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UI Automation of a WPF Custom Control

Microsoft UI Automation provides a single, generalized interface that automation clients can use to examine or operate the user interfaces of a variety of platforms and frameworks. UI Automation enables both quality-assurance (test) code and accessibility applications such as screen readers to examine user-interface elements and simulate user interaction with them from other code. For information about UI Automation across all platforms, see Accessibility.

This topic describes how to implement a server-side UI Automation provider for a custom control that runs in a WPF application. WPF supports UI Automation through a tree of peer automation objects that parallels the tree of user interface elements. Test code and applications that provide accessibility features can use automation peer objects directly (for in-process code) or through the generalized interface provided by UI Automation.

This topic contains the following sections.

WPF controls support UI Automation through a tree of peer classes that derive from AutomationPeer. By convention, peer class names begin with the control class name and end with "AutomationPeer". For example, ButtonAutomationPeer is the peer class for the Button control class. The peer classes are roughly equivalent to UI Automation control types but are specific to WPF elements. Automation code that accesses WPF applications through the UI Automation interface does not use automation peers directly, but automation code in the same process space can use automation peers directly.

Elements implement an automation peer class if they accept interface activity from the user, or if they contain information needed by users of screen-reader applications. Not all WPF visual elements have automation peers. Examples of classes that implement automation peers are Button, TextBox, and Label. Examples of classes that do not implement automation peers are classes that derive from Decorator, such as Border, and classes based on Panel, such as Grid and Canvas.

The base Control class does not have a corresponding peer class. If you need a peer class to correspond to a custom control that derives from Control, you should derive the custom peer class from FrameworkElementAutomationPeer.

Automation peers must run in a partial-trust environment. Code in the UIAutomationClient assembly is not configured to run in a partial-trust environment, and automation peer code should not reference that assembly. Instead, you should use the classes in the UIAutomationTypes assembly. For example, you should use the AutomationElementIdentifiers class from the UIAutomationTypes assembly, which corresponds to the AutomationElement class in the UIAutomationClient assembly. It is safe to reference the UIAutomationTypes assembly in automation peer code.

After locating an automation peer, in-process code can navigate the peer tree by calling the object's GetChildren and GetParent methods. Navigation among WPF elements within a control is supported by the peer's implementation of the GetChildrenCore method. The UI Automation system calls this method to build up a tree of subelements contained within a control; for example, list items in a list box. The default UIElementAutomationPeer.GetChildrenCore method traverses the visual tree of elements to build the tree of automation peers. Custom controls override this method to expose children elements to automation clients, returning the automation peers of elements that convey information or allow user interaction.

All classes that derive from UIElement and ContentElement contain the protected virtual method OnCreateAutomationPeer. WPF calls OnCreateAutomationPeer to get the automation peer object for each control. Automation code can use the peer to get information about a control’s characteristics and features and to simulate interactive use. A custom control that supports automation must override OnCreateAutomationPeer and return an instance of a class that derives from AutomationPeer. For example, if a custom control derives from the ButtonBase class, then the object returned by OnCreateAutomationPeer should derive from ButtonBaseAutomationPeer.

When implementing a custom control, you must override the "Core" methods from the base automation peer class that describe behavior unique and specific to your custom control.

Override the OnCreateAutomationPeer method for your custom control so that it returns your provider object, which must derive directly or indirectly from AutomationPeer.

Automation peers simplify some implementation aspects of server-side UI Automation providers, but custom control automation peers must still handle pattern interfaces. Like non-WPF providers, peers support control patterns by providing implementations of interfaces in the System.Windows.Automation.Provider namespace, such as IInvokeProvider. The control pattern interfaces can be implemented by the peer itself or by another object. The peer's implementation of GetPattern returns the object that supports the specified pattern. UI Automation code calls the GetPattern method and specifies a PatternInterface enumeration value. Your override of GetPattern should return the object that implements the specified pattern. If your control does not have a custom implementation of a pattern, you can call the base type's implementation of GetPattern to retrieve either its implementation or null if the pattern is not supported for this control type. For example, a custom NumericUpDown control can be set to a value within a range, so its UI Automation peer would implement the IRangeValueProvider interface. The following example shows how the peer's GetPattern method is overridden to respond to a PatternInterface.RangeValue value.

		Public Overrides Function GetPattern(ByVal patternInterface As PatternInterface) As Object
			If patternInterface = PatternInterface.RangeValue Then
				Return Me
			End If
			Return MyBase.GetPattern(patternInterface)
		End Function

A GetPattern method can also specify a subelement as a pattern provider. The following code shows how ItemsControl transfers scroll pattern handling to the peer of its internal ScrollViewer control.

Public Class Class1
    Public Overrides Function GetPattern(ByVal patternInterface__1 As PatternInterface) As Object
        If patternInterface1 = PatternInterface.Scroll Then
            Dim owner As ItemsControl = DirectCast(MyBase.Owner, ItemsControl)

            ' ScrollHost is internal to the ItemsControl class
            If owner.ScrollHost IsNot Nothing Then
                Dim peer As AutomationPeer = UIElementAutomationPeer.CreatePeerForElement(owner.ScrollHost)
                If (peer IsNot Nothing) AndAlso (TypeOf peer Is IScrollProvider) Then
                    peer.EventsSource = Me
                    Return DirectCast(peer, IScrollProvider)
                End If
            End If
        End If
        Return MyBase.GetPattern(patternInterface1)
    End Function
End Class

To specify a subelement for pattern handling, this code gets the subelement object, creates a peer by using the CreatePeerForElement method, sets the EventsSource property of the new peer to the current peer, and returns the new peer. Setting EventsSource on a subelement prevents the subelement from appearing in the automation peer tree and designates all events raised by the subelement as originating from the control specified in EventsSource. The ScrollViewer control does not appear in the automation tree, and scrolling events that it generates appear to originate from the ItemsControl object.

Automation code gets information about your control by calling public methods of the peer class. To provide information about your control, override each method whose name ends with "Core" when your control implementation differs from that of that provided by the base automation peer class. At a minimum, your control must implement the GetClassNameCore and GetAutomationControlTypeCore methods, as shown in the following example.

		Protected Overrides Function GetClassNameCore() As String
			Return "NumericUpDown"
		End Function

		Protected Overrides Function GetAutomationControlTypeCore() As AutomationControlType
			Return AutomationControlType.Spinner
		End Function

Your implementation of GetAutomationControlTypeCore describes your control by returning a ControlType value. Although you can return ControlType.Custom, you should return one of the more specific control types if it accurately describes your control. A return value of ControlType.Custom requires extra work for the provider to implement UI Automation, and UI Automation client products are unable to anticipate the control structure, keyboard interaction, and possible control patterns.

Implement the IsContentElementCore and IsControlElementCore methods to indicate whether your control contains data content or fulfills an interactive role in the user interface (or both). By default, both methods return true. These settings improve the usability of automation tools such as screen readers, which may use these methods to filter the automation tree. If your GetPattern method transfers pattern handling to a subelement peer, the subelement peer's IsControlElementCore method can return false to hide the subelement peer from the automation tree. For example, scrolling in a ListBox is handled by a ScrollViewer, and the automation peer for PatternInterface.Scroll is returned by the GetPattern method of the ScrollViewerAutomationPeer that is associated with the ListBoxAutomationPeer.Therefore, the IsControlElementCore method of the ScrollViewerAutomationPeer returns false, so that the ScrollViewerAutomationPeer does not appear in the automation tree.

Your automation peer should provide appropriate default values for your control. Note that XAML that references your control can override your peer implementations of core methods by including AutomationProperties attributes. For example, the following XAML creates a button that has two customized UI Automation properties.

<Button AutomationProperties.Name="Special" 
    AutomationProperties.HelpText="This is a special button."/>

The interfaces implemented by a custom provider are explicitly declared if the owning element derives directly from Control. For example, the following code declares a peer for a Control that implements a range value.

Public Class RangePeer1
    Inherits FrameworkElementAutomationPeer
    Implements IRangeValueProvider
End Class

If the owning control derives from a specific type of control such as RangeBase, the peer can be derived from an equivalent derived peer class. In this case, the peer would derive from RangeBaseAutomationPeer, which supplies a base implementation of IRangeValueProvider. The following code shows the declaration of such a peer.

Public Class RangePeer2
    Inherits RangeBaseAutomationPeer
End Class

For an example implementation, see NumericUpDown Custom Control with Theme and UI Automation Support Sample.

Automation clients can subscribe to automation events. Custom controls must report changes to control state by calling the RaiseAutomationEvent method. Similarly, when a property value changes, call the RaisePropertyChangedEvent method. The following code shows how to get the peer object from within the control code and call a method to raise an event. As an optimization, the code determines if there are any listeners for this event type. Raising the event only when there are listeners avoids unnecessary overhead and helps the control remain responsive.

			If AutomationPeer.ListenerExists(AutomationEvents.PropertyChanged) Then
				Dim peer As NumericUpDownAutomationPeer = TryCast(UIElementAutomationPeer.FromElement(nudCtrl), NumericUpDownAutomationPeer)

				If peer IsNot Nothing Then
					peer.RaisePropertyChangedEvent(RangeValuePatternIdentifiers.ValueProperty, CDbl(oldValue), CDbl(newValue))
				End If
			End If
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