iOS App Development Flashcards

Question 1

What is Swift and what is it mainly used for?

Accepted Answer

Swift is a modern, type-safe programming language created by Apple for building apps on iOS, iPadOS, macOS, watchOS, and tvOS. It is designed to be readable and safe, catching many common errors at compile time. Swift code can interoperate with Objective-C, which allowed developers to adopt it gradually in existing projects.

Question 2

What is the difference between a constant and a variable in Swift?

Accepted Answer

In Swift, a constant is declared with let and its value cannot change after it is set. A variable is declared with var and can be reassigned later. Swift encourages using let by default, since immutable values make code easier to reason about and reduce unexpected state changes.

Question 3

What is an optional in Swift and why does it exist?

Accepted Answer

An optional is a type that either holds a value of a given type or holds nil to represent the absence of a value. Swift uses optionals to make missing data explicit, so the compiler requires you to handle the empty case. You can unwrap an optional safely with if let or guard let.

Question 4

How do Swift structs differ from classes in terms of value and reference behavior?

Accepted Answer

Structs are value types, so assigning or passing one creates an independent copy. Classes are reference types, so multiple variables can point to the same instance, and changes made through one are visible through the others. Structs cannot inherit from other structs, while classes support inheritance.

Question 5

What is protocol-oriented programming in Swift and why is it used?

Accepted Answer

Protocol-oriented programming is a style where you define behavior with protocols and build functionality by conforming types to them, rather than relying mainly on class inheritance. Swift protocols can be adopted by structs, enums, and classes alike. Protocol extensions can supply default implementations, letting many types share behavior without a shared base class.

Question 6

What does the Swift compiler's type inference allow you to omit?

Accepted Answer

Type inference lets the compiler deduce a variable's type from the value assigned to it, so you can omit explicit type annotations. For example, writing let count = 10 makes count an Int automatically. Explicit annotations are still needed when the compiler cannot infer the intended type.

Question 7

What is a Swift closure and how does it capture values?

Accepted Answer

A closure is a self-contained block of code that can be stored in a variable or passed to a function. Closures can capture and keep references to variables from the surrounding scope, even after that scope has finished running. Capture lists can control how values are captured, which helps avoid retain cycles.

Question 8

How does Swift's error handling with throws and do-catch work?

Accepted Answer

Swift marks functions that can fail with the throws keyword, and callers must acknowledge that possibility with the try keyword. Errors are handled with do-catch blocks, where a try inside the do block that fails transfers control to a matching catch clause. Using try? turns a failure into nil, while try! crashes on failure.

Question 9

What is SwiftUI and how does it differ from UIKit at a high level?

Accepted Answer

SwiftUI is Apple's declarative framework for building user interfaces, where you describe what the screen should look like for a given state. UIKit is an imperative framework in which you create and mutate view objects step by step. SwiftUI views are value types that are re-evaluated when state changes, while UIKit views are long-lived objects.

Question 10

What is a property wrapper like @State in SwiftUI used for?

Accepted Answer

The @State property wrapper stores simple value data owned by a view and refreshes the view when that value changes. The state is kept in storage managed by SwiftUI, so it survives the view struct being recreated. State works best for data the view itself owns, such as whether a toggle is on.

Question 11

What is the purpose of Auto Layout constraints in UIKit?

Accepted Answer

Auto Layout uses constraints to describe relationships between views, such as edges aligned to a superview or one view's width equal to another's. Constraints let the layout adapt automatically to different screen sizes, orientations, and text sizes. The layout system solves the constraint equations at runtime to compute each view's frame.

Question 12

How does a UITableView reuse cells to save memory?

Accepted Answer

A table view keeps a queue of reusable cells and dequeues one for each row that appears on screen. Scrolled-off cells return to the queue for reuse, so memory stays low for long lists. Developers register a cell identifier and dequeue cells by that identifier, which avoids creating a new cell for every row.

Question 13

What is the purpose of a SwiftUI modifier and how is one applied?

Accepted Answer

A modifier is a method that returns a new view with an added or changed property. Modifiers are chained onto a view one after another, and each call wraps the previous result. Because the order matters, applying padding before a background produces a different result than applying it after.

Question 14

What is a NavigationStack in SwiftUI used for?

Accepted Answer

A NavigationStack manages a stack of views so that users can move forward into detail screens and back out again. You add destinations with navigationDestination and can push values onto a path to navigate programmatically. The stack keeps track of the current screen and provides the familiar back button automatically.

Question 15

Why should you keep view logic separate from data models in an iOS app?

Accepted Answer

Separating view logic from data models makes each part easier to test, reuse, and change independently. Models hold the app's data and rules, while views only display that data and forward user actions. When a model is pure Swift, it can be tested without launching the interface.

Question 16

What does the body property of a SwiftUI View represent?

Accepted Answer

The body property is a required computed property that returns the view's content as a description of what should appear on screen. SwiftUI calls body whenever the view needs to be rendered or updated, so it should be free of side effects. You can compose many smaller views inside body to keep it readable.

Question 17

What is the app lifecycle in iOS and why does it matter?

Accepted Answer

The app lifecycle describes the states an app moves through, such as not running, active, inactive, and background, as the user switches tasks. Apps must respond correctly to these transitions, for example by saving unsaved work before entering the background. Understanding lifecycle events prevents lost data and wasted battery.

Question 18

What is the role of the App protocol and @main in a SwiftUI app?

Accepted Answer

The App protocol defines the entry point of a SwiftUI app and describes its scenes, such as the main window. Marking a type with @main tells the system that this type launches the app. The body of the App type returns one or more scenes, and each scene hosts a view hierarchy.

Question 19

What does a scenePhase environment value tell a SwiftUI view?

Accepted Answer

The scenePhase value reports whether a scene is active, inactive, or in the background. A view reads it with the Environment property wrapper and can react when it changes, for example by pausing a timer or saving a draft.

Question 20

What is the difference between a push and a modal presentation in navigation?

Accepted Answer

A push adds a screen onto a navigation stack, and the user returns with a back button. A modal presentation shows a screen on top of the current context, and it is designed for self-contained tasks. Push suits moving through related details, while modal suits creating something new or completing a separate step.

Question 21

How do you pass data from one screen to another in SwiftUI?

Accepted Answer

You pass data to a child screen through an initializer parameter, using a Binding when the child needs to change it. The parent owns the source of truth, usually a @State or observable model, and passes it down to the child. Passing data down keeps the flow predictable.

Question 22

What is a deep link and how does an app handle one?

Accepted Answer

A deep link is a URL that opens a specific screen inside an app rather than just launching the app to its home screen. The app receives the URL through a system callback, parses its path or parameters, and navigates to the matching view.

Question 23

Why would you use a tab bar instead of a single long navigation path?

Accepted Answer

A tab bar gives users persistent access to several top-level sections of an app, so they can switch between areas without losing their place. Each tab usually keeps its own navigation history, so returning to a tab shows the screen the user last viewed.

Question 24

What does single source of truth mean for app state?

Accepted Answer

A single source of truth means each piece of data has exactly one owner that holds the authoritative value, while other parts of the app read or reference it instead of keeping copies. This prevents views from showing conflicting information. In SwiftUI, the owner is often a state property or a model object, and child views receive bindings to it.

Question 25

What is the difference between UserDefaults and a database for storing app data?

Accepted Answer

UserDefaults stores small pieces of data, such as preferences and flags, as key-value pairs. A database, such as Core Data or SQLite, is better suited to large, structured, or frequently queried collections of records. UserDefaults should not hold passwords or tokens, which belong in the Keychain.

Question 26

What is Codable and what problem does it solve?

Accepted Answer

Codable is a Swift protocol that lets types convert themselves to and from external formats such as JSON. By conforming a struct to Codable, you can decode incoming data into typed values and encode values back into data for storage or sending. The compiler synthesizes the implementation when all stored properties are themselves Codable.

Question 27

How does URLSession fetch data from a web service asynchronously?

Accepted Answer

URLSession sends network requests and returns results without blocking the main thread. With modern Swift, you call its data method using await inside an async function, which returns the response bytes and metadata. You should check the HTTP status code, since a successful network call can still carry an error response.

Question 28

What is Core Data used for in an iOS app?

Accepted Answer

Core Data is Apple's framework for managing an app's object graph and persisting it, typically to a local SQLite store. It lets you define entities and relationships, then fetch and save objects using managed object contexts. Core Data also tracks changes so that unsaved edits can be discarded or committed.

Question 29

What is SwiftData and how does it relate to Core Data?

Accepted Answer

SwiftData is a newer persistence framework from Apple that uses Swift macros and types to declare models directly in code. Developers annotate classes with the Model macro and query stored objects with the Query property wrapper in SwiftUI. Core Data remains supported and is still widely used in existing projects.

Question 30

Why should network calls be made off the main thread?

Accepted Answer

The main thread handles user interface updates and touch input, so blocking it with a slow network call makes the app appear frozen. Using asynchronous APIs lets the app keep responding while the request completes. UI changes should still be made on the main actor after the data arrives.

Question 31

What is the purpose of Keychain Services in iOS?

Accepted Answer

Keychain Services is the system facility for storing small pieces of sensitive data, such as passwords and tokens. Data in the Keychain is encrypted and protected by the device's security mechanisms, and access can be restricted by conditions such as device unlock state. Apps should store secrets there rather than in files or UserDefaults.

Question 32

What does it mean for a network request to be idempotent, and why does it matter?

Accepted Answer

An idempotent request produces the same server-side result no matter how many times it is repeated, such as a GET or a PUT that sets a value. This matters because apps may retry requests after timeouts or dropped connections, and repeating a non-idempotent request, such as one that creates an order, could duplicate effects.

Question 33

What is unit testing and what does XCTest provide?

Accepted Answer

Unit testing checks that small, isolated pieces of code, such as a function or a model method, behave as expected for given inputs. XCTest is Apple's built-in framework for writing these tests in Xcode, with test classes and assertions such as XCTAssertEqual. Tests run quickly and help catch regressions when code changes.

Question 34

What is the difference between a unit test and a UI test?

Accepted Answer

A unit test exercises code directly without launching the full interface, so it runs fast and isolates failures to one component. A UI test launches the app and interacts with it the way a user would, tapping buttons and checking visible text. UI tests cover broader flows but run more slowly.

Question 35

What is the purpose of breakpoints and the debugger in Xcode?

Accepted Answer

Breakpoints pause a running app at a chosen line so you can inspect variables, the call stack, and the state of the program at that moment. The debugger lets you step over, step into, or continue execution. Conditional breakpoints pause only when an expression is true, which helps isolate rare bugs.

Question 36

Why is a provisioning profile needed to run an app on a physical device?

Accepted Answer

A provisioning profile links your app's bundle identifier, signing certificate, and allowed devices so the system can verify that the app is authorized to run. Xcode can manage signing automatically for most developers, creating and updating profiles. Without a valid profile, the device will refuse to install or launch the build.

Question 37

What is the purpose of Instruments in Xcode?

Accepted Answer

Instruments is Xcode's performance analysis tool, used to profile an app's CPU usage, memory allocations, energy impact, and rendering behavior. You run the app under a chosen template and record a trace that shows where time and resources are spent. Instruments helps locate slow code, memory leaks, and excessive work on the main thread.

Question 38

What is TestFlight used for when releasing an app?

Accepted Answer

TestFlight is Apple's service for distributing pre-release builds of an app to testers before the public App Store release. Developers upload a build through App Store Connect, invite testers, and collect feedback and crash reports. Internal testers from the development team can be added quickly, while external testers may require a review of the build first.

Question 39

What is the purpose of App Store Connect in the release process?

Accepted Answer

App Store Connect is Apple's web portal where developers manage app records, metadata, pricing, builds, and submissions for review. Developers upload build archives from Xcode, provide screenshots and descriptions, and submit the app for App Review. Once approved, the developer chooses a release option, such as releasing manually or automatically.

Question 40

How do you find and fix a retain cycle in a Swift closure?

Accepted Answer

A retain cycle occurs when a closure strongly captures an object that also holds the closure, so neither can be freed. You can fix it with a capture list that marks self as weak or unowned. The Memory Graph Debugger in Xcode helps find such cycles by showing objects that remain alive.

iOS App Development Flashcards

1Swift language fundamentals

2SwiftUI and UIKit views

3App lifecycle and navigation

4Data storage and networking

5Testing, debugging, and distribution

Learning Resources