UIKit has been the backbone of iOS app development for years.

Its imperative approach offers granular control over every UI element, making it a powerful tool for complex and highly customized interfaces.

Strengths:

-> Flexibility: Unparalleled customization options for UI elements.
-> Maturity: A well-established framework with a vast ecosystem of libraries and tools.
-> Performance: Generally offers superior performance, especially for computationally intensive tasks.

Use Cases:

-> Complex UIs: Apps demanding intricate layouts, custom animations, or high performance.

-> Legacy Apps: Migrating existing UIKit apps incrementally.

-> Performance-Critical Components: Specific UI elements requiring optimal performance.

SwiftUI: Declarative Design, Simplified

SwiftUI introduces a declarative paradigm, focusing on describing the desired UI state rather than the steps to achieve it. This approach accelerates development, enhances code readability, and leverages the power of Swift's language features.

Strengths:
-> Rapid Development: Declarative syntax and live previews boost development speed.

-> Interoperability: Seamless integration with UIKit for gradual adoption.

-> Cross-Platform: Shared codebase for iOS, macOS, watchOS, and tvOS.

Use Cases:
New Projects: Building apps from scratch with a modern approach.
Prototyping: Quickly creating interactive prototypes.

Simple UIs: Apps with straightforward layouts and moderate complexity.

When to Choose Which?

The decision between SwiftUI and UIKit often hinges on project-specific requirements:

Opt for SwiftUI when:
1/ Rapid prototyping and development time are crucial.
2/ Cross-platform compatibility is a priority.
3/ The UI is relatively simple and can benefit from declarative syntax.

Consider UIKit when:
1/ Uncompromising performance is required.
2/ The app demands highly customized and complex UI elements.
3/ You're working with a large existing UIKit codebase.

In many cases, a hybrid approach is feasible. Combining the strengths of both frameworks can lead to optimal results.

For instance, you might use SwiftUI for the app's core UI and integrate UIKit components where necessary for performance or customization.