Objective-C Interview Questions for iOS Developers with Answers

Here’s a list of commonly asked interview questions for iOS developers focused on Objective-C, along with demonstrative and concise answers:

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1. What is Objective-C?

Answer:

Objective-C is an object-oriented programming language that extends C by adding Smalltalk-style messaging. It’s primarily used for macOS and iOS development.

2. Explain the difference between id and instancetype.

Answer:

• id: A generic pointer to any Objective-C object. It doesn’t enforce type safety.

Example:

id myObject = @"Hello";

• instancetype: A compile-time type that returns the same type as the class.

Example:

+ (instancetype)sharedInstance;

3. What is a category in Objective-C?

Answer:

A category is a way to add methods to an existing class without modifying the original implementation.

Example:

@interface NSString (Reverse)

- (NSString *)reversedString;

@end

4. What are protocols in Objective-C?

Answer:

Protocols are similar to interfaces in other languages. They define methods that a class must implement.

Example:

@protocol MyProtocol

- (void)doSomething;

@end

5. What is the purpose of @synthesize and @dynamic?

Answer:

• @synthesize: Automatically generates getter and setter methods for a property.

Example:

@synthesize name = _name;

6. How does memory management work in Objective-C?

Answer:

Objective-C uses Automatic Reference Counting (ARC) to manage memory. Objects are automatically retained or released based on strong, weak, or unowned references.

Example:

@property (strong, nonatomic) NSString *name;

@property (weak, nonatomic) id delegate;

7. What is the difference between copy and assign properties?

Answer:

• copy: Creates a new copy of the object (used for immutable objects like NSString).

• assign: Directly assigns the reference (used for primitive data types like int).

8. Explain message passing in Objective-C.

Answer:

Message passing is how methods are called in Objective-C. Instead of direct calls, messages are sent to objects using square brackets.

Example:

[object performAction];

9. What is the use of NS_ASSUME_NONNULL_BEGIN and NS_ASSUME_NONNULL_END?

Answer:

These macros specify that all pointers between them are non-null by default unless marked with _Nullable.

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

Answer:

Exceptions are handled using @try, @catch, and @finally blocks.

Example:

@try {

    // Code that might throw an exception

}

@catch (NSException *exception) {

    NSLog(@"Exception: %@", exception.reason);

}

@finally {

    // Cleanup code

}

11. What is Key-Value Coding (KVC) in Objective-C?

Answer:

KVC allows accessing an object’s properties dynamically at runtime using keys.

Example:

[object setValue:@"value" forKey:@"key"];

NSString *value = [object valueForKey:@"key"];

12. Explain Key-Value Observing (KVO).

Answer:

KVO lets objects observe changes to specified properties of other objects.

Example:

[object addObserver:self forKeyPath:@"property" options:NSKeyValueObservingOptionNew context:nil];

13. What is the difference between nil, Nil, and NULL in Objective-C?

Answer:

• nil: Null pointer to an Objective-C object.

• Nil: Null pointer to a class.

• NULL: Null pointer for C pointers.

14. What is a block in Objective-C?

Answer:

A block is a chunk of code that can be passed around and executed later.

Example:

void (^simpleBlock)(void) = ^{

    NSLog(@"This is a block!");

};

simpleBlock();

15. Explain the use of @autoreleasepool.

Answer:

@autoreleasepool is used to manage temporary objects in memory, ensuring they are released promptly.

Example:

@autoreleasepool {

    NSString *temp = [NSString stringWithFormat:@"Hello"];

}

16. What is the difference between weak and assign?

Answer:

• weak: Non-owning reference to an object that is automatically set to nil when the object is deallocated.

• assign: Non-owning reference, but not set to nil automatically.

17. How does NSObject work in Objective-C?

Answer:

NSObject is the root class of most Objective-C classes. It provides fundamental methods like alloc, init, and dealloc.

18. What is the purpose of respondsToSelector:?

Answer:

It checks if an object can respond to a specific method at runtime.

Example:

if ([object respondsToSelector:@selector(methodName)]) {

    [object methodName];

}

19. What is dynamic method resolution?

Answer:

Objective-C allows adding methods to classes at runtime using resolveInstanceMethod.

Example:

+ (BOOL)resolveInstanceMethod:(SEL)sel {

    // Add method implementation dynamically

    return YES;

}

20. What are the differences between class and instancetype?

Answer:

• class: Specifies the return type as a class type.

• instancetype: Returns an instance of the class in which it is declared.