In any application that you write, you’ll probably manipulate data. Some of this data you may want, or need, to keep—even after your application quits or the system restarts. Perhaps it is data that your application needs to run, or perhaps it is data that your users create and may wish to return to at a later time. If you want that data to be there when your application quits and is later restarted, you’ll have to store that information in a persistent form—that is, as a file.
In this chapter, you will learn some of the concepts that you’ll need to store and retrieve data in files in Mac OS X. You’ll also learn the tasks every application that stores and retrieves data should perform. As a final step, you’ll add code to the Moon Travel Planner application to open, display, save, and close an itinerary file for planning a trip to the moon.
In our discussion of opening and saving files, we will focus on storing and retrieving user data, which most applications must do at some time or another. To store and retrieve user data on a physical storage device, there are two things that you must do:
Communicate with the physical storage device to transfer the data and organize it on disk.
Allow users to choose the data to retrieve and the locations in which to store data, and to make other, similar choices about the data that you are storing for the users.
If your application needs to store its own data that doesn’t need to be visible to users, ...