The
Ambient object gives you information about selected
properties (such as
ForeColor, BackColor, and
Font)
of your custom control's container. The
AmbientChanged event fires whenever one
of these selected properties changes, thus giving your control the chance to react to changes
in its container.
A common use of the Ambient object would be to synchronize
the custom control's background color with its container's in the
AmbientChanged event, as in Listing 13.1. Notice that the AmbientChanged
event receives a single parameter indicating which container property has just changed. We
check the parameter to see if a property we are interested in has changed and, if so, we
synchronize our control with the value of the same property of the Ambient
object.
LISTING 13.1
CHECKING TO SEE WHICH CONTAINER PROPERTY HAS CHANGED IN THE AMBIENTCHANGED EVENT PROCEDURE
Private Sub UserControl_AmbientChanged
_
(PropertyName
As String)
If UCase$(PropertyName) =
"BACKCOLOR" Then
BackColor
= Ambient.BackColor
End If
End Sub
The LocaleID Ambient property is a number representing
the country or language of the developer. For example, 1033 represents U.S. English (these
numbers are often referred to in hex notation in documentation).
The UserMode property (a Boolean
property) is an important Ambient object property for the control
author. It tells you whether the current control instance is in design or run mode. When UserMode
is True, the control instance is in a running application. When it's False, the control
instance is in a design-time instance of its container.
You can check UserMode in the UserControl's
Paint and Resize events or in a custom property's
Property procedures, for example, to vary the behavior of your control.
You can give your control one behavior if it's in a running application
(Ambient.UserMode = True) and another when it's in the VB design environment
(Ambient.UserMode = False). In the example of Listing 13.2, we implement a property that
is writeable at design time but not at runtime by checking Ambient.UserMode
in the Property Let procedure.
LISTING 13.2
THIS PROPERTY IS ONLY WRITEABLE AT DESIGN TIME
Property Let MyProp (sValue as String)
If Not Ambient.UserMode Then
m_MyProp
= sValue
End If
End Property
Some Ambient properties don't represent states of
the container per se but rather states of
your control relative to the container. For instance, the
DisplayName Ambient property represents the name by which your control
is known to the container (you should use Ambient.Displayname
in error messages that you display in your control's code). Likewise the
DisplayAsDefault Ambient property is True if the developer has set your
control to be the Default control on its container (Default=True).
