| Technote TE 08 | August 1989 |
When a cdev is called by the Control Panel, the cdev's 'DITL' resource is concatenated to the Control Panel's 'DITL'. The Control Panel then lets the Dialog Manager update the window. If the cdev contains an item of type EditText, the Dialog Manager allocates and activates a TEHandle to be used for displaying and editing text. All of this action happens before the cdev gets the initDev message from the Control Panel.
As detailed in The Control Panel chapter, if an error occurs from which a cdev cannot recover, the cdev should dispose of any private memory and return the appropriate error code or a NIL value to the Control Panel. The Control Panel then grays out the cdev's area, displays the appropriate error dialog, and then deletes the items that were added to its 'DITL'.
All of this is fine, except that the TEHandle does not get deallocated. The EditText items get thrown away, including the strings in the item list that the Dialog Manager would use to store text entered into the EditText field, but the TEHandle stays there and stays active. Figure 1 illustrates what this would look like.
Figure 1-Erroneous Insertion Point
So the Dialog Manager, knowing that it allocated a TEHandle for an item that was visible, goes merrily on its way flashing the insertion point. The problem is not simply one of appearance. If a user hits a key, the Dialog Manager tries to process the key-down event just as if the EditText item was still available, and this series of events causes a rather nasty crash.
Fortunately, the solution for this problem is a very simple one. If an
EditText item is hidden with a _HideDItem call, the Dialog
Manager does not consider it active and will not try to process key-down events
for it. So if your cdev contains EditText items, part of your error
handling should be to first hide the EditText items with a call to
_HideDItem before returning an error code or a NIL as the
cdev's function result.
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