Mobile app development has become more dynamic than ever since companies realized that a mobile strategy is what wins consumers. Developing an iOS app is a particularly promising endeavor because it means creating a product that targets a group with high purchasing power and a high level of engagement with their mobile devices. SwiftUI and UIKit are the primary Apple native frameworks for building user interfaces, both fully supported by Xcode, Apple's official integrated development environment (IDE). In this article, I will compare and contrast SwiftUI and UIKit to help developers make an informed choice of which of the frameworks suits their project better.

Overview of SwiftUI and UIKit

UIKit and SwiftUI are front-end frameworks for iOS development. UIKit, a modular framework built in Objective-C, first introduced by Apple in 2008, was originally designed to build user interfaces for the small touch screens of the iPhone. It is now used to build the core infrastructure for iOS, iPadOS, or tvOS apps. It offers a wide range of UI components that make interface development more convenient and intuitive.

SwiftUI was released by Apple in 2019 to streamline iOS interface development. Apple designated it “...a modern way to declare user interfaces for any Apple platform”.  It came with version 13 of the iOS SDK, so it supports iOS 13 and later versions. It's based on the Swift programming language and is intended for building UIs for applications across the Apple ecosystem, including iOS, macOS, tvOS, and watchOS, with a single set of tools and APIs.

Declarative SwiftUI and Imperative UIKit

The main factor that determines different developer experiences when working with these two frameworks is that SwiftUI and UIkit use different programming paradigms.

SwiftUI works with declarative syntax. Declarative programming involves describing desired results without explicitly listing the steps to be taken. It is considered more modern than the imperative paradigm. With SwiftUI's declarative code syntax, it is easier to translate design concepts into code because you only have to describe what your interface or its element should look like and what it should do. Simply write a sequence of statements that describe an item you want to see in the UI and describe elements that determine the appearance of that element: its positioning, colors, and font, etc. SwiftUI automatically performs the actions required to implement the UI state. Even animation is simple. It can be added with only a few lines of code and enriched with ready-to-use effects. At runtime, the system manages all of the steps required to achieve smooth movement, including user interaction and state changes mid-animation. Drawing on state and data binding, SwiftUI performs automatic updates when the underlying data changes, cutting down on repetitive code writing.

By contrast, UIKit functions through an imperative method, in which developers directly instruct the program what tasks to complete and manually handle UI updates during its entire lifecycle. To make an object show up on a user's screen, you need to first create it, specify details like title, font, color, and size, and then include it in the view hierarchy.

UIKit’s Complexity & Customization and SwiftUI's Simplicity & Quicker Development

Newcomers to iOS UI development may find the work with UIKit more complex compared to SwiftUI due to the code verbosity of the former. However, a developer who is at ease with coding may see an imperative approach as a positive aspect. One potential benefit of having to describe each state or transition is that it gives more control over the project.

For the same reason, UIKit is regarded as more flexible, allowing users to customize a wide range of elements and create complex animations, video content, or other high-resource-consuming elements for bespoke interfaces.

UIKit differs from SwiftUI in that it doesn't use reactive programming and dependency injection. So, developers have to come up with their own solutions. Sometimes, they might need to use extra tools from other sources to get around limits. However, there are lots of external libraries for UIKit that can help with all kinds of tasks.

SwiftUI and UIKit both have lots of pre-built UI components. SwiftUI provides buttons, sliders, and navigation bars. These components can be customized and combined to build prototypes. With live preview, developers do not have to compile and run an app to see how UI changes as they change the code. SwiftUI lets developers turn design prototypes faster into code, shortening the app-building process.

UIKit provides built-in features and tools such as buttons, search bars, tables, input management tools, auto layout, dynamic type, collection views, touch interactions, ready-to-use effects, and drag-and-drop functionality. One of the main advantages of this framework - customization - comes from its pool of traditional customization options, i.e., view controllers, interface builders, and storyboards, delegates, and data sources. Unfortunately, the development process with UIKit can take much more time (compared to SwiftUI). Therefore, it would be difficult to meet the modern standard of releasing new devices every year.

SwiftUI is all about making UI development easier with a more declarative approach, but it lacks the customization capabilities of UIKit. While it's great for getting things done with less effort and quickly, SiwftUI may not provide the same level of precise control over the UI's functional and visual elements as the imperative syntax. If you need to prioritize rapid development with fewer lines of code over customization, consider SwiftUI.

SwiftUI's Cross-platform integration and UIKit's Legacy Support

SwiftUI boasts cross-platform compatibility. This feature allows developers to create UI for different Apple devices without wasting time and resources. However, when it comes to maintenance and updates to existing applications, developers' choices may be limited to UIKit. This is because many existing apps have been built with UIKit. Therefore, completely converting them to SwiftUI can be time-consuming and costly. This inconvenience can be avoided by integrating SwiftUI into existing UIKit projects, which doesn’t entail code rewriting.

However, App Store statistics show that 76% of all iPhones released in the last four years are running iOS 17, 20% of iPhones are running iOS 16, and only 4% are running an earlier operating system. So if you are building a new app rather than maintaining or updating existing ones, you can probably choose SwiftUI for faster development and prototyping.

UIKit’s Established Community and SwiftUI’s Accessibility:

UIkit is an old, well-established development framework backed by extensive official Apple documentation, a rich community, and third-party resources, including libraries, YouTube tutorials, blogs, and more. The downside is that UIKit is an imperative framework, so it has a steep learning curve. The lack of knowledge can negatively affect the final product. UIKit requires more coding knowledge and experience, but it provides more control and flexibility, making it appealing to experienced developers or those creating highly customized and complex interfaces.

SwiftUI is the ideal choice for those new to iOS development. Its declarative nature and intuitive syntax make it easy to learn and use. The development process with SwiftUI is undoubtedly more streamlined. Developers can focus on the "what" of their app design, rather than the "how". The framework handles the underlying flow without the need for intervention or control by a programmer.

Conclusion

To summarize, the SwiftUI framework is a great option for projects where rapid deployment and modern UI/UX are key. Conversely, the UIKit framework is perfect for creating complex applications with lots of intricate and sophisticated features. What's more, you don't have to choose SwiftUI or UIKit once and stick with it forever. Furthermore, you can mix parts and interface elements from each of these tools to create something truly unique. For instance, you can place UIKit views and view controllers inside SwiftUI views and vice versa. It is important to have the right UI development framework for your project. The key to success is choosing the tool that best fits your project's specific needs and goals.