called PlotPanel, which positions child elements according to two hard-coded x- and y-coordinates. In this example, x and y are both set to 50; therefore, all child elements are positioned at that location on the x and y axes.
Public Class PlotPanel Inherits Panel 'Override the default Measure method of Panel. Protected Overrides Function MeasureOverride(ByVal availableSize As System.Windows.Size) As System.Windows.Size Dim childSize As Size = CType(availableSize, Size) For Each child As UIElement In InternalChildren child.Measure(childSize) Next Return MyBase.MeasureOverride(availableSize) End Function Protected Overrides Function ArrangeOverride(ByVal finalSize As System.Windows.Size) As System.Windows.Size For Each child As UIElement In InternalChildren Dim x As Double = 50 Dim y As Double = 50 child.Arrange(New Rect(New System.Windows.Point(x, y), child.DesiredSize)) Next Return MyBase.ArrangeOverride(finalSize) End Function End Class
For the complete sample, see Create a Simple Custom Panel Behavior Sample.
Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter EditionThe Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.