Objective-C Interview Questions and Answers for 10 years experience

1. What is Objective-C, and why is it important in iOS development?

   • Answer: Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. It's important in iOS development because it was the primary language used for building iOS and macOS applications before Swift's rise. While Swift is now preferred, understanding Objective-C is crucial for maintaining legacy codebases and grasping foundational iOS concepts.

2. Explain the difference between `@interface` and `@implementation` in Objective-C.

   • Answer: `@interface` declares the public interface of a class, specifying its properties and methods. It's the contract that other parts of the codebase will interact with. `@implementation` provides the actual code that defines the behavior of those methods. It's where the implementation details are hidden from the public interface.

3. What are properties in Objective-C, and how do they differ from instance variables?

   • Answer: Properties are a high-level way to access instance variables. They encapsulate access methods (getter and setter) and can have attributes like `readonly`, `nonatomic`, `strong`, `weak`, `copy`, etc., which control how the property is accessed and managed. Instance variables are the underlying storage for the property’s value. Properties provide better encapsulation and management compared to directly accessing instance variables.

4. Explain the difference between `strong`, `weak`, and `assign` properties.

   • Answer: `strong`: Retains the object, preventing it from being deallocated as long as the property holds a reference. `weak`: Holds a reference to the object but doesn't retain it, preventing retain cycles. The pointer becomes nil when the object is deallocated. `assign`: Directly assigns the value, without any memory management. Mostly used for primitive data types.

5. What is a retain cycle and how can you prevent it?

   • Answer: A retain cycle occurs when two or more objects retain each other, preventing them from being deallocated even when they are no longer needed. This leads to memory leaks. To prevent it, use `weak` properties where appropriate, typically in situations where one object observes another.

6. Describe the role of the `dealloc` method.

   • Answer: The `dealloc` method is called by the runtime right before an object is deallocated. It's used to release any resources held by the object, like memory allocated dynamically, file handles, or network connections. It's important to clean up properly to avoid memory leaks and resource exhaustion.

7. What are categories and extensions in Objective-C? What are their differences?

   • Answer: Categories add methods to an existing class without subclassing. Extensions add properties and methods to a class but are limited to the current compilation unit (file). Categories are useful for adding functionality to classes you don't control, while extensions are used for internal implementation details.

8. Explain the concept of protocols in Objective-C.

   • Answer: Protocols define a set of methods that a class can adopt. They provide a way to achieve polymorphism and loose coupling. Classes adopting a protocol agree to implement the methods declared in the protocol.

9. What is the difference between `id` and `NSObject`?

   • Answer: `id` is a generic object pointer that can point to any Objective-C object. `NSObject` is the root class of the Objective-C class hierarchy. All Objective-C objects inherit from `NSObject`. `id` offers flexibility but lacks type safety, while `NSObject` provides type checking at compile time.

10. Explain the role of the ARC (Automatic Reference Counting) memory management system.

    • Answer: ARC automatically handles memory management in Objective-C by tracking object references and releasing objects when they are no longer needed. This reduces the risk of memory leaks and simplifies the development process.

11. What is KVO (Key-Value Observing)? Provide an example of its usage.

    • Answer: KVO is a mechanism that allows objects to be notified of changes to properties of other objects. For example, a view controller might observe a model object's property to update its UI automatically when the property changes. It's implemented using `addObserver:forKeyPath:options:context:` and related methods.

12. What is KVC (Key-Value Coding)? Explain its use cases.

    • Answer: KVC is a mechanism that allows you to access and modify object properties using strings instead of direct method calls. This is useful for data binding, reflection, and working with external data sources where property names are known only at runtime.

13. How do you handle concurrency in Objective-C? What are some common techniques?

    • Answer: Concurrency can be handled using Grand Central Dispatch (GCD) and NSOperationQueue. GCD provides low-level primitives for managing tasks and threads, while NSOperationQueue offers a higher-level abstraction that manages dependencies and priorities between operations.

14. What are blocks in Objective-C and how are they used?

    • Answer: Blocks are anonymous functions that capture their surrounding context. They are used for callbacks, asynchronous operations, and creating closures.

15. Explain the concept of singletons in Objective-C. How do you implement a thread-safe singleton?

    • Answer: A singleton is a design pattern that ensures a class has only one instance and provides a global point of access to it. Thread safety is crucial in multithreaded environments. A common approach involves using a dispatch_once block in GCD.

16. Describe your experience with memory management in Objective-C, both with and without ARC.

    • Answer: (This answer should be tailored to your actual experience. It should detail your experience with manual reference counting, using retain/release, autorelease pools, and the transition to ARC.)

17. How do you handle exceptions in Objective-C?

    • Answer: Objective-C uses exceptions for handling runtime errors, however, it is not widely used in iOS Development. Error Handling is usually done through NSError objects and checking for return values.

Thank you for reading our blog post on 'Objective-C Interview Questions and Answers for 10 years experience'.We hope you found it informative and useful.Stay tuned for more insightful content!