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UIElement::Measure Method

Updates the DesiredSize of a UIElement. Parent elements call this method from their own MeasureCore implementations to form a recursive layout update. Calling this method constitutes the first pass (the "Measure" pass) of a layout update.

Namespace:  System.Windows
Assembly:  PresentationCore (in PresentationCore.dll)

public:
void Measure(
	Size availableSize
)

Parameters

availableSize
Type: System.Windows::Size

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 participating in layout 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. Querying an element via code-behind checking of property values may not immediately reflect changes to properties that interact with the sizing or layout characteristics (the Width property, for example).

NoteNote

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 visual tree of the element performing layout; 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.

To implement custom Panel behaviors, the example uses the MeasureOverride and ArrangeOverride methods. Each method returns the Size data that is necessary to position and render child elements.

   public: 
      ref class PlotPanel : Panel {

      public: 
         PlotPanel () {};

      protected: 
         // Override the default Measure method of Panel 
         virtual Size MeasureOverride(Size availableSize) override
         {
             Size^ panelDesiredSize = gcnew Size();

             // In our example, we just have one child.  
             // Report that our panel requires just the size of its only child. 
             for each (UIElement^ child in InternalChildren)
             {
                 child->Measure(availableSize);
				 panelDesiredSize = child->DesiredSize;
             }
             return *panelDesiredSize ;
         }

      protected: 
         virtual System::Windows::Size ArrangeOverride (Size finalSize) override 
         {
            for each (UIElement^ child in InternalChildren)
            {
               double x = 50;
               double y = 50;
               child->Arrange(Rect(Point(x, y), child->DesiredSize));
            }
            return finalSize;
         };
      };

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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