Building Cocoa Applications: A Step by Step Guide

Chapter 8. Events and Responders

Our Calculator already handles events such as mouseclicks on buttons and menu items, but all this was automatic; we haven’t had to write any specific code to handle these mouse events. In this chapter we’ll learn more about events and the chain of objects that Cocoa uses to respond to events. At the end of the chapter, we’ll see how to “catch” events from the keyboard in our Calculator application. This is the final chapter of the Calculator application.

Events and the NSResponder Chain

There are seven basic kinds of events that Cocoa developers need to be concerned about:

Mouse events

Generated by pressing, clicking, or moving the mouse

Keyboard events

Generated by a keypress or release

Tracking rectangle and cursor-update events

Generated when the cursor crosses the boundary of a predefined rectangular area (tracking rectangle) in a window

Periodic events

Generated to notify an application that a certain time interval has elapsed

AppKit-defined events

Generated by the Application Kit when a window is moved, resized, or exposed, or when the application is activated or deactivated

System-defined events

Generated by the system — for example, when the power is turned off

NSApplication-defined events

Custom events defined and generated by your application to be inserted into the event queue

Of these, the mouse and keyboard events are usually the most important for developers.

What Is an Event?

A Cocoa event is a message and a corresponding object that the Window ...