Top 10 iOS Interview Questions

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June 7, 2023 11 min readInterview

As an iOS developer we need to keep learning and experimenting new things as per industry's current standards. This will help us land in a better workplace having a strong technical iOS team.

But along with learning various concepts we should not forget the basics of the 'Swift' language and iOS development. These basics reflect our strength, they show how strong we are in `iOS development' and they help us land our next job.

We have listed down 10 must know iOS Interview Questions that will help an iOS developer prepare for the interviews.

Q1. How can we reduce the iOS app launch time?

There are several ways to reduce app launch time in iOS. Here are some suggestions.

Improving launch code:

Make sure that your code is efficient and doesn’t include unnecessary processes or calculations that could slow down the app launch. You can use profiling tools like Instruments to identify optimization areas. For example, if you’re doing something during the launch that can happen later, do it later.

// Preferred way
private lazy var settingsViewController: MySettingsViewController = {
    return MySettingsViewController()
}()

// Can be avoid
private func onSettingsButtonPressed() {
    present(self.settingsViewController, animated: true)
}

Use background threads:

If something doesn’t need to happen on the main thread, offload it onto a background thread so the main thread can show your UI. Be careful, because much code is not thread-safe or is designed to run only on the main thread.

Compromise on behavior:

At the end of the day, the more your app is trying to do when it launches, the slower it will launch. Negotiate with your team to see if there’s anything user-facing you’re willing to change or remove to get a faster launch time.

Reduce dependencies:

The more dependencies your app has, the longer it will take to load. Look for ways to reduce dependencies in your app.

Use Launch Screens:

Launch Screens are placeholder screens that display when your app launches. By using Launch Screens, you can create the illusion that your app is launching faster, even if there is some delay in loading the app’s content.

Optimize Images and Videos:

Images and videos can be large and slow to load. Optimize them by reducing their size and compressing them without compromising quality

Q2. How do you implement dynamic type and font scaling in an iOS app?

Dynamic type and font scaling are important features in iOS apps that allow users to customize the font size and style according to their preferences. Here are the steps to implement dynamic type and font scaling in an iOS app:

Choose a font family that supports dynamic type:

When selecting a font for your app, choose a font family that includes various font styles and weights, and supports dynamic type. Some of the popular font families that support dynamic type are San Francisco, Avenir, and Helvetica Neue.

Use the system font APIs:

To implement dynamic type, use the system font APIs provided by iOS, such as UIFont.preferredFont(forTextStyle:), UIFontMetrics.default.scaledFont(for:), and UIFontMetrics.default.scaledValue(for:). These APIs automatically adjust the font size and weight based on the user’s preferred text size in the Settings app.

Configure font scaling in Interface Builder:

If you are using Interface Builder to design your app’s UI, you can configure font scaling for labels and text views by setting the font to "preferred font" and selecting a text style from the "text style" dropdown menu.

Q3. What is Conditional Conformance? What are the benefits of applying Conditional Conformance?

Conditional conformance in Swift is a feature that allows a type to conform to a protocol under certain conditions, such as when its generic parameters meet certain constraints. It was introduced in Swift 4.

Here are some examples of how conditional conformance can be used in Swift:

Example 1:

extension Dictionary: CustomStringConvertible where Key: CustomStringConvertible {
    var description: String {
        return "{" + self.map { "\($0.key.description): \($0.value.description }
}
let names = ["Alice": 25, "Bob": 30, "Charlie": 35]
print(names) // Output: {"Alice: 25, "Bob": 30, "Charlie": 35

In this example, we extend the Dictionary type to conform to the CustomStringConvertible protocol if its Key and Value types also conform to CustomStringConvertible. This allows us to print a dictionary with CustomStringConvertible keys and values using the default string representation.

Example 2:

struct ValueWrapper<T> {
    let value: T
}
extension ValueWrapper where T == String {
    var stringValue: String {
        return self.value
    }
}
let wrapped = ValueWrapper(value: "This is a string")
print(wrapped.stringValue) // This is a string

Conditional conformance has several benefits in Swift:

  • Reusability: With conditional conformance, we can define a single implementation of a protocol for multiple types that meet certain conditions. This makes our code more reusable and reduces duplication.
  • Flexibility: Conditional conformance allows us to define different implementations of a protocol for different types based on their characteristics. This gives us more flexibility in how we design our types and protocols.
  • Convenience: With conditional conformance, we can add conformance to a protocol for a type without modifying its original implementation. This makes it easier to add functionality to our types without introducing breaking changes.
  • Clarity: Conditional conformance makes our code more clear and expressive by explicitly stating the conditions under which a type conforms to a protocol. This makes it easier to understand how our types and protocols work together.
  • Type safety: Conditional conformance ensures type safety by allowing us to restrict conformance to only those types that meet certain conditions. This helps prevent bugs and makes our code more reliable.

Q4. What are the Background modes in iOS? How do they work?

In iOS, there are several background modes that allow apps to continue functioning even when they are not actively running in the foreground. Here are some common background modes in iOS and how they work:

Audio

This background mode allows apps to continue playing audio even when they are not in the foreground. For example, a music streaming app can continue playing music while you use other apps or lock your device.

Location Updates:

This background mode allows apps to track your location even when they are not in the foreground. For example, a navigation app can continue to provide turn-by-turn directions while you use other apps.

Background Fetch:

This background mode allows apps to periodically fetch new content or data in the background. For example, a news app can fetch new articles in the background so that they are ready for you to read when you open the app.

Remote Notifications:

This background mode allows apps to receive and process remote notifications in the background. For example, a messaging app can receive and display new messages even when it is not in the foreground.

VoIP:

This background mode allows apps to receive and handle VoIP (Voice over Internet Protocol) calls in the background. For example, a video conferencing app can continue to receive and handle calls even when it is not in the foreground.

Background Processing:

The Background Processing allows apps to perform long-running tasks in the background, even if the app is not actively running in the foreground. This is useful for apps that need to perform tasks like uploading or downloading large amounts of data, or processing images or videos.

External Accessory:

The External Accessory allows apps to communicate with external hardware accessories in the background, even if the app is not actively running in the foreground. This is useful for apps that need to interact with accessories like fitness trackers, heart rate monitors, or car audio systems.

How they work?

When an app uses one of these background modes, it is still running in the background, but it may be using less resources than when it is in the foreground. The exact behavior of an app in the background depends on the specific background mode it is using and how it is designed.

Q5. What is the @UIApplicationMain attribute for?

The @UIApplicationMain attribute is used in Swift to designate the entry point for an iOS, iPadOS, or macOS app that uses a graphical user interface (GUI). When you add the @UIApplicationMainattribute to a class that conforms to the UIApplicationDelegate protocol, it tells the compiler that this class contains the main entry point for the application. This class must also include a main()function that serves as the starting point for the app. The UIApplicationDelegate protocol defines methods that manage the application’s life cycle, such as launching, backgrounding, and terminating the app. By conforming to this protocol, your app’s main class can implement these methods to respond appropriately to changes in the app’s state. The class that you add the @UIApplicationMain attribute to must conform to the UIApplicationDelegate protocol, which defines a set of methods that manage the app’s life cycle. These methods include application(:didFinishLaunchingWithOptions:), applicationDidEnterBackground(:), applicationWillEnterForeground(_:), and others. Your app’s main class can implement these methods to customize the behavior of the app when it’s launched, enters the background, or returns to the foreground. If you need to access the UIApplication object from your app’s main class, you can do so by using the UIApplication.shared singleton instance.

Q6. What is Threads Explosion in Swift? What are the means of prevention?

"Thread explosion" in Swift refers to a situation where a large number of threads are created, leading to performance issues, such as increased CPU usage, memory usage, and decreased responsiveness of the application.

In Swift, threads can be created using the Grand Central Dispatch (GCD) framework, which provides a simple way to create and manage threads. However, if not used properly, GCD can result in thread explosion. Some of the common causes of thread explosion include:

  • Creating too many threads: If you create too many threads, it can overwhelm the system and cause performance issues.
  • Creating threads in a loop: If you create threads in a loop, it can quickly lead to thread explosion.
  • Creating threads for short-lived tasks: If you create threads for short-lived tasks, it can result in a lot of threads being created and destroyed, leading to performance issues.

To prevent thread explosion in Swift, you can follow these best practices:

  • Use thread pooling: Instead of creating a new thread for each task, use a thread pool to manage a fixed number of threads.
  • Use asynchronous programming: Instead of creating threads for short-lived tasks, use asynchronous programming to avoid unnecessary thread creation.
  • Use Dispatch Queues: Dispatch Queues are a more efficient way of managing threads, as they provide a simple way to manage tasks without the need for explicit thread creation.
  • Use Quality of Service (QoS): Use QoS to prioritize tasks and avoid creating unnecessary threads.

By following these best practices, you can prevent thread explosion and ensure that your Swift application performs efficiently.

Q7. Explain the difference between Generics and Opaque types in Swift?

In Swift, generics and opaque types are two features used for creating flexible and reusable code. Both of these features allow you to write code that can work with different types without having to write separate code for each type. However, there are some differences between the two.

Generics

Generics in Swift allow you to define functions, types, and protocols that can work with any type. You can use placeholders, called type parameters, to represent the types that will be used at runtime. For example, consider the following generic function:

func printArray<T>(array: [T]) { for item in array {
    print(item) }
}
let intArray = [1, 2, 3, 4, 5]
let stringArray = ["apple", "banana", "orange"] print(intArray) // [1, 2, 3, 4, 5]

Opaque types

Opaque types allow you to hide the specific implementation of a type while still providing a way to work with it. With opaque types, you define a protocol that specifies the behavior of the type, but you don’t provide the implementation details.

The implementation is hidden and can be changed without affecting the code that uses the opaque type. Here’s an example:

protocol Displayable {
    func display()
}

struct Person: Displayable {
    func display() {
        print("Name: John") }
}
struct Car: Displayable {
    func display() {
        print("Make: Toyota, Model: Corolla") 14 }
}

func printItem(item: some Displayable) {
    item.display()
}

let john = Person()
let corolla = Car()
printItem(item: john)
// Output: Name: John

printItem(item: corolla)
// Output: Make: Toyota, Model: Corolla

In summary, generics allow you to write code that can work with any type, while opaque types allow you to hide the implementation details of a type and provide a way to work with it. Both features are useful for creating flexible and reusable code.

Q8. How can we fix non-smooth scrolling issues?

Here are some more points to address non-smooth scrolling issues in Swift:

Avoid heavy computations on the main thread: Heavy computations can cause non- smooth scrolling. Performing long-running tasks like network requests or file I/O on the main thread can cause non-smooth scrolling. You can use GCD or Operation queues to perform these tasks asynchronously on a background thread.

Minimize layout updates: Frequent layout updates can cause non-smooth scrolling. You can minimize layout updates by setting the UIView’s clipsToBounds property to true, setting the backgroundColor property to a solid color, and avoiding the use of transparency.

Use instruments to identify bottlenecks: Xcode’s Instruments is a powerful tool that can help you identify performance bottlenecks in your app. You can use the Time Profiler instrument to identify which parts of your code are causing non-smooth scrolling.

Optimize image loading: Loading large images can cause non-smooth scrolling. You can optimize image loading by resizing images to match the size of the image view they’re being displayed in, using compression, or using a library like SDWebImage or Kingfisher to handle image loading and caching.

Implement pagination: If your app is displaying a large amount of data, it can cause non- smooth scrolling issues. One way to address this is to implement pagination, which means loading and displaying data in small chunks or pages. This can significantly improve scrolling performance.

Implement cell reuse: When using UITableView or UICollectionView, ensure that you’re reusing the cells instead of creating new ones each time. This can improve scrolling performance and reduce memory usage.

Lazy loading: Instead of loading all the content at once, consider lazy loading or infinite scrolling to load the content gradually as the user scrolls. This will ensure that the app only loads what’s necessary and reduces the overall memory footprint.

By following these steps, you can improve the scrolling performance of your app and provide a better user experience.

Q9. What is a dynamic and static framework?

In iOS development, a framework is a precompiled collection of code, resources, and libraries that can be used by other programs. There are two types of frameworks in iOS development: dynamic frameworks and static frameworks.

Static Framework:

A static framework is a library of compiled code that is linked directly into an application at compile-time. This means that the code in the framework becomes a part of the application and cannot be changed once the application is compiled. Static frameworks are useful for small projects where the code does not change frequently.

Dynamic Framework:

A dynamic framework is a library of compiled code that is linked to an application at runtime. This means that the code in the framework can be changed without having to recompile the application. Dynamic frameworks are useful for larger projects where the code changes frequently.

Here are some examples of dynamic framework in iOS:

  • UIKit.framework: provides the basic UI elements for iOS apps.
  • CoreGraphics.framework: provides low-level graphics primitives for drawing on the screen.
  • CoreData.framework: provides data persistence for iOS apps.

Static frameworks are included in the app bundle at compile time, whereas dynamic frameworks are loaded at runtime. When using a dynamic framework, Xcode includes a reference to the framework in the app bundle, but the actual binary code is stored separately on the device or simulator and is loaded into memory when needed. This makes it possible to update the framework separately from the app itself, without needing to recompile the app.

In general, dynamic frameworks are more flexible and easier to update than static frameworks. However, they come with some performance overhead as they need to be loaded at runtime, while static frameworks provide better performance as they are already linked into the application.

Q10. What is the relation between threads and GCD in iOS Swift?

In iOS Swift, threads and Grand Central Dispatch (GCD) are both mechanisms used to manage concurrent execution of code. GCD is a high-level API provided by Apple that simplifies the creation and management of concurrent tasks, while threads are a lower-level abstraction that provide more fine-grained control over the execution of code.

Under the hood, GCD uses threads to execute tasks concurrently, but it abstracts away many of the low-level details of thread management. GCD allows developers to create queues of tasks that are executed concurrently, without the need to manage threads explicitly. GCD automatically manages the creation, scheduling, and execution of threads to ensure optimal performance.

In summary, threads and GCD are related in that GCD uses threads to execute tasks concurrently, but GCD provides a higher-level abstraction that simplifies concurrent programming by managing threads for you. As a developer, you can choose to use threads directly if you need fine-grained control over concurrency, but in many cases, GCD provides a simpler and more efficient way to write concurrent code in iOS Swift.

We hope that these questions will help you strengthen the basics of iOS development. We have launched our new e-book with Top 100 iOS Interview Questions & Answers. You can grab your copy now!

We at Swift Anytime have the mission to make learn iOS development the way everyone enjoys it. You can check out our articles on SwiftUI, Swift, iOS Interview Questions and get started with your iOS journey today.​​

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