Assembly: PresentationCore (in PresentationCore.dll)
For WPF framework-level element deriving scenarios, behavior for should not (and cannot, unless you shadow) be changed. Instead, you should override the ArrangeOverride implementation in your class. Your ArrangeOverride implementation is called internally by as part of default WPF framework-level layout operations. Your ArrangeCore implementation should also call ArrangeOverride on each child element, if it has child elements.
For WPF core-level element deriving scenarios, the behavior for should not (and cannot, unless you shadow) be changed. Instead, you should override ArrangeCore in your class. Your ArrangeCore implementation is called internally by as part of default WPF framework-level layout operations. However, this assumes you are using WPF framework-level layout and its layout system, which is often not the case if you are specifically deriving elements at the WPF core-level from the UIElement base element class. Your ArrangeCore implementation should also call on each child element, if it has child elements. Note that the WPF core-level scenario implies that you are not using a FrameworkElement derived class, because FrameworkElement seals ArrangeCore.
Computation of WPF framework-level layout positioning in Windows Presentation Foundation (WPF) consists of a Measure call and an call. During the Measure call, the layout system determines an element's size requirements using a provided Size (availableSize) argument. During the call, the layout system finalizes the size and position of an element's bounding box. For more information, see Layout.
availableSize can be any number from zero to infinity. Elements to be laid out return the minimum Size they require through the availableSize parameter.
When a layout is first instantiated, it always receives a Measure call before . However, after the first layout pass, it may receive an call without a Measure; this can happen when a property that affects only is changed (such as alignment), or when the parent receives an without a Measure. A Measure call will automatically invalidate an call.
Layout updates generally occur asynchronously (at a time determined by the layout system). An element might not immediately reflect changes to properties that affect element sizing (such as Width).
Layout updates can be forced by using the UpdateLayout method; however, calling this function is not recommended, as it is usually unnecessary and can cause poor performance. In many situations where calling UpdateLayout might be appropriate, the layout system will probably already be processing updates. The layout system can process layout changes in a manner that can optimize all necessary updates as part of a package.
The layout system keeps two separate queues of invalid layouts, one for Measure and one for . The layout queue is sorted based upon the order of elements in the visual tree. Elements higher in the tree are at the top of the queue, in order 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 valid.
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 panelDesiredSize As Size = New Size() ' In our example, we just have one child. ' Report that our panel requires just the size of its only child. For Each child As UIElement In InternalChildren child.Measure(availableSize) panelDesiredSize = child.DesiredSize Next Return panelDesiredSize 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 finalSize End Function End Class