Intro

All across iOS dev blogs and posts you read about AppDelegate being deprecated in the next major iOS release.

  • AppDelegate has been a go-to for app lifecycle handling since iOS 2.
  • From iOS 13, SceneDelegate lives alongside AppDelegate handling per-scene lifecycle.

Do the methods applicationWillEnterForeground and sceneWillEnterForeground actually behave the same?

Difference 1 - App-wide vs per-scene calls

  • AppDelegate’s will enter foreground
    • Fires once when the entire app moves from background to foreground
  • SceneDelegate’s will enter foreground
    • Fires once per scene as each one enters foreground

This behaviour is reflected in each method’s signature (look on arguments) ⤵️

// AppDelegate
func applicationWillEnterForeground(_ application: UIApplication) {}

// SceneDelegate
func sceneWillEnterForeground(_ scene: UIScene) {}

Difference 2 - App start behaviour

On a fresh lunch:

  • AppDelegate’s will enter foreground function is not called
  • SceneDelegate’s will enter foreground function is called

diff-2

Takeaways

  • Migrate with care - when transferring logic from AppDelegate to SceneDelegate, remember it now runs per scene and may fire multiple times.
  • Do not assume - even though both delegates exposes “will enter foreground” method, their behaviour isn’t identical (app start behaviour 👀).

Remember - AppDelegate will still stick around for a while. You don’t need to panic and plan ASAP refactors. Always check docs and verify behaviour in your app before making crucial changes.

PDF version ⤵️ Will enter foreground or won’t?

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