Visual Basic.NET How to Program (PDF)

Chapter 12 Graphical User Interface Concepts: Part 1 513
event. Line 32 sets shouldPaint to True. Unlike MouseMove events, the system generates
a MouseDown event only once while the mouse button is down.
When the user releases the mouse button (to complete a “click” operation), the system
generates a single MouseUp event. FrmPainter_MouseUp handles the MouseUp
event (lines 36–41). Line 40 sets shouldPaint to False.
12.10 Keyboard-Event Handling
This section explains the handling of key events, which are generated when keys on the keyboard
are pressed and released. Such events can be handled by any control that inherits
from System.Windows.Forms.Control. There are two types of key events. The
first is event KeyPress, which fires when a key representing an ASCII character is
pressed (determined by KeyPressEventArgs property KeyChar). ASCII is a 128character
set of alphanumeric symbols, a full listing of which can be found in Appendix E,
ASCII Character Set.
However the KeyPress event does not enable us to determine whether modifier keys
(e.g., Shift, Alt and Control) were pressed. It is necessary to handle the second type of key
events, the KeyUp or KeyDown events, to determine such actions. Class KeyEventArgs
contains information about special modifier keys. The key’s Key enumeration value can
be returned, providing information about a wide range of non-ASCII keys. Often, modifier
keys are used in conjunction with the mouse to select or highlight information.
KeyEventHandler (event argument class KeyEventArgs) and KeyPress–
EventHandler (event argument class KeyPressEventArgs) are the delegates for
the two classes. Figure 12.31 lists important information about key events.
Keyboard Events, Delegates and Event Arguments
Key Events (Delegate KeyEventHandler, event arguments KeyEventArgs)
KeyDown Generated when key is initially pressed.
KeyUp Generated when key is released.
Key Events (Delegate KeyPressEventHandler, event arguments KeyPressEventArgs)
KeyPress Generated when key is pressed. Occurs repeatedly
while key is held down, at a rate specified by the operating
system.
Class KeyPressEventArgs Properties
KeyChar Returns the ASCII character for the key pressed.
Handled Indicates whether the KeyPress event was handled.
Class KeyEventArgs Properties
Alt Indicates whether the Alt key was pressed.
Control Indicates whether the Control key was pressed.
Shift Indicates whether the Shift key was pressed.
Fig. 12.31 Keyboard events, delegates and event arguments (part 1 of 2).