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8 - Exclusive Code
Now we've given each object an awareness of all others, how does this provide exclusivity? For that, we need to intercept button presses with our MsgHandler() function: S32 cpaRadioButton::MsgHandler(MSG_TYPE type,CViewable*from,S32 data)
{
switch (type)
{
case MSG_PEN_DOWN:
case MSG_PEN_DOUBLECLICK:
SetDownStatus(true);
// send message to parent
GetParent()->MsgHandler(MSG_BUTTON_SELECT,this,true);
break;
case MSG_PEN_UP:
case MSG_PEN_TRACK:
// ignore these
break;
default:
// all others messages processed
// by CRadioButton class as usual
return CRadioButton::MsgHandler(type,from,data);
}
return 1;
}
Our code is very straightforward in that
we really only need to monitor for button
down events (our PEN messages). We then call
SetDownStatus() and send a message to our
parent so it behaves like a normal button.
The parent message has available to it the
state (always on), as well as the button
object pointer, which gives it access to
the object's tag. The tag value is a carryover
from my programming on other systems, where
I've found a tag value a handy place to associated
numbers with objects. For instance, the tag
could contain a value associated with each
choice, making relating the button choice
to the end result that much easier (using
Tag(value) to set it in initialization).
If we happen to want to access the radio buttons outside of the MsgHandler() call, there is a static function cpaRadioButton::GetGroupDownButton(int group), which returns a pointer to the current 'down' button in the specified group.
Tying all this together is the SetDownStatus() call, which walks the linked list looking for group id matches (if positive - negative groups are not allowed), and turning off all in the group except the current item:
void cpaRadioButton::SetDownStatus(BOOL status)
{
if (false==status) // trying to 'unselect'? Don't allow that
return;
if ( m_groupId>=0 ) // valid group id?
{
// walk list, turning off all others with identical ids
cpaRadioButton*ptr=s_firstObject;
while ( NULL!=ptr )
{
// check if from our group and down - if so, set up
if ( ptr->m_groupId==m_groupId && ptr->GetDownStatus() )
ptr->CRadioButton::SetDownStatus(false);
ptr=ptr->m_nextObject;
}
}
CRadioButton::SetDownStatus(status);
}
As the final part of the example program's
display, we have the bottom showing three
columns of three radio buttons, with each
column allowing a single 'on' button.
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