objective-c-patterns

name	objective-c-patterns
description	Use when writing or reviewing Objective-C — including adding features to existing ObjC pages — debugging retain cycles, reasoning about blocks or ARC, using runtime features, or bridging Objective-C with Swift.

Objective-C Patterns

When to Use

Use this skill when the task involves Objective-C language behavior rather than app-specific product logic:

• Writing or reviewing Objective-C classes, categories, or protocols
• Debugging ownership bugs, leaks, or EXC_BAD_ACCESS
• Choosing between delegates, blocks, target-action, KVO, or runtime hooks
• Evaluating swizzling, forwarding, associated objects, or other dynamic behavior
• Designing Objective-C and Swift boundaries for mixed-language codebases

Start with the router, then open only the references needed for the current question.

Operating Rules

• Prefer simple Objective-C language features before reaching for runtime mutation.
• Nil-check optional blocks before invocation, for example if (completion) { completion(...); }.
• Treat retain cycles as ownership design bugs, not cleanup chores.
• Prefer inheritance, composition, or explicit integration points before any hook or swizzle.
• Use swizzling only when a direct override, wrapper, delegate, or explicit integration point is unavailable.
• Prefix category APIs with a project-specific abbreviation (e.g. xx_), keep categories lightweight, and move complex stateful logic into real classes.
• Prefix private methods with _.
• In getter methods, build the object into a local variable and assign _ivar as the final statement; see references/coding-conventions.md.
• Wrap multi-property object initialization in a ({ }) statement expression to group creation and configuration; see references/coding-conventions.md.
• Check respondsToSelector: before invoking any optional delegate method; see references/coding-conventions.md.
• Prefer RACObserve over raw KVO when the project depends on ReactiveObjC; see references/kvc-kvo-and-associations.md.
• Group methods with #pragma mark - and split very large implementations into focused categories when it improves readability.
• Place - (void)dealloc near the top of @implementation so teardown stays visible during reviews.
• Guard uncertain collection indexing before subscripting, and prefer firstObject or lastObject for boundary access.
• Use early returns for nil, invalid state, or failed preconditions instead of nesting conditionals.
• Route bridging questions to the interop reference before proposing mixed-language APIs.
• Keep dynamic behavior local and easy to remove. If a pattern cannot be explained in a few sentences, it is probably the wrong default.
• For crash or leak triage, confirm the symptom first, then narrow to ownership, threading, or runtime mutation before editing code.

Topic Router

Consult the matching reference before giving implementation advice:

Topic	Reference
Blocks, captures, ARC ownership	references/blocks-and-memory.md
Runtime APIs and swizzling	references/runtime-and-swizzling.md
Forwarding and dynamic dispatch	references/message-forwarding.md
KVC, KVO, RAC observation	references/kvc-kvo-and-associations.md
Coding conventions, getter patterns, delegate guards	references/coding-conventions.md
Queues, locks, legacy async work	references/threading-and-gcd.md
Swift bridging boundaries	references/objc-swift-interop.md
Crash and leak triage	references/diagnostics.md

Memory Checklist

Use this checklist when ownership or lifecycle is unclear:

• Identify who owns the object now, and who should own it after the change.
• Check whether the relationship is one-shot, repeated, or long-lived.
• Keep delegates weak unless the platform contract says otherwise.
• Treat stored blocks as owners of the objects they capture.
• Break cycles by redesigning the relationship first, not by sprinkling nil assignments later.
• Verify notification, KVO, timer, display link, and dispatch source teardown paths.
• For bridged Swift APIs, confirm nullability and collection element types are explicit.

Retain-Cycle Workflow

1. Find the strong ownership loop.
2. Name the expected owner for each node in the loop.
3. Decide which edge should become non-owning or shorter-lived.
4. Replace the pattern with a better boundary:
   • delegate for long-lived callbacks
   • block for one-shot completion
   • injected coordinator or service for shared orchestration
5. Use weak capture only where the ownership model truly requires it.
6. Re-check deallocation with a breakpoint, Instruments, or a temporary dealloc log.

Typical loop shapes:

• controller -> view model -> completion block -> controller
• owner -> timer or display link -> target -> owner
• object A -> object B -> delegate-like property -> object A
• object -> associated object -> block -> object

Quick Diagnostics

Symptom	Likely cause	Reference
Object never deallocates	Block capture or delegate ownership bug	references/blocks-and-memory.md
EXC_BAD_ACCESS after async callback	Object deallocated earlier than expected	references/diagnostics.md
Strange behavior after adding category hooks	Swizzle collision or wrong method exchange timing	references/runtime-and-swizzling.md
Message sent to wrong helper object	Forwarding chain too broad or incomplete	references/message-forwarding.md
Observer crash on property change	KVO lifecycle mismatch	references/kvc-kvo-and-associations.md
UI state updates from background queue	Missing main-thread hop	references/threading-and-gcd.md
Swift sees awkward Objective-C API	Missing nullability or poor naming surface	references/objc-swift-interop.md

Common Mistakes

• Using associated objects to avoid creating a real subclass or stored property owner.
• Swizzling framework methods without collision guards, original implementation checks, or a rollback plan.
• Assuming [weak self] is always correct. Weak capture is a tool, not the ownership model.
• Using blocks for long-lived relationships that should be delegates or explicit collaborators.
• Invoking optional blocks without checking whether they are non-nil.
• Hiding complex behavior in broad categories instead of a subclass or helper object.
• Using raw subscripting on uncertain arrays or sets when firstObject, lastObject, or bounds checks would make the contract explicit.
• Forwarding selectors broadly enough to hide missing APIs or typos.
• Mixing KVO, manual ivar mutation, and direct setter bypasses without understanding notification behavior.
• Exposing Swift generics, structs, or async-only APIs directly to Objective-C callers that cannot consume them safely.
• Treating intermittent crashes as "ARC bugs" before checking lifetime, queueing, and swizzle interactions.