Note on iOS application launch process

2 minute read
ios

Entry point

  • When the app launches, system needs to know where to find the compiled binary inside app’s bundle 💊 Get name of the binary from Info.plist file with Executable file key (CFBundleExecutable), whose value is usually the same as $(PRODUCT_NAME) Build Setting
  • System locates, loads binary file and links any needed frameworks
  • Where systems call into binary’s code to make it run?

With Objective-C app → entry point is main()

  • setup memory management environment - @autoreleasepool {}
  • call built-in UIApplicationMain function to get the app running
1int main(int argc, char * argv[]) {
2    NSString * appDelegateClassName;
3    @autoreleasepool {
4        // Setup code that might create autoreleased objects goes here.
5        appDelegateClassName = NSStringFromClass([AppDelegate class]);
6    }
7    return UIApplicationMain(argc, argv, nil, appDelegateClassName);
8}

with Swift app

  • notice the @main attribute on AppDelegate.swift, which does all thing that has been done on main() Objective-C app.
  • Can create a custom main function. Create new file with the name as main.swift:
1import UIKit
2
3UIApplicationMain(
4    CommandLine.argc,
5    CommandLine.unsafeArgv,
6    nil,
7    NSStringFromClass(AppDelegate.self)
8)
  • From Swift 5.3, can create custom struct in a file with any name, just required to give it a static function main and trigger UIApplicationMain:
 1import UIKit
 2
 3@main
 4struct EntryPoint {
 5    static func main() {
 6        UIApplicationMain(
 7            CommandLine.argc,
 8            CommandLine.unsafeArgv,
 9            nil,
10            NSStringFromClass(AppDelegate.self)
11        )
12    }
13}

Events of UIApplicationMain

  1. It creates shared application instance
  2. Create application instance’s delegate. See above the 4th argument, passing into NSStringFromClass(AppDelegate.self) specifies that AppDelegate will be the app delegate class.
  3. Turn to app delegate, call application(_:didFinishLaunchingWithOptions:), can do some initializations here
  4. Create UISceneSession, UIWindowScene and app’s window scene delegate. Entry Application Scene Manifest in the file Info.plist declare the class of the window scene delegate instance.
  5. If there is storyboard defined in Info.plist, UIApplicationMain will load and find the initial view controller and the associated class, UIApplicationMain will initialize it
  6. Create app’s window. This window is assigned to scene delegate’s window. Then, assign the initial view controller instance to window’s rootViewController property
  7. Call scene delegate method scene(_:willConnectTo:options:)
  8. Now, time to make UI to appear, UIApplicationMain calls makeKeyAndVisible of UIWindow
  9. Window is about to appear. window turn to the root view controller, the main view of that VC will be obtained. If VC get its view from nib, it is loaded and its objects are instantiated and initialized.
  10. Time for some methods of view controller lifecycle will be called, as viewDidLoad(), root view controller’s main view is placed into window, which is visible to user.

UIApplicationMain has still been running, waiting for user to interact , maintain the event loop