WPF Concepts—Routed Events

Microsoft .NET Framework, ASP.NET, Visual C# (CSharp, C Sharp, C-Sharp) Developer Training, Visual Studio

Routed Events

Just as WPF adds more infrastructure on top of the simple notion of .NET properties, it also adds more infrastructure on top of the simple notion of .NET events. Routed events are events that are designed to work well with a tree of elements. When a routed event is raised, it can travel up or down the visual and logical tree, getting raised on each element in a simple and consistent fashion, without the need for any custom code.

Event routing helps most applications remain oblivious to details of the visual tree (which is good for restyling) and is crucial to the success of WPF's element composition. For example, Button exposes a Click event based on handling lower-level MouseLeftButtonDown and KeyDown events. When a user presses the left mouse button with the mouse pointer over a standard Button, however, they are really interacting with its ButtonChrome or TextBlock visual child. Because the event travels up the visual tree, the Button eventually sees the event and can handle it. Similarly, for the VCR-style Stop Button in the preceding chapter, a user might press the left mouse button directly over the Rectangle logical child. Because the event travels up the logical tree, the Button still sees the event and can handle it as well. (Yet if you really wish to distinguish between an event on the Rectangle versus the outer Button, you have the freedom to do so.)

Therefore, you can embed arbitrarily complex content inside an element like Button or give it an arbitrarily complex visual tree, and a mouse left-click on any of the internal elements still results in a Click event raised by the parent Button. Without routed events, producers of the inner content or consumers of the Button would have to write code to patch everything together.