Computer Programming Concepts and Visual Basic David I. Schneider

■ EVENTS
In the previous section we drew a parallel between objects and controls and showed how to define
properties and methods for classes. In addition to the two predefined events for classes,
Initialize and Terminate, other events can be defined by the programmer to communicate
changes of properties, errors, and the progress of lengthy operations. Such events are called
user-defined events. The statement for triggering an event is located in the class module and
the event is dealt with in the form code. Suppose the event is named UserDefinedEvent and
has the arguments arg1, arg2, and so on. In the class module, the statement
Public Event UserDefinedEvent(arg1, arg2, ...)
should be placed in the (Declarations) section of (General), and the statement
RaiseEvent UserDefinedEvent(arg1, arg2, ...)
should be placed at the locations in the class module code at which the event should be triggered.
In the form code, an instance of the class, call it object1, must be declared with a statement
of the type
Private WithEvents object1 As CClassName
in order to be able to respond to the event. That is, the keyword WithEvents must be inserted
into the standard declaration statement. The header of an event procedure for object1 will be
Private Sub object1_UserDefinedEvent(par1, par2, ...)
EXAMPLE 3
Consider the circle class defined in Example 3 of Section 12.1. Add a user-defined event that is triggered
whenever the center of a circle changes. The event should have parameters to pass the center and radius
of the circle. The form code should use the event to determine if part (or all) of the drawn circle will fall
outside the form. If so, the event procedure should display the message “Circle Off Screen” in a label and
cause all future circles to be drawn in red.
SOLUTION:
Let’s call the event PositionChanged.
Object Property Setting
frmCircles BorderStyle 0-None
cmdMove Caption Move and Show
Circle
lblCaution BorderStyle 1-Fixed Single
Caption, (blank)
cmdQuit Caption, Quit
‘Class module for CCircle
Private m_x As Integer ‘Dist from center of circle to left side of form
Private m_y As Integer ‘Distance from center of circle to top of form
Private m_r As Single ‘Radius of circle
Public Event PositionChanged(x As Integer, y as Integer, r As Single)
’Event is triggered by a change in the center of the circle
Private Sub Class_Initialize()
Collections and Events 371