18 Essential iOS Interview Questions

iOS has a layered architecture. The four layers that make up the iOS Architecture each provide a programming foundation for building applications that run on top of the hardware.
The layers between the Application Layer and the Hardware Layer will improve communication.
A lower-level layer provides all applications' necessary services.
In contrast, graphics and user interface services are provided by an upper-level layer (also known as a high-level layer).

The iPhone OS stack's bottom layer, the Core OS (or Application) Layer, rests directly on top of the device's hardware.

This layer offers low-level networking, access to external accessories, and other functions in addition to the essential operating system services, including memory management, processing of file systems and threads, and memory management.

The service layer's job is to design the services that users or layers above need. Other essential features include iCloud storage, Grand Central Dispatch, in-app purchases, and block objects. The addition of ARC Automatic Reference Counting has enhanced the service layer.

It manages media types like graphics, audio, and video. Thanks to the media layer, we can utilize the entire system's audio, video, and graphic capabilities.

The application layer is another name for the Cocoa Touch Layer. When applications are built, this is where frameworks are made. Additionally, it serves as the operating system's user interface for iOS users. This includes the ability to touch and move.

The Atomic Property, the default property, guarantees that the getter will return a valid value, and the setter will set a valid value. The getter/setter of a specific property can only be accessed by one thread at a time, and all other threads must wait until the first thread releases the getter/setter.

Thus, atomic properties are promises that will always yield a completely initialized object. This is the default state for synthesized properties, so even while it's a good idea to specify atomic to avoid any potential ambiguity, your properties will still be atomic if you leave it off. However, it is slow despite being thread-safe since it ensures the procedure is finished.

Non-atomic attributes allow for simultaneous access by many threads to the getter and setter methods, which increases the risk of inconsistent values. They offer improved access but no assurance of a profit.

So, setting a property to nonatomic can help you improve speed if you know that getting an uninitialized value won't cause problems (for example, if all access to the property is already synchronized via another method).

An iOS application experiences several states as it is being used. The lifecycle state of an application refers to each of these states. The five possible states for an iOS app are listed below:

• Not running: An application that is in the Not Running state has either not been launched or has been shut down or closed by the system.
• Inactive: While the app is leaving or entering its active state, it briefly enters an inactive state. It is currently not ready to take user input or events despite operating in the foreground. This indicates that the application is currently inactive.
• Active: The Active state shows that the app is receiving events and running in the foreground. Foreground apps often operate in this mode by default, and the User Interface is usable.
• Background: Although the application's user interface is hidden in this mode, the iOS system still processes it in the background. Typically, applications go through this stage before being suspended.
• Suspended: When an application is suspended, code is not executed, but it is still running in the background. It does remain in the system memory. However, the system may destroy programs that are in the suspended state without warning when memory is limited.

Note: Commonly asked iOS interview question.

Deep links are URL (Uniform Resource Locator) or universal links that take consumers straight to an app rather than a website or store. Deep links are commonly created using the URL system, but Apple's new Universal Links feature unites your website and app under a single link.

To deep link, you must create a clickable link that launches your program and a clever link that directs them to the desired resource. These connections take users directly to in-app places, saving them the time and effort to search for those pages themselves and greatly enhancing their user experience.

"Running many tasks simultaneously" is what concurrency means. iOS devices can perform background processes (such as downloading or processing data) while keeping a responsive user interface thanks to concurrency.

Grand Central Dispatch (or GCD) and Operations are two iOS features that let you manage multiple activities at once (formally known as NSOperation). iOS has three methods to accomplish concurrency:

• Dispatching lines: They are employed for sequential or concurrent task execution and first-in, first-out (FIFO) task management. This is a simple method for managing your application's concurrent and asynchronous (not occurring simultaneously) operations.
• Threads: A program's ability allows one set of instructions to run independently from other programs. A single program can run many code paths concurrently, thanks to threads. Having a thread is extremely helpful when you need to complete a time-consuming activity without interfering with the remainder of the program's execution.
• Operation Queues: Operation queue objects are called based on their readiness and priority. Operation queues, constructed on top of GCD, are high-level abstractions of queueing models (Grand Central Dispatch). Therefore, it is possible to run tasks concurrently in an object-oriented fashion, just like GCD.

SpriteKit is a toolkit to help game creators generate animated 2D elements and objects for casual games more quickly and easily. It enables two-dimensional drawing of forms, particles, text, photos, and movies.

SceneKit is an iOS framework adapted from OS X and aids in the production of 3D visuals. You can create 3D animated sceneries and effects for your iOS games and applications using SceneKit.

Note: Commonly asked iOS interview question.

With assign, a reference is established between two objects without adding to the source's retain count, which measures how many objects are "hanging onto" one another. Instead of copying or storing the value, it simply assigns it to the instance variable.

Example:

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

Instead of duplicating or storing the data, Assign will create a setter in this case that directly assigns the value to the instance variable.

Retain method can be used to construct references between different objects and raise the source object's retain count.

Example:

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

Until you are finished utilising an object, a retain message prohibits its deallocation.

Today's fastest-growing programming language is Swift, which Apple developed. Swift is the standard for iOS development, outperforming the well-known Objective-C by a wide margin. It's a completely original language used to produce software for Apple's operating systems.

You can discover type inference, optional, generics, and other higher-order functions in Swift because it incorporates all the characteristics of other contemporary languages. It works with watchOS, tvOS, macOS, and iOS.

The Swift programming language was created to make it simple for developers to create and maintain accurate programs. It provides the following features:

• Swift is a productive programming language. It is crucial to test code before using it in production. Before being utilized in production, Apple Swift strips out any dangerous code.
• Swift's syntax is straightforward and user-friendly, as expected by engineers. Swift's grammar characteristics let you create more expressive code. Swift's straightforward syntax makes it simpler to read and write. Swift code is simpler for developers to create since it resembles plain English more, so it takes less time searching for incorrect code.
• Swift is fully interoperable with many platforms, including iOS, macOS, tvOS, watchOS, Linux, and many more. This implies that you can create software that works with all operating systems.
• Swift is an open-source framework. Swift needed to be an open-source piece of technology to become a standard programming language. All Apple platforms are supported by Swift, which also makes programming simpler, faster, and safer.
• Swift provides complete compatibility with Objective-C. The Swift syntax allows programmers to import frameworks from Objective-C. Swift programming can make use of Objective-C libraries and classes.

Generics are a key component of Swift, and a sizable portion of the standard library is written in generic code. For instance, Swift's 'Array' and 'Dictionary' types are examples of generic collections. Using generic code, you can write reusable, adaptable types and functions that work with any data type. A clearer and less detailed code can be written about the underlying data.

In the example above, we wrote a function called Swapping() to swap numbers. It requires the int type parameters x and y. As seen in the output below, the values of x and y are switched following the swapping.

Before Swapping: 10 and 50 
After Swapping: 50 and 10

Note: Commonly asked iOS interview question.

With KVC (Key-Value Coding), object properties can be accessed at runtime using strings rather than by knowing the names of the properties in advance while developing the application.

KVO (Key-Value Observing) is one of the techniques for tracking program state changes in Swift and Objective-C. KVO makes it possible for other objects to keep track of changes to instance variables if an object has them.

Testing-driven development (TDD) is a technique that software engineers can utilize. In TDD, programmers first plan the software's features before creating test cases for each feature and putting it into use. We can learn more about the effectiveness of the implementation (does it work) and the design quality by using test-driven development.

The test case should initially fail because the code has not been implemented, known as the “red phase”. The "green phase" next stage involves writing code to ensure the test case passes and doesn't interfere with existing test cases or components.

The developer should then optimize efficiency and clean up and maintain the codebase before refactoring the implementation of the code. Then, the process should be repeated each time a fresh test case is introduced.

There is no assurance that the block being enqueued will be executed on the main thread when the code above dispatches work using NSOperationQueue's function addOperationWithBlock.

You'll see that the block's body has an updated version of the UILabel's content. Undefined behavior may result from UI updates not being carried out on the main thread. This code may take a while to start acting incorrectly, but UI updates must always occur on the main thread.

Changing the block's body so that the update is re-enqueued using the main thread's queue is the simplest way to resolve the potential problem:

[waitQueue addOperationWithBlock:^{
    [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        self.alert.text = @"Thanks!";
    }];
}];

Note: Commonly asked iOS interview question.

Dynamic dispatch, put simply, is the process by which a program determines at runtime which implementation of a specific method or function it needs to call. Dynamic dispatch decides whether to invoke the subclass implementation of the method or the parents when a subclass overrides a method of its superclass.

Data persistence saves information on the hard drive so that it can be accessed and not changed when the app is opened again. Data storage techniques range from straightforward to complex:

• For storing data in between, data structures like arrays, dictionaries, sets, and others are ideal.
• Keychains and NSUserDefaults are both straightforward key-value stores. Keychains is secure however, NSUserDefaults is not.
• Data can be stored (serialized or not) to or read from a disc using a file or disc storage using NSFileManager.
• Complex querying mechanisms work well when implemented using relational databases like SQLite.

"areEqual" will be output by the code.

Although it might seem clear, it's not. Checking if two pointers point to the same object is equivalent to comparing their values. If and only if they genuinely point to the same object, pointers will have the same value (whereas pointers to different objects will not have the same value, even if the objects they point to have the same value).

FirstUserName and secondUserName are references to string objects in the code. Even though the objects they point to each have the same value, one may easily believe they are pointing to separate string objects.

Since both pointers are pointing to the same address and the iOS compiler reuses references to string objects with the same value instead of allocating identical string objects again, the condition evaluates to true.

Note: Commonly asked iOS interview question.

Apple's iOS support both synchronous and asynchronous tasks. During synchronous operations, each task is carried out separately. As a result, the last task must be finished before moving on to additional duties. Asynchronous processes take place in parallel. You will be informed when tasks in the background are finished. You may handle several requests simultaneously with asynchronous programming, which allows you to finish more jobs faster.

Completion handlers are functions that other functions send as parameters. Since we cannot predict when asynchronous processes will finish, they are accustomed to dealing with their responses. When an action, such as an API call is finished, completion handlers alert the program. The program is alerted that subsequent action must be taken.

The preceding code creates a precise CompletionHandler class with a single function called count that counts from 0 to 50. A network request to https://scaler.com will be sent whenever it hits 25 (a random value). The request will print the Received response once it has been fulfilled.

As soon as the code executes, you will see all the numbers displayed in the console; the Received response won't appear until all the other numbers have been printed.

A retain cycle is exemplified in this situation.

Both, the children array and each TTChild object added to it will remain in the parent. Each newly generated child object will likewise keep its parent, ensuring that even after the last external reference to the parent has been removed, the retain count on the parent will still be higher than zero, and it won't be deleted.

This can be fixed by declaring the child's reference to the parent as a weak reference, as shown below:

@interface TTChild : NSObject

@property (weak, atomic) TTParent *parent; 

@end.

When the target is finally destroyed, a weak reference will be set to nil and will not increase the keep count for the target.

Note: Commonly asked iOS interview question.