How Window Controllers Work

A controller object (in this case, an instance of the NSWindowController class) manages a window; this object is usually stored in a nib file. This management entails the following:

• Loading and displaying the window

• Closing the window when appropriate

• Customizing the window’s title

• Storing the window’s frame (size and location) in the defaults database

• Cascading the window in relation to other document windows of the application

A window controller can manage a window by itself or as a participant in AppKit’s document-based architecture, which also includes the NSDocument and NSDocumentController classes. In this architecture, a window controller is created and managed by a document (an instance of an NSDocument subclass) and, in turn, keeps a reference to the document. For a discussion of this architecture, see Document-Based App Programming Guide for Mac.

The relationship between a window controller and a nib file is important. Although a window controller can manage a programmatically created window, it usually manages a window in a nib file. The nib file can contain other top-level objects, including other windows, but the window controller’s responsibility is this primary window. The window controller is usually the owner of the nib file, even when it is part of a document-based application.

For simple documents—that is, documents with only one nib file containing a window—you need do little directly with NSWindowController objects. AppKit creates one for you. However, if the default window controller is not sufficient, you can create a custom subclass of NSWindowController.

For documents with multiple windows or panels, your document must create separate instances of NSWindowController (or of custom subclasses of NSWindowController), one for each window or panel. An example is a CAD application that has different windows for side, top, and front views of drawn objects. What you do in your NSDocument subclass determines whether the default NSWindowController object or separately created and configured NSWindowController objects are used.