Proper way to exit iPhone application?

On the iPhone there is no concept of quitting an app. The only action that should cause an app to quit is touching the Home button on the phone, and that's not something developers have access to.

According to Apple, your app should not terminate on its own. Since the user did not hit the Home button, any return to the Home screen gives the user the impression that your app crashed. This is confusing, non-standard behavior and should be avoided.

Have you tried exit(0)?

Alternatively, [[NSThread mainThread] exit], although I have not tried that it seems like the more appropriate solution.

Its not really a way to quit the program, but a way to force people to quit.

UIAlertView *anAlert = [[UIAlertView alloc] initWithTitle:@"Hit Home Button to Exit" message:@"Tell em why they're quiting" delegate:self cancelButtonTitle:nil otherButtonTitles:nil];
[anAlert show];

After some tests, I can say the following:

• using the private interface : [UIApplication sharedApplication] will cause the app looking like it crashed, BUT it will call - (void)applicationWillTerminate:(UIApplication *)application before doing so;
• using exit(0); will also terminate the application, but it will look "normal" (the springboard's icons appears like expected, with the zoom out effect), BUT it won't call the - (void)applicationWillTerminate:(UIApplication *)application delegate method.

My advice:

1. Manually call the - (void)applicationWillTerminate:(UIApplication *)application on the delegate.
2. Call exit(0);.

You should not directly call the function exit(0) as it will quit the application immediately and will look like your app is crashed. So better to show users a confirmation alert and let them do this themselves.

Swift 4.2

func askForQuit(_ completion:@escaping (_ canQuit: Bool) -> Void) {
    let alert = UIAlertController(title: "Confirmation!", message: "Do you want to quit the application", preferredStyle: .alert)
    alert.addAction(UIAlertAction(title: "Yes", style: UIAlertAction.Style.default, handler: { (action) in
        alert.dismiss(animated: true, completion: nil)
        completion(true)
    }))
    alert.addAction(UIAlertAction(title: "No", style: UIAlertAction.Style.cancel, handler: { (action) in
        alert.dismiss(animated: true, completion: nil)
        completion(false)
    }))
    self.present(alert, animated: true, completion: nil)
}

/// Will quit the application with animation
func quit() {
    UIApplication.shared.perform(#selector(NSXPCConnection.suspend))
    /// Sleep for a while to let the app goes in background
    sleep(2)
    exit(0)
}

Usage:

self.askForQuit { (canQuit) in
     if canQuit {
         self.quit()
     }
}

This has gotten a good answer but decided to expand a bit:

You can't get your application accepted to AppStore without reading Apple's iOS Human Interface Guidelines well. (they retain the right to reject you for doing anything against them) The section "Don't Quit Programmatically" http://developer.apple.com/library/ios/#DOCUMENTATION/UserExperience/Conceptual/MobileHIG/UEBestPractices/UEBestPractices.html is an exact guideline in how you should treat in this case.

If you ever have a problem with Apple platform you can't easily find a solution for, consult HIG. It's possible Apple simply doesn't want you to do it and they usually (I'm not Apple so I can't guarantee always) do say so in their documentation.

Hm, you may 'have to' quit the application if, say, your application requires an internet connection. You could display an alert and then do something like this:

if ([[UIApplication sharedApplication] respondsToSelector:@selector(terminate)]) {
    [[UIApplication sharedApplication] performSelector:@selector(terminate)];
} else {
    kill(getpid(), SIGINT); 
}

In addition to the above, good, answer I just wanted to add, think about cleaning up your memory.

After your application exits, the iPhone OS will automatically clean up anything your application left behind, so freeing all memory manually can just increase the amount of time it takes your application to exit.

Swift 4.2 (or older)

Library called Darvin can be used.

import Darwin

exit(0) // Here you go

NB: This is not recomanded in iOS applications.

Doing this will get you crash log.

In iPadOS 13 you can now close all scene sessions like this:

for session in UIApplication.shared.openSessions {
    UIApplication.shared.requestSceneSessionDestruction(session, options: nil, errorHandler: nil)
}

This will call applicationWillTerminate(_ application: UIApplication) on your app delegate and terminate the app in the end.

But beware of two things:

• This is certainly not meant to be used for closing all scenes. (see https://developer.apple.com/design/human-interface-guidelines/ios/system-capabilities/multiple-windows/)

• It compiles and runs fine on iOS 13 on an iPhone, but seems to do nothing.

More info about scenes in iOS/iPadOS 13: https://developer.apple.com/documentation/uikit/app_and_environment/scenes