Visual Basic and WPF Event Handling
For the Microsoft Visual Basic .NET language specifically, you can use the language-specific Handles keyword to associate event handlers with instances, instead of attaching event handlers with attributes or using the AddHandler method. However, the Handles technique for attaching handlers to instances does have some limitations, because the Handles syntax cannot support some of the specific routed event features of the WPF event system.
Using "Handles" in a WPF Application
The event handlers that are connected to instances and events with Handles must all be defined within the partial class declaration of the instance, which is also a requirement for event handlers that are assigned through attribute values on elements. You can only specify Handles for an element on the page that has a Name property value (or x:Name Directive declared). This is because the Name in XAML creates the instance reference that is necessary to support the Instance.Event reference format required by the Handles syntax. The only element that can be used for Handles without a Name reference is the root-element instance that defines the partial class.
You can assign the same handler to multiple elements by separating Instance.Event references after Handles with commas.
You can use Handles to assign more than one handler to the same Instance.Event reference. Do not assign any importance to the order in which handlers are given in the Handles reference; you should assume that handlers that handle the same event can be invoked in any order.
To remove a handler that was added with Handles in the declaration, you can call RemoveHandler.
You can use Handles to attach handlers for routed events, so long as you attach handlers to instances that define the event being handled in their members tables. For routed events, handlers that are attached with Handles follow the same routing rules as do handlers that are attached as XAML attributes, or with the common signature of AddHandler. This means that if the event is already marked handled (the Handled property in the event data is True), then handlers attached with Handles are not invoked in response to that event instance. The event could be marked handled by instance handlers on another element in the route, or by class handling either on the current element or earlier elements along the route. For input events that support paired tunnel/bubble events, the tunneling route may have marked the event pair handled. For more information about routed events, see Routed Events Overview.
Limitations of "Handles" for Adding Handlers
Handles cannot reference handlers for attached events. You must use the add accessor method for that attached event, or typename.eventname event attributes in XAML. For details, see Routed Events Overview.
For routed events, you can only use Handles to assign handlers for instances where that event exists in the instance members table. However, with routed events in general, a parent element can be a listener for an event from child elements, even if the parent element does not have that event in its members table. In attribute syntax, you can specify this through a typename.membername attribute form that qualifies which type actually defines the event you want to handle. For instance, a parent Page (with no Click event defined) can listen for button-click events by assigning an attribute handler in the form Button.Click. But Handles does not support the typename.membername form, because it must support a conflicting Instance.Event form. For details, see Routed Events Overview.
Handles cannot attach handlers that are invoked for events that are already marked handled. Instead, you must use code and call the handledEventsToo overload of AddHandler(RoutedEvent, Delegate, Boolean).
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Do not use the Handles syntax in Visual Basic code when you specify an event handler for the same event in XAML. In this case, the event handler is called twice. |
How WPF Implements "Handles" Functionality
When a Extensible Application Markup Language (XAML) page is compiled, the intermediate file declares Friend WithEvents references to every element on the page that has a Name property set (or x:Name Directive declared). Each named instance is potentially an element that can be assigned to a handler through Handles.
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Within Microsoft Visual Studio, IntelliSense can show you completion for which elements are available for a Handles reference in a page. However, this might take one compile pass so that the intermediate file can populate all the Friends references. |