OS X comes with built in calendaring, a system-wide address book, a keychain for storing secrets, and a variety of other standard APIs. Use these, rather than rolling your own, and your program will work with all of the user's other programs. Opera on OS X uses the keychain, so all of your Safari passwords work if you switch to Opera. FireFox uses its own, so users have to manually transfer them over. This makes it a lot easier for users to switch to Opera than to FireFox, and to switch back if they don't like it.
I've expanded this tip, as I will do each Friday in January, into a full article: Cocoa Tips: Don't Reimplement Standard Functionality.
For more tips, please see my new book,
Cocoa Programming Developer's Handbook,
available in print and electronic formats.
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