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Safe cdevs


This Technical Note describes a potential problem with Control Panel devices (cdevs) that contain EditText fields and presents a way to avoid it.

[Aug 01 1989]


The Control Panel chapter in Inside Macintosh, Volume 5 describes, in detail, how run-time errors are handled by the Control Panel and a cdev. There is, however, a potential problem with cdevs that contain EditText items that this chapter does not cover.

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.

Erroneous Insertion Point

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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Inside Macintosh, Volume I, The Dialog Manager

Inside Macintosh, Volume IV, The Dialog Manager

Inside Macintosh, Volume V, The Control Panel

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Acrobat version of this Note (100K).


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