Programming WPF, 2nd Edition

Chapter 4. Input

A user interface wouldn't be much use if it couldn't respond to user input. In this chapter, we will examine the input handling mechanisms available in WPF. There are three main kinds of user input for a Windows application: mouse, keyboard, and ink.^[19] Any user interface element can receive input—not just controls. This is not surprising, because controls rely entirely on the services of lower-level elements like Rectangle and TextBlock in order to provide visuals. All of the input mechanisms described in the following sections are, therefore, available on all user interface element types.

Raw user input is delivered to your code through WPF's routed event mechanism. There is also a higher-level concept of a command—a particular action that might be accessible through several different inputs such as keyboard shortcuts, toolbar buttons, and menu items.

Routed Events

The .NET Framework defines a standard mechanism for managing events. A class may expose several events, and each event may have any number of subscribers. WPF augments this standard mechanism to overcome a limitation: if a normal .NET event has no registered handlers, it is effectively ignored.

Consider what this would mean for a typical WPF control. Most controls are made up of multiple visual components. For example, suppose you give a button a very plain appearance consisting of a single Rectangle, and provide a simple piece of text as the content. (Chapter 9 describes how to customize a control's appearance.) ...