On creating a new application through Xcode, it simplifies the task of the developer by generating several files for us that are required for successful execution of the application. The two important files that are autogenerated by Xcode are application delegate files. The question is what is the usage of application delegate files? The answer is quite simple: there are several important events that happen in the life of an application. The two most important events are launching and termination of the application. The application needs to know when these events happen or are about to happen. The iPhone OS notifies about these events through Application Delegate by calling its appropriate methods. iPhone OS calls the
applicationDidFinishLaunching method when it finishes the launch procedure and calls
applicationWillTerminate when the application is terminated, so as to close any open files.
Let's have a look at the code of the autogenerated files...