Visual Basic.NET How to Program (PDF)

510 Graphical User Interface Concepts: Part 1 Chapter 12
To respond to the user’s clicks, the program must handle the Click event. Inside the
event handler, picImage_Click, we use an Integer (imageNumber) to store the
image we want to display. We then set the Image property of picImage to an Image
(line 28–30). Although class Image is discussed in Chapter 16, Graphics and Multimedia,
we now overview method FromFile, which takes a String (the image file) and creates
an Image object.
To find the images, we use class Directory (namespace System.IO, specified on
line 4) method GetCurrentDirectory (line 29). This returns the current directory of
the executable file as a String. To access the images subdirectory, we append
“\images\” and the file name to the name of the current directory. We use image-
Number to append the proper number, enabling us to load either image0, image1 or
image2. The value of Integer imageNumber stays between 0 and 2 because of the
modulus calculation in line 25. Finally, we append ".bmp" to the filename. Thus, if we
want to load image0, the String becomes “CurrentDir\images\image0.bmp”,
where CurrentDir is the directory of the executable.
12.9 Mouse-Event Handling
This section explains the handling of mouse events, such as clicks, presses and moves,
which are generated when the mouse interacts with a control. Mouse events can be handled
for any control that derives from class System.Windows.Forms.Control. Mouseevent
information is passed through class MouseEventArgs, and the delegate used to
create mouse-event handlers is MouseEventHandler. Each mouse-event-handling
method requires an Object and a MouseEventArgs object as arguments. For example,
the Click event, which we covered earlier, uses delegate EventHandler and event arguments
EventArgs.
Class MouseEventArgs contains information related to the mouse event, such as
the x- and y-coordinates of the mouse pointer, the mouse button pressed Right, Left or
Middle), the number of times the mouse was clicked and the number of notches through
which the mouse wheel turned. Note that the x- and y-coordinates of the MouseEventArgs
object are relative to the control that generated the event. Point (0,0) represents the
upper-left corner of the control. Several mouse events are described in Fig. 12.29.
Mouse Events, Delegates and Event Arguments
Mouse Events (Delegate EventHandler, event arguments EventArgs)
MouseEnter Generated if the mouse cursor enters the area of the control.
MouseLeave Generated if the mouse cursor leaves the area of the control.
Mouse Events (Delegate MouseEventHandler, event arguments MouseEventArgs)
MouseDown Generated if the mouse button is pressed while its cursor is over the
area of the control.
MouseHover Generated if the mouse cursor hovers over the area of the control.
Fig. 12.29 Mouse events, delegates and event arguments (part 1 of 2).