| Technote TB 31 | August 1988 |
Changes since August 1988: Updated to reflect known bugs in the posCntl and thumbCntl messages and the Control Manager _TrackControl call.
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.
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.
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).
_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.
Further Reference:
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