--- name: objective-c description: Expert Objective-C assistance covering message passing, runtime dynamic method resolution, ARC, and Foundation/Cocoa. Use when maintaining Apple legacy codebases, developing native iOS/macOS frameworks, or C++ interop. --- # Objective-C Objective-C is in **maintenance mode**. New features are rare, but it powers the Apple ecosystem's foundation. It allows mixing C++ (`.mm`) loosely. ## When to Use - **Legacy iOS and macOS Codebase Maintenance**: Maintaining established Apple applications built before Swift. - **C/C++ and Swift Bridging (Objective-C++)**: Acting as a high-performance interoperability bridge between native C++ engines and modern Swift. - **Dynamic Runtime Introspection**: Leveraging the dynamic Objective-C runtime for method swizzling and dynamic message forwarding. - **High-Performance Audio & Core Audio Frameworks**: Interfacing with low-level Apple media frameworks where C pointer interop is required. ## Quick Start ```objc #import @interface Person : NSObject @property (nonatomic, copy) NSString *name; @property (nonatomic, assign) NSInteger age; - (void)sayHello; @end @implementation Person - (void)sayHello { NSLog(@"Hello, my name is %@", self.name); } @end ``` ## Core Concepts ### Dynamic Message Passing (`[receiver message]`) Method calls in Objective-C are dynamic messages resolved at runtime via `objc_msgSend`: ```objc #import @interface OrderService : NSObject - (BOOL)processPayment:(double)amount forCustomer:(NSString *)customer; @end @implementation OrderService - (BOOL)processPayment:(double)amount forCustomer:(NSString *)customer { NSLog(@"Charging %@: $%.2f", customer, amount); return amount > 0.0; } @end // Invocation syntax OrderService *service = [[OrderService alloc] init]; [service processPayment:150.0 forCustomer:@"Alice"]; ``` ### Automatic Reference Counting (ARC) & Nullability Annotations Manages object memory automatically at compile time with strong/weak ownership semantics: ```objc @property (nonatomic, strong) NSString *accountNumber; @property (nonatomic, weak) id delegate; // Prevents retain cycles @property (nonatomic, copy) void (^completionHandler)(BOOL success); ``` ### Objective-C++ (`.mm`) Bridging Seamlessly mixes C++ standard library types with Cocoa objects in the same file: ```objc // ProcessorBridge.mm (Objective-C++) #include #import "ProcessorBridge.h" @implementation ProcessorBridge { std::vector _signalBuffer; } - (void)addSample:(double)sample { _signalBuffer.push_back(sample); } @end ``` ## Common Patterns ### Safe Block Self-Capture to Prevent Retain Cycles **Problem**: Strong reference cycles in asynchronous blocks cause permanent memory leaks under ARC. **Solution**: Use the `weakSelf / strongSelf` dance in completion blocks: ```objc __weak typeof(self) weakSelf = self; [self.networkClient fetchProfileWithCompletion:^(NSDictionary *data) { __strong typeof(weakSelf) strongSelf = weakSelf; if (!strongSelf) return; [strongSelf updateUIWithData:data]; }]; ``` ## Best Practices **Do**: - Add Nullability Annotations (`_Nonnull`, `_Nullable`): Ensure smooth, idiomatic bridging into modern Swift codebases. - Use Weak References for Delegates: Mark all delegate properties as `weak` to eliminate memory retain cycles. - Use Objective-C Generics: Parameterize collections (`NSArray *`) for compile-time type safety. - Wrap Bridged Code with Modern Swift Interfaces: Plan incremental migrations by creating Swift wrapper classes. **Don't**: - Start new greenfield Apple applications in Objective-C: Build all new iOS/macOS applications in Swift and SwiftUI. - Invoke methods on deallocated objects without weak-strong dancing: Use `__weak typeof(self) weakSelf = self;` in asynchronous blocks. - Use manual retain/release (MRR): Ensure modern ARC (`-fobjc-arc`) is enabled across all targets. ## Troubleshooting | Error | Cause | Solution | | :--------------------------------------------- | :--------------------------------------------------------- | :--------------------------------------------------------------------------------------------------- | | `unrecognized selector sent to instance ...` | Invoking a method not implemented by the receiving object. | Verify method name and colons (e.g. `doAction:` vs `doAction`), or check with `respondsToSelector:`. | | `EXC_BAD_ACCESS (code=1, address=...)` | Dereferencing deallocated memory or zombie object. | Enable Zombie Objects in Xcode scheme diagnostics to locate deallocated reference. | | `Property with 'copy' attribute is not copied` | Custom setter failed to call `[newValue copy]`. | Implement custom setters using `_prop = [newValue copy];`. | ## References - [Apple Developer Documentation](https://developer.apple.com/) --- Source attribution: G1Joshi Original project: https://github.com/G1Joshi/Agent-Skills/blob/6a74cf03a8a86dc7d993ead3091b41123dbd0181/skills/languages/objective-c/SKILL.md License: MIT License information: https://opensource.org/license/mit Original upstream instructions; downloaded from Dave AI. ## Upstream license notice MIT License Copyright (c) 2026 G1Joshi Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. # Dave AI resource metadata Creator / publisher: G1Joshi Original project: https://github.com/G1Joshi/Agent-Skills/blob/6a74cf03a8a86dc7d993ead3091b41123dbd0181/skills/languages/objective-c/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering Roles: Backend Developer, Full-Stack Developer, Mobile Developer Specialties: Backend, Cross Platform, Mobile Tools: Tool not specified Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: f6d948d2d8a06aea56ac51942e879afa5b6e48c4c6a94160917e346784bb67a8