iOS Developers

          iOS Interview questions for beginners

1. What is the difference between atomic and nonatomic?

atomic: It is the default behavior. If an object is declared as atomic then it becomes thread-safe.

Thread-safe means, at a time only one thread of a particular instance of that class can

have the control over that object.

nonatomic:  It is not thread-safe. You can use the non-atomic property attribute to specify

that synthesized accessors simply set or return a value directly, with no guarantees about

what happens if that same value is accessed simultaneously from different threads. For this

reason, it’s faster to access a non-atomic property than an atomic one.

2. What is @Dynamic?

It tells the compiler that getter and setter are not implemented by the class but by some other class. May be super class or child class.

Example: Core Data

The Managed Object classes have properties defined by using @dynamic.

3. What are Properties?

Properties enables us to expose all the fields in your class so that you can control how values

are set or returned.

4. What is Delegate?

A delegate is an object that acts on behalf of, or in coordination with, another object when that object encounters an event in a program.

5. What is Protocol?

A protocol is a group of related properties and methods that can be implemented by any class.

Creating Protocols:

StreetLegal.h

#import <Foundation/Foundation.h>

@protocol StreetLegal <NSObject>

- (void)signalStop;

- (void)signalLeftTurn;

- (void)signalRightTurn;

@end

Using Protocols:

Bicycle.h

#import <Foundation/Foundation.h>

#import "StreetLegal.h"

@interface Bicycle : NSObject <StreetLegal>

- (void)startPedaling;

- (void)removeFrontWheel;

- (void)lockToStructure:(id)theStructure;

@end

6. What are Strong and Weak properties?

A 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 (A owns, or

needs to exist B). 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.

7. Explain different App States in iOS?

Not running - The app has not been launched or was running but was terminated by the system.

Inactive        - The app is running in the foreground but is currently not receiving events.

Active           - The app is running in the foreground and is receiving events.

Background - The app is in the background and executing code.

Suspended    -  The app is in the background but is not executing code.

8.  What are Web Services and Explain?

Web Service is a collection of protocols used for exchanging data between applications. Generally in iOS we use web services like SOAP and REST API.

9. Explain SQLite functions in iOS?

sqlite3_open(): This function creates and opens an empty database with the specified

filename argument. If the database already exists it will only open the database.

sqlite3_close(): This function should be used to close a previously opened SQLite database

connection.

sqlite3_prepare_v2(): To execute an SQL statement it first needs to be compiled into

byte-code and that is exactly what this function is doing.

sqlite3_step( ): This function deletes a previously prepared SQL statement from

memory.

sqlite3_finalize(): This function deletes a previously prepared SQL statement from

memory.

sqlite3_exec(): Combines the functionality of sqlite3_prepare_v2(), sqlite3_step() and

sqlite3_finalize() into a single function call.

sqlite3_column_<type>(): This returns information about a single column of the

current result row of a query. Typical values for <type> are text and int.

10. What is Singleton class in iOS?

The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance. In Singleton patterns two types are there.

1. Structural Patterns

2. Behavioral Patterns.

@interface SomeManager : NSObject

             + (id)singleton;

 @end

 @implementation SomeManager

            + (id)singleton {    

                                 static id sharedMyManager = nil; 

                                 @synchronized([MyObject class]){ 

                                                     if (sharedMyManager == nil) { 

                                                                         sharedMyManager = [[self alloc] init]; 

                                                      } 

                                 }

                                return sharedMyManager;

            }

 @end

//using block

+ (id) singleton {

    static SomeManager *sharedMyManager = nil;

    static dispatch_once_t  onceToken;

    dispatch_once(&onceToken, ^{

        sharedMyManager = [[self alloc] init];

    });

    return sharedMyManager;

}

11. Explain MVC Design Pattern?

MVC means Model View Controller. It is a design pattern that defines how to separate out logic when implementing user interfaces

The Model represents data in an application and can be implemented using any NSObject, including data collections like NSArray and NSDictionary. 

In iOS, Apple provides UIView as a base class for all Views, UIViewController is provided to support the Controller which can listen to events in a View and update the View when data changes.

Some of the pitfalls that people hit are bloated UIViewController and not separating out code into classes beyond the MVC format.

12. What is Notification?

Notifications is a process of broadcasting messages to an interested parties. We use Notification to send messages in one to many ways.

13. Explain Difference between Delegate and Notification?

Both are used for sending values and messages to interested parties. A delegate is for one-to-one communication and is a pattern promoted by Apple. In delegation the class raising events will have a property for the delegate and will typically expect it to implement some protocol. The delegating class can then call the delegates protocol methods.

Notification allows a class to broadcast events across the entire application to any interested parties. The broadcasting class doesn't need to know anything about the listeners for this event, therefore notification is very useful in helping to decouple components in an application.

[NSNotificationCenter defaultCenter]

        postNotificationName:@"TestNotification"

        object:self];

14. What are Blocks in iOS?

Blocks are a way of defining a single task or unit of behavior without having to write an entire Objective-C class.You can create a blocks like

 myBlock = ^{

    NSLog(@"This is a block");

 }

15. What are Categories?

Categories enables us to add method to an existing class without need to subclass it. You can also use a category to override the implementation of existing class.

16.What is retain?

retain is required when the attribute is a pointer to an object. The setter method will

increase retain count of the object, so that it will occupy memory in auto release pool.

17. What is pool drain?

The drain keyword is used to release the NSAutoreleasePool.

18. What is Autolayout?

AutoLayout is way of laying out UIViews using a set of constraints that specify the location and size based relative to other views or based on explicit values. AutoLayout makes it easier to design screens that resize and layout out their components better based on the size and orientation of a screen. Constraints include:

• setting the horizontal/vertical distance between 2 views
• setting the height/width to be a ratio relative to a different view
• a width/height/spacing can be an explicit static value

19. What is Multi-threading in iOS?

NSThread creates a low-level thread which can be started by calling start method.

NSThread* myThread = [[NSThread alloc] initWithTarget:self

                                        selector:@selector(myThreadMainMethod:)

                                        object:nil];

[myThread start]; 

NSOperationQueue allows a pool of threads to be created and used to execute NSOperations in parallel. NSOperations can also be run on the main thread by asking NSOperationQueue for the mainQueue.

NSOperationQueue* myQueue = [[NSOperationQueue alloc] init];

[myQueue addOperation:anOperation];

[myQueue addOperationWithBlock:^{

   /* Do something. */

}];

GCD or Grand Central Dispatch is a modern feature of Objective-C that provides a rich set of methods and API's to use in order to support common multi-threading tasks. GCD provides a way to queue tasks for dispatch on either the main thread, a concurrent queue (tasks are run in parallel) or a serial queue (tasks are run in FIFO order).

dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_async(myQueue, ^{

    printf("Do some work here.\n");

});

20. What is Garbage Collection?

Garbage Collection is a Memory Management feature. It manages the allocation and release of the memory to your applications. When the garbage collector performs a collection, it checks for objects in the managed heap that are not executed by the applications.

21. What is Synchronous and Asynchronous web request?

Synchronous web request means user will not get control if thread is executing until task will complete. If you want to download data from server continuously then you can use Synchronous web request. It is Thread-Safe.

Asynchronous web request means user will get control automatically even thread is executing. It is not Thread-Safe.

22. What is NSURLSession?

NSURLSession is the replacement for NSURLConnection from iOS 9. It is same as NSURLRequest and NSURLCache. NSURLSession has three sub classes.

1. NSURLSessionDataTask

2. NSURLSessionUploadTask and

3. NSURLSessionDownloadTask

NSURL *url=[NSURL URLWithString:"http://example.com"];

NSURLRequest *request=[NSURLRequest requestWithURL:url];

NSURLSession *session=[NSURLSession sharedSession];

NSURLSessionDataTask *task=[session dataTaskWithRequest:request

                                                                     completionHandler:^(NSData *data,

                                                                                                           NSURLResponse *response,

                                                                                                           NSError *error)

{

            //Do Something

}];

23. Difference between SQLite and Core Data?

SQLite:

1. It is not a relational database.

2. It is light-weighted (It contains embedded SQL Engine)

3. It works as a part of app.

Core Data: 

1. Operates on objects stored in memory.

2. Non-transactional, single threaded, single user (unless you create an entire abstraction around Core Data which provides these things).

24. Explain about Core Data?

1. Core Data is not a relational database. It is a framework that lets developers store or retrieve data in database in an object oriented way.

2. With Core Data, you can easily map the objects in your apps to the table records in the database without knowing any SQL.

Core Data will work mainly basis of three ways.

1. Managed Object Model- It describes the schema represented by a collection of objects(entities). In Xcode Managed Object Model is defined in a file with extension .xcmodeld.

2. Persistent Store Coordinator- By default SQLite is the persistent store in iOS.Core Data allows developers to setup multiple stores containing different entities. Persistent store coordinator is the party responsible to manage different persistent object stores and save the objects to the stores. You will not interact with persistent store coordinator directly when you are using Core Data.

3. Managed Object Context- Whenever you need to fetch and save objects in persistent store, the context is the component responsible.

25. Explain about Notifications?  

Notifications is a process of broadcasting messages to an interested parties. We use Notification to send messages in one to many way. Notifications will work by using device tokens. There are two types of notifications.

1. Local Notifications.

2. Push Notifications.

In Local Notifications we will schedule time to broadcast notifications. We will place Local Notifications code in AppDelegate file.

[[NSNotificationCentre defaultCentre] postNotificationName:@"Testing"

                                                               object:self];

If you want to receive notifications from server then you need to enable Push Notification feature. Your app will receive notifications from server if any event occurs.

26. What is Automatic Reference Counting (ARC)?  

ARC is a compiler-level feature that simplifies the process of managing the lifetimes of Objective-C objects. Instead of you having to remember when to retain or release an object, ARC evaluates the lifetime requirements of your objects and automatically inserts the appropriate method calls at compile time.

27. What is posing in iOS?

Objective-C permits a class to entirely replace another class within an application. The replacing class is said to “pose as” the target class. All messages sent to the target class are then instead received by the posing class. 

28. InApp purchase product type

1. Consumable products must be purchased each time the user needs that item. For example, one-time services are commonly implemented as consumable products.

2. Non-consumable products are purchased only once by a particular user. Once a non-consumable product is purchased, it is provided to all devices associated with that user’s iTunes account. Store Kit provides built-in support to restore non-consumable products on multiple devices.

3. Auto-renewable subscriptions are delivered to all of a user’s devices in the same way as non-consumable products. However, auto-renewable subscriptions differ in other ways. When you create an auto-renewable subscription in iTunes Connect, you choose the duration of the subscription. The App Store automatically renews the subscription each time its term expires. If the user chooses to not allow the subscription to be renewed, the user’s access to the subscription is revoked after the subscription expires. Your application is responsible for validating whether a subscription is currently active and can also receive an updated receipt for the most recent transaction.

4. Free subscriptions are a way for you to put free subscription content in Newsstand. Once a user signs up for a free subscription, the content is available on all devices associated with the user’s Apple ID. Free subscriptions do not expire and can only be offered in Newsstand-enabled apps.

29. What is KVC and KVO? Give an example of using KVC to set a value.

KVC stands for Key-Value Coding. It's a mechanism by which an object's properties can be accessed using strings at runtime rather than having to statically know the property names at development time.

KVO stands for Key-Value Observing and allows a controller or class to observe changes to a property value.

Let's say there is a property name on a class:

@property (nonatomic, copy) NSString *name;

We can access it using KVC:

NSString *n = [object valueForKey:@"name"];

And we can modify its value by sending it the message:

[object setValue:@"Mary" forKey:@"name"];

30. What is the Responder Chain?

When an event happens in a view, for example a touch event, the view will fire the 

event to a chain of UIResponder objects associated with the UIView. The first

UIResponder is the UIView itself, if it does not handle the event then it

continues up the chain to until UIResponder handles the event. The chain will

include UIViewControllers, parent UIViews and their associated

UIViewControllers, if none of those handle the event then the UIWindow is asked

if it can handle it and finally if that doesn't handle the event then the

UIApplicationDelegate is asked.

31. Enlist the methods to achieve Concurrency in iOS.

The following listed are the methods to achieve concurrency functionality in iOS:

1. Threads

2. Dispatch Queues

3. Operation Queues

32. Differentiate between Release and Pool Drain.

The release keyword is used to free a memory location in the system which is not being

utilized. The drain keyword is used to release the NSAutoreleasePool.

33. What is a Collection?

A Collection is a Foundation Framework Class that is used to Manage and Store the group of

Objects. The primary role of a Collection is to store Objects in the form of

either a Set, a Dictionary or an Array.

34. Explain the significance of AutoRelease.

When you send an Object AutoReleasemessage, it gets added to the Local AutoRelease Pool. 

When the AutoRelease Pool gets destroyed, the Object will receive a Release message. The 

Garbage Collection functionality will destroy the object if it has the RetainCount as Zero.

35. What is the First Responder and Responder Chain?

A Responder Chain is a hierarchy of Objects that can respond to the events received. The

first object in the ResponderChain is called the First Responder.





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