10 iOS Developer interview questions and answers

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Name four important data types found in Objective-C.

Four data types that you’ll definitely want your developer to be aware of are as follows:

  • NSString: Represents a string.
  • CGfloat: Represents a floating point value.
  • NSInteger: Represents an integer.
  • BOOL: Represents a boolean.

How proficient are you in Objective-C and Swift? Can you briefly describe their differences?

When Swift was first launched in 2014, it was aptly described as “Objective-C without the C.” By dropping the legacy conventions that come with a language built on C, Swift is faster, safer, easier to read, easier to maintain, and designed specifically for the modern world of consumer-facing apps. One of the most immediately visible differences is the fact that Objective-C lacks formal support for namespaces, which forces Objective-C code to use two- or three-letter prefixes to differentiate itself. Instead of simple names like “String,” “Dictionary,” and “Array,” Objective-C must use oddities like “NSString,” “NSDictionary,” and “NSArray.” Another major advantage is that Swift avoids exposing pointers and other “unsafe” accessors when referring to object instances. That said, Objective-C has been around since 1983, and there is a mountain of Objective-C code and resources available to the iOS developer. The best iOS developers tend to be pretty well versed in both, with an understanding that Swift is the future of iOS development.

What are UI elements and some common ways you can add them to your app?

Buttons, text fields, images, labels, and any other elements that are visible within the application are called UI elements. These elements may be interactive and comprise the user interface (hence the term "UI") of an application. In iOS development, UI elements can be quickly added through Xcode’s interface builder, or coded from scratch and laid out using NSLayoutConstraints and Auto Layout. Alternatively, each element can also be positioned at exact coordinates using the UIView "(id)initWithFrame:(CGRect)frame" method.

Explain the purpose of the reuseIdentifier in the UITableViewCell constructor:

  (id)initWithStyle:(UITableViewCellStyle)style reuseIdentifier:(NSString *)reuseIdentifier

The reuseIdentifier tells UITableView which cells may be reused within the table, effectively grouping together rows in a UITableView that differ only in content but have similar layouts. This improves scroll performance by alleviating the need to create new views while scrolling. Instead the cell is reused whenever dequeueReusableCellWithIdentifier: is called.

What are some common execution states in iOS?

The common execution states are as follows:

  • Not Running: The app is completely switched off, no code is being executed.
  • Inactive: The app is running in the foreground without receiving any events.
  • Active: The app is running in the foreground and receiving events.
  • Background: The app is executing code in the background.
  • Suspended: The app is in the background but is not executing any code.

What is the purpose of managed object context (NSManagedObjectContext) in Objective-C and how does it work?

Managed object context exists for three reasons: life-cycle management, notifications, and concurrency. It allows the developer to fetch an object from a persistent store and make the necessary modifications before deciding whether to discard or commit these changes back to the persistent store. The managed object context tracks these changes and allows the developer to undo and redo changes.

Determine the value of “x” in the Swift code below. Explain your answer.

  var a1 = [1, 2, 3, 4, 5]
  var a2 = a1
  a2.append(6)
  var x = a1.count

In Swift, arrays are implemented as structs, making them value types rather than reference types (i.e., classes). When a value type is assigned to a variable as an argument to a function or method, a copy is created and assigned or passed. As a result, the value of "x" or the count of array "a1" remains equal to 5 while the count of array "a2" is equal to 6, appending the integer "6" onto a copy of the array "a1." The arrays appear in the box below.",

  a1 = [1, 2, 3, 4, 5]
  a2 = [1, 2, 3, 4, 5, 6]

Find the bug in the Objective-C code below. Explain your answer.

  @interface HelloWorldController : UIViewController

  @property (strong, nonatomic) UILabel *alert;

  @end

  @implementation HelloWorldController

  - (void)viewDidLoad {
    CGRect frame = CGRectMake(150, 150, 150, 50);
    self.alert = [[UILabel alloc] initWithFrame:frame];
    self.alert.text = @"Hello...";
    [self.view addSubview:self.alert];
    dispatch_async(
      dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
      ^{
        sleep(10);
        self.alert.text = @"World";
      }
    );
  }
  @end

All UI updates must be performed in the main thread. The global dispatch queue does not guarantee that the alert text will be displayed on the UI. As a best practice, it is necessary to specify any updates to the UI occur on the main thread, as in the fixed code below:

  dispatch_async(
  dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
  ^{
  sleep(10);
  dispatch_async(dispatch_get_main_queue(), ^{
  self.alert.text = @"World";
});
});

Explain the difference between raw and associated values in Swift.

This question tests the developer’s understanding of enumeration in Swift. Enumeration provides a type-safe method of working with a group of related values. Raw values are compile time-set values directly assigned to every case within an enumeration, as in the example detailed below:

enum Alphabet: Int {
  case A = 1
  case B
  case C
}

In the above example code, case "A" was explicitly assigned a raw value integer of 1, while cases "B" and "C" were implicitly assigned raw value integers of 2 and 3, respectively. Associated values allow you to store values of other types alongside case values, as demonstrated below:

enum Alphabet: Int {
  case A(Int)
  case B
  case C(String)
}

You’ve just been alerted that your new app is prone to crashing. What do you do?

This classic interview question is designed to see how well your prospective programmer can solve problems. What you’re looking for is a general methodology for isolating a bug, and their ability to troubleshoot issues like sudden crashes or freezing. | In general, when something goes wrong within an app, a standard approach might look something like this: | | ------ | | Determine the iOS version and make or model of the device. | | Gather enough information to reproduce the issue. | | Acquire device logs, if possible. | | Once you have an idea as to the nature of the issue, acquire tooling or create a unit test and begin debugging. | A great answer would include all of the above, with specific examples of debugging tools like Buglife or ViewMonitor, and a firm grasp of software debugging theory—knowledge on what to do with compile time errors, run-time errors, and logical errors. The one answer you don’t want to hear is the haphazard approach—visually scanning through hundreds of lines of code until the error is found. When it comes to debugging software, a methodical approach is a must.

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  • $45 hourly
    Andrew L.
    iOS Developer
    • 5.0
    • (4 jobs)
    Kyiv, KYIV CITY
    vsuc_fltilesrefresh_TrophyIcon iOS Development
    JavaScript
    Node.js
    Tablet
    Objective-C
    Java
    Hybrid App Development
    Firebase
    Mobile App Development
    Android
    iOS
    Flutter
    Smartphone
    Smartwatch
    React Native
    Xamarin
    Swift
    Native App Development
    Android App Development
    Kotlin
    🚀 Greetings! With over 11 years in the mobile development realm, I am a full-cycle mobile developer equipped to construct entire mobile applications from the ground up, spanning from design to implementation. Here's a snapshot of the technologies I wield: ✅ Languages: Swift, Objective-C, Java, Kotlin, Javascript, Dart ✅ Frameworks: React Native, Flutter, Node.js ✅ Databases: Firebase, MySQL, MS SQL, Cassandra, MongoDB, PostgreSQL, Redis ✅ SCMs: Github, Gitlab, BitBucket ✅ CI/CD tools: Jenkins, TeamCity, CircleCI, GitLabCI, GitHub Actions, Bitbucket Pipelines, Docker, Kubernetes ✅ Testing Approaches: TDD, BDD My expertise extends to microservices and event-driven architectures, with a wealth of experience in handling scalable applications. Beyond coding, I excel in working with legacy code, optimizing and automating processes, and crafting efficient workflows. Proficient and skilled in both design and mobile development, I can transform your ideas into market-ready solutions. Here are some of my specialties: 🍔 Food/Retail Delivery and Pickup Platforms for Drivers, Staff, and Customers 👥 Social Networks for Friends/Groups with Chat, Feeds, Profiles, Signup, and Log in 🔄 Buy and Sell Marketplace Apps with Commissions and Payments 📊 Task Management, Expense Management, and more. Let's collaborate to bring your visions to life and make a significant impact in the digital landscape! 💻✨
  • $70 hourly
    Tudor-Mihai A.
    iOS Developer
    • 5.0
    • (9 jobs)
    Brasov, BRAŞOV
    vsuc_fltilesrefresh_TrophyIcon iOS Development
    Stripe
    RxSwift
    Apple Xcode
    RESTful Architecture
    Mobile App Development
    Firebase
    In-App Purchases
    Node.js
    Swift
    - fully embraced SwiftUI; - big on modular codebases and clean architecture flavors - usually taking the lead in development and taking initiative product wise - ish at designing screens out of just ideas, but great as a critical eye over provided design
  • $150 hourly
    Abdus Sami H.
    iOS Developer
    • 5.0
    • (5 jobs)
    Toronto, ON
    vsuc_fltilesrefresh_TrophyIcon iOS Development
    C++
    Scala
    Machine Learning
    Django
    Linux
    JavaScript
    Vue.js
    Python
    Python Script
    Flutter
    jQuery
    Object-Oriented Programming
    PyTorch
    I have been working on pythonic frameworks since 2010. I have extensive experience in web-applications, technical writing, scientific content development, programming languages such as python, JavaScript, Scala and C++. I'm flexible with my working hours and am happy to work closely with any existing freelancers you work with. I look forward to hearing from you!
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