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This Technical Note describes known bugs in the Control Manager which affect
control definition functions ('CDEF' resources).
[Aug 01 1988]
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Introduction
The Control Manager chapter of Inside Macintosh, Volume I-309, describes
how to write a control definition function ('CDEF' resource). This
Note assumes a basic understanding of this chapter, specifically of the various
messages which are sent in the message parameter.
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drawCntl (0) and autoTrack (8)
When a 'CDEF' is called with either the message drawCntl or
autoTrack, it is possible for the high word of the param
parameter to contain undefined data which could result in the failure of
routines that rely upon all 32 bits of param being defined.
'CDEF' resources should only consider the low word of the
param parameter when dealing with the drawCntl and
autoTrack messages.
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posCntl (5) and thumbCntl (6)
According to Inside Macintosh, the Control Manager calls a
'CDEF' with the posCntl message and the thumbCntl
message if an application does custom dragging of an indicator (a thumb), but
not if it does default dragging. This is not true. The Control Manager calls
a 'CDEF' with the posCntl message if an application does
default dragging, which is exactly the opposite of the way it is documented.
The 'CDEF' receives the thumbCntl message regardless of which
type of dragging an application does, however, the results are used only for
default dragging (they are ignored for custom dragging).
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_TrackControl
When a user clicks on your control, you normally call _TrackControl,
which is supposed to return zero if the user does not change the control's
setting or the part code if the user does change the setting. For
'CDEF' resources that implement custom dragging,
_TrackControl returns zero whether or not the user changes the
control's setting. To work around this problem, you must use another method to
find out if the user has changed the control's setting, such as comparing the
control's value before and after the call to _TrackControl.
Back to top References
Inside Macintosh, Volume I-309, The Control Manager
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Downloadables
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Acrobat version of this Note (48K).
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