What is Objective C execution order after UIApplicationMain in Main?

Question

Could someone please explain how control of execution flows in an iOS application? I know that UIApplicationMain is called first from main. Then what? What is the relationship between my defined methods and main? Is it all event-driven or can there be some structured sequence?

I don't mean to be so vague, I just need to know where to start. Perhaps I'm looking at this in the wrong way.

For example, in C++ I would do something like:

#include "myMethods.h"
int main (int argc, char * const argv[]) {

   Method1(); // Initialization
   Method2(); // Opening views and options
   Method3(); // Meat of the program

   return 0;
}

Thanks in advance.

Answer 1

So, as you mentioned, the main() function in main.m is the starting point, which then calls UIApplicationMain(). If you check the docs, you'll see that UIApplicationMain takes four arguments:

• argc,
• *argv[],
• *principalClassName
• *delegateClassName.

The first two of those are just the argument count and variable list passed from main(). But the third and fourth arguments are pointers to NSStrings. The third argument specifies which class should be UIApplication. Unless you intend on subclassing UIApplication, you specify nil for the third argument. The fourth argument specifies which class should be UIApplication's delegate class, which will respond to anything specified in the UIApplicationDelegate Protocol. You don't have to muck with this directly, as it's included in all of the Xcode templates:

int main(int argc, char *argv[])
{
    @autoreleasepool {
    return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

Don't let the NSStringFromClass([AppDelegate class])) bit throw you. That's just a fancy way of specifying the fourth argument so that the right delegate gets called should you later change the name of AppDelegate.m.

UIApplication starts the main event loop and calls -application:didFinishLaunchingWithOptions:, one of the methods its delegate has to handle. Take a look at AppDelegate.m, and you'll find some template code for this method. This is where you can start customizing, creating things that need to be in place before the UIWindow and other instances of UIView get created:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    self.window.rootViewController = self.viewController;
    [self.window makeKeyAndVisible];
    return YES;
}

So, now the application window and root view controller are defined, and the app is off and running.

All of this, and quite a bit more, is excellently explained here: http://oleb.net/blog/2012/02/app-launch-sequence-ios-revisited/

Answer 2

As you said UIApplicationMain creates an application execution in the system. Among the stuff the application loading process does, I assume you are interested in what is relevant to a specific application. Also I assume a typical case, which is illustrated in many project templates that Xcode provides.

The application loading process looks into the application's information property list. There it finds 'Main nib file base name', and the UIApplication instance of your application loads the corresponding nib file from the application bundle. This nib file specifies an application delegate class, and tells to connect an instance of the class to the delegate property of your UIApplication instance.

Depending on the main nib file, other objects may be created and connected as well, for example, the application's window, the main view controller, etc.

Now the loading sequence ends, and everything is all event-driven, starting from your application delegate class to get the famous -applicationDidFinishLaunching: message.

Answer 3

From Apple Documents -

The application life cycle constitutes the sequence of events that occurs between the launch and termination of your application. In iOS, the user launches your application by tapping its icon on the Home screen. Shortly after the tap occurs, the system displays some transitional graphics and proceeds to launch your application by calling its main function. From this point on, the bulk of the initialization work is handed over to UIKit, which loads the application’s main nib file and readies the event loop.

Application Life Cycle