Using Alerts and Panels
With every program, you’ll need to design the unique features of your program while letting the Cocoa framework worry about making your program look and behave like a standard OS X program. Two common features of nearly every OS X program are alerts and panels.
An alert typically pops up on the screen to inform the user such as asking the user to verify the deletion of a file or to alert the user of a problem. A panel displays common user interface items such as a print panel that OS X programs typically display to give you options for printing, or an open panel for selecting a file.
By using alerts and panels, you can add common OS X user interface elements without having to create your own user interface ...