Understanding application flow of iOS 7 apps. What happens on launch?

Question

I'm new to Objective-C and iOS programming. I've read through a few of Apple's "Getting Started" docs and I've downloaded a few sample applications to try and get familiar with what is going on in the app. What I'm confused about is what exactly happens from the application's launch. I see in main.m that UIApplicationMain gets called and invokes the AppDelegate. From what I've read, now, if applicationDidFinishLaunching returns true, the app will enter the main event loop.

Is this all correct? If so, how do I tie my code to events? Where do I create instances of my classes when an event occurs?

Finally, from my understanding, Xcode now creates all template apps with storyboards. However, some applications I am looking at are a bit older and don't use storyboards, but I don't see .xib or .nib files anywhere in the directory, but the applications successfully build. Am I wrong in assuming that applications require these files?

Answer 1

An application does not require a .xib or .nib to run. Many programmers do not use any of the visual design tools that generate storyboards or .nib files. You can define your UI completely in code if you wish.

The call to UIApplicationMain in main.m creates a singleton UIApplication object, sets up an event loop, and sets up your UIApplicationDelegate which works in tandem with UIApplication so you can customize the behavior of your app at key points in its lifecycle. UIApplication manages the event loop for you. It receives events from the system and dispatches them to your code for handling.

In order to really understand event handling you need to understand the responder chain. If you look at the superclass of your app delegate you will see it is UIResponder. This is the interface that lets objects respond to and handle events. It is the superclass of UIApplication and UIView and handles most of the raw event handling for you. Events follow a defined path through your code. In the simplest case, touch events, UIApplication pops an event from the event loop and hands it off to UIWindow. UIWindow will perform a hit test to try and deliver it directly to the UIView under the finger. It gets more complicated and motion events take a different path but the point is that much of the raw event processing is handled for you by UIKit. All you really need to do is setup your UIWindow and its rootViewController and events will follow a specific delivery path through all your UIKit responders. You can read about the responder chain in detail here https://developer.apple.com/library/ios/documentation/EventHandling/Conceptual/EventHandlingiPhoneOS/event_delivery_responder_chain/event_delivery_responder_chain.html.

Answer 2

When your app starts, the following delegate methods will get called in this order:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions;

- (void)applicationDidBecomeActive:(UIApplication *)application;

In addition, your initial view controller will get created and shown and the following methods will get called (assuming you are using storyboards or xib files):

-(void)viewDidLoad;

-(void) viewWillAppear:(BOOL)animated;

As for "tying your code to events", what type of events are you talking about? What are you trying to accomplish?

You no longer need to use xib files if you are using storyboards, however, you can if you want to.