App Resizability

App Resizability Skill

When to Use

Use this skill whenever the request is about any of the following, whether or not it names an API:

• Getting an app ready for, or optimizing it for, the foldable iPhone Duo.
• An app that must adapt when its window changes size while it runs.
• Split-screen multitasking, Stage Manager, or a resizable scene.
• Any deprecated API named in the Scope section below.

The request does not have to be technical. "Get my app ready for the iPhone Duo", "optimize this app for the foldable iPhone", and "make this app resize properly" all activate this skill. iPhone Duo is the foldable iPhone, so treat questions about iPhone Duo or the foldable iPhone as the same request. Treat a request about the foldable iPhone Duo as a request for every task in the Task Registry, because a screen that changes size exposes all of them at once.

Purpose

Modernize UIKit apps to behave correctly on modern iOS by:

• Eliminating references to legacy shared-state APIs
• Migrating from application lifecycle to scene lifecycle
• Supporting dynamic scene sizing and multi-window environments

Scope

This skill performs specific, targeted modernizations in both Swift and Objective-C codebases:

• Replace legacy shared-state APIs with context-appropriate modern APIs
• Migrate to scene-based lifecycle
• Update apps to support a resizable user interface by removing usage of:
  • main screen (UIScreen.mainScreen, UIScreen.main)
  • interface orientation (interfaceOrientation)
  • user interface idiom (userInterfaceIdiom, UI_USER_INTERFACE_IDIOM())
  • assumptions of symmetric safe areas (safeAreaLayoutGuide, safeAreaInsets)
  • application lifecycle in place of scene lifecycle (UIApplicationDelegate)

Core Principles

1. Closest to consumer — Prefer information nearest the point of use (e.g., view's trait collection over window's).

2. Always apply a replacement when the target API is present. A TODO alone is a failure. An empty diff for a file containing the target API is also a failure. If the file contains the target deprecated API and a concrete replacement is feasible under any pattern in the active task's reference file, apply it. Only skip when the target API appears exclusively inside dead code (#if 0/#endif). When uncertain between two valid replacements, pick the one that best fits the user's request rather than producing an empty diff. Never silently skip a file: if you are unwilling to apply a change, talk to the user about possible options — never produce no output for it. Do not get stuck weighing edge cases on simple files; when the substitution is obvious, apply it and move on.

3. TODOs must be actionable. State why the change is needed, what the replacement would look like, and any lifecycle or threading concerns. Place the TODO on its own line above the unchanged code, never inline. A vague TODO ("fix this later") is worse than none, because it consumes review attention and informs nobody.

   Indent every line of an inserted comment to match the line it precedes. A continuation line at a different indent leaves the block misaligned. Re-emitting the following line to realign it edits a line you were not asked to touch.

4. Don't add a redundant TODO when an existing annotation already covers the migration. If the call site already has a #pragma clang diagnostic ignored paired with a bug-report reference, an existing // TODO, or a deprecation comment that points at the migration, do not add another one. Only add a new TODO when it provides additional migration guidance not present in the existing annotation.

5. Ask the user before making a risky code change; fall back to a TODO only when interactive guidance is unavailable. When a replacement risks breaking callers or changing observable behavior (e.g., changing a method signature in a header that other modules import; substituting width > height for orientation when left-vs-right matters), the first move is to ask the user how to proceed. Only when the skill is running non-interactively, or when the user explicitly declines to provide guidance, drop a TODO and move on. This does not apply to standard, drop-in safe replacements specified by the active task's reference file — those must be applied per Core Principle 2.

6. Honor explicit user instructions; otherwise apply the defaults from the task reference file. When the user asks for a specific approach — a particular attribute, parameter name, parameter position, trait source, or fallback behavior — use that exactly. Don't silently substitute what you consider the modern equivalent. When the user is general ("modernize this app", "fix UIScreen.main usages"), apply the defaults from the active task's reference file.

7. Never replace dynamic values with literals — Always keep replacements dynamic.

8. Preserve control flow — Prefer drop-in replacements that maintain the original code structure. Only add guard/early-return patterns when a direct substitution does not work. When editing code around control flow (if/else, switch/case/default, do/catch), verify that the branching structure is preserved after your edit. Never collapse an if/else into sequential execution: both bodies then run unconditionally, which is a critical bug. Keep every branch whose condition is sound, and replace only the deprecated value inside it. Remove a branch only when the active task names that condition itself as the defect and states that the correct behavior is unconditional. In that case delete the condition, keep the body that matches the correct behavior, and name the branch you removed in your summary so a reviewer can check it.

9. Stay in scope — no opportunistic cleanup. Only modify lines containing the target deprecated API for the active task. Do NOT also fix other deprecation that happens to live nearby. Do NOT trim trailing whitespace, reformat blank lines, or "clean up" surrounding formatting. Even if you see an obvious modernization opportunity on an adjacent line, leave it alone — each task is independent and out-of-scope edits convert a successful in-scope change into a warning.

10. Extract repeated expressions — When the same replacement value is used multiple times in a scope, extract it into a named local variable.

11. Never walk global scene/window state — Never use UIApplication.shared, UIDevice.current, UIScreen.main, or other shared objects as a replacement. If no local object is available, modify the method to accept a new parameter and update its callers to pass one.

12. Complete patterns — atomic, never partial — Every multi-part pattern the active task defines requires ALL of its parts applied together as a single unit. When the active task requires both an API replacement AND a reactive update (e.g., trait change observation), these form a single atomic change — never apply one without the other. If you cannot complete every part the task specifies, do not apply a partial change — either complete the pattern or skip with an explicit reason.

13. Preserve unrelated guards and fallbacks. When removing a reference to the target API, change ONLY that reference. Do not simultaneously delete respondsToSelector: checks, nil guards, if (x != nil) defenses, version checks (#available, @available), or any other defensive logic that wraps the call site — unless the user explicitly asks for it. Each guard exists for an independent reason (selector availability across SDK versions, nil-window safety, feature flags); the modernization touches only the value you came to replace, not the surrounding control flow.

14. Off-target replacement guard. Before editing any line, verify two things: (a) the line contains the target deprecated API for the active task, and (b) you're editing the deprecation the user asked about — not a nearby line that "looks similar."

Prerequisites

Run all three checks once per project, before Phase 1. These checks are read-only: add no key, change no value, and delete no key, in the Info.plist or in the build settings. Report what you read, then continue with the workflow. An app that fails a check cannot resize, whatever the source code says.

Read each key in the target Info.plist and in the build settings. Build settings live in the project file and in any .xcconfig. The build merges every source, so read them all before you report. A key missing from one source proves nothing. When two sources set the same key, the project file wins.

Skip all three checks when the request names files rather than a project. Say nothing about them. Do not search the file system for a project.

Both keys in the last row stay exactly as you found them. Never delete UIRequiresFullScreen, because deletion makes the app resizable at once and the layouts may not be ready. Never add UIRequiresFullScreenIgnoredStartingWithVersion, because its value decides which releases keep the old behavior, and that is the developer's decision. A value of 27 or earlier makes the app fully resizable on iOS 27, which is the opposite of a safe step. The key comes out after the app handles resizing. Confirming that means running the app and resizing it. One view controller can break while the rest is correct.

Workflow