Optimizing App Launch

Launch phases

1. System InterfaceSeparate in two phases:

• DYLD3Dynamic Linker: loads shared libraries and frameworks

• libSystem Init: initializes app low level components

1. Static Runtime init

• Separate in two phases:

  • Initializes the language runtime (objc/swift)

  • Expose dedicate initialization API in frameworks.

  • Reduce impact to launch by avoiding +[Class load]

  • Use [Class initialize] to lazily conduct static initialization

1. UIKit init

• This is when the system initializes UIApplication and UIApplicationDelegate

1. Application init

• Invokes UIApplicationDelegate app lifecycle callbacks UIApplicationWillFinishLaunching / didFinish / DidBecomeActive

• (iOS 13 and later) We now get UISceneDelegate UI lifecycle callbacks for each scene: willConnectToSession:, sceneWillEnterForeground:, sceneDidBecomeActive.

1. Initial Frame Render

• this is when the app views are created, their layout is performed, and they’re drawn. loadView viewDidLoad layoutSubviews

• tips:

  • reduce the number of views launched at the beginning

  • flat view hierarchies and lazily load views not needed during launch

1. Extended

• apps specific logic

How to test our app launch time

We have an instrument entirely dedicated to analyze launch times: this will split the app launch in all the sections seen above and highlight our code that executes during this phase.

The new XCTest Metrics is also a very useful and lightweight way to make sure to not incur in some huge slowdowns during development.