ArcGIS Engine Developer Guide

THE V ISUAL BASIC 6 ENVIRONMENT
perform some action when the user clicks the control. You can also add events
that certain objects generate. VBA and Visual Basic let you declare a variable with
the keyword WithEvents. WithEvents tells the development environment that the
object variable will be used to respond to the object’s events. This is sometimes
referred to as an “event sink”. The declaration must be made in a class module or
a form. Here’s how you declare a variable and expose the events of an object in
the Declarations section:
Private WithEvents m_pViewEvents as Map
Visual Basic only supports one outbound interface (marked as the default outbound
interface in the IDL) per coclass. To get around this limitation, the
coclasses that implement more than one outbound interface have an associated
dummy coclass that allows access to the secondary outbound interface. These
coclasses have the same name as the outbound interface they contain, minus the I.
Private WithEvents m_pMapEvents as MapEvents
Once you’ve declared the variable, search for its name in the Object combo box
at the top left of the code window. Then, inspect the list of events to which you
can attach code in the Procedure/Events combo box at the top right of the code
window.
Not all procedures of the outbound event interface need to be stubbed out, as
Visual Basic will stub out any unimplemented methods. This is different from
inbound interfaces, in which all methods must be stubbed out for compilation to
occur.
Before the methods are called, the hookup between the event source and sink
must be made. This is done by setting the variable that represents the sink to the
event source.
Set m_pMapEvents = MapControl1.Map
Pointers to valid objects as parameters
Some ArcGIS methods expect interfaces for some of their parameters. The
interface pointers passed can point to an instanced object before the method call
or after the method call is completed.
For example, if you have a polygon (pPolygon) whose center point you want to
find, you can write code as follows:
Dim pArea As IArea
Dim pPt As IPoint
Set pArea = pPolygon ' QI for IArea on pPolygon
Set pPt = pArea.Center
You don’t need to create pPt because the Center method creates a Point object for
you and passes back a reference to the object via its IPoint interface. Only methods
that use client-side storage require you to create the object prior to the
method call.
Passing data between modules
When passing data between modules it is best to use accessor and mutator functions
that manipulate some private member variable. This provides data encapsulation,
which is a fundamental technique in object-oriented programming. Public
variables should never be used.
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