Inside Macintosh: Files
An activation procedure controls the highlighting of dialog items that are defined by your application and can receive keyboard input.
An activation procedure should have the following form:
PROCEDURE MyActivateProc (theDialog: DialogPtr; itemNo: Integer;
activating: Boolean; myDataPtr: Ptr);
Your activation procedure controls the highlighting of dialog items that are defined by your application and can receive keyboard input. Ordinarily, you need to supply an activation procedure only if your application builds a list from which the user can select entries. The Standard File Package supplies the activation procedure for the file display list and for all TextEdit fields. You can also use the activation procedure to keep track of which field is receiving keyboard input, if your application needs that information.
Your application is responsible for removing the highlighting when one of its fields becomes inactive and for adding the highlighting when one of its fields becomes active. The Standard File Package can handle the highlighting of all TextEdit fields, even those defined by your application.