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Best iOS Interview Questions and Answers


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Best iOS Interview Questions and Answers
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1) What is the difference between retain & assign?

Assign creates a reference from one object to another without increasing the source’s retain count.
if (_variable != object)
{
[_variable release];
_variable = nil;
_variable = object;
}

Retain creates a reference from one object to another and increases the retain count of the source object.

if (_variable != object)
{
[_variable release];
_variable = nil;
_variable = [object retain];
}

2) Does Objective-C contain private methods?

NO, there is nothing called a private method in Object-C programming. If a method is defined in .m only then it becomes protected. If in .h,it is public.

If you really want a private method then you need to add a local category/ unnamed category/ class extension in the class and add the method in the category and define it in the class.m

3) What is Abstract class in Cocoa?

Cocoa doesn’t provide anything called abstract. We can create a class abstract that gets checked only at runtime while it is not checked at compile time.

@interface AbstractClass : NSObject
@end
@implementation AbstractClass
+ (id)alloc{
if (self == [AbstractClass class]) {
NSLog(@"Abstract Class can’t be used");
}
return [super alloc];
@end

4) What is the difference between atomic and nonatomic properties? Which is the default for synthesized properties?

Properties specified as atomic always return a fully initialized object. This also happens to be the default state for synthesized properties. But, if you have a property for which you know that retrieving an uninitialized value is not a risk (e.g. if all access to the property is already synchronized via other means), then setting it to nonatomic can give you better performance than atomic.

5) What is Core Data ?

Core Data is not an ORM or object-relational mapper. Nor is it a database. Instead, Core Data is an object graph manager which also has the ability to persist object graphs to a persistent store, on a disk.

6) Features of Swift?

  • Faster, easier to use Strings that retain Unicode correctness and add support for creating, using and managing substrings
  • Smart key paths for type-safe, efficient, extensible key value coding for Swift types
  • Enhancements to creating and manipulating Dictionary and Set types
  • Extends support of archival and serialization to struct and enum types and enables type-safety for serializing to external formats such as JSON and plist
  • Enforced exclusive access to memory

7)Explain about MVC?

The Model-View-Controller (MVC) design pattern assigns objects in an application one of three roles: model, view, or controller. The pattern defines not only the roles objects play in the application, it defines the way objects communicate with each other. Each of the three types of objects is separated from the others by abstract boundaries and communicates with objects of the other types across those boundaries. The collection of objects of a certain MVC type in an application is sometimes referred to as a layer—for example, model layer.

8) Difference between let and var?

  • Let is an immutable variable, meaning that it cannot be changed
  • Var is a mutable variable, meaning that it can be changed

9) Whats the difference between weak and strong ?

strong property is one where you increment the reference count of the object. If object A has a strong reference to B, and no other object is referencing B, B has count 1 Now, if B wants to have a reference to A, we would want to use a weak reference.

Weak references don’t increment the reference count of the object. So in this particular case, if A has no other objects referencing it but B, A’s count would be 0 given B’s weak reference.

10) What is a memory leak?

memory leak is a type of resource leak that occurs when a computer program incorrectly manages memory allocations in such a way that memory which is no longer needed is not released. In object-oriented programming, a memory leak may happen when an object is stored in memory but cannot be accessed by the running code.

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