Building Cocoa Applications: A Step by Step Guide

Events and the NSApplication Object

After an event is translated into an NSResponder method, your program’s NSApplication object sends the corresponding message to the appropriate NSWindow (or other) object within your application. The particular object in your application that receives the message is determined by the type of message, as shown in Table 8-2.

Table 8-2. Important NSResponder methods

NSResponder method	Sent to
mouseDown:,rightMouseDown:	The window where the mouse-down event occurred
mouseUp:,rightMouseUp:	The window where the original mouse-down event occurred
mouseDragged:,rightMouseDragged:	The window where the mouse-dragged event occurred
mouseEntered:, mouseExited:	The object that was specified when the tracking rectangle was created (see Chapter 18)
keyDown:	The key window
keyUp:	The same window that received the key-down event
performKeyEquivalent:	The key window

As we saw earlier, each one of these NSResponder methods has an NSEvent object as its only argument. This NSEvent object contains essentially the same information that the Mac OS X Window Server passes to the NSApplication object, but it’s in a form that Cocoa programmers can handle.

Responders and the NSResponder Chain

As we mentioned earlier, the NSView, NSWindow, and NSApplication classes are all subclasses of the NSResponder class. NSResponder is the main class for handling events. It’s called an abstract superclass because its functionality is used via instances ...