Why is SwiftUI not backwards compatible?

SwiftUI is not backwards compatible primarily because it is designed to leverage the unique features and capabilities of each Apple platform. While SwiftUI enables code sharing across platforms, it is not a "write once, run everywhere" solution. Each platform has its own specialized APIs and design considerations, as described in the Human Interface Guidelines. This means that while you can share a lot of code, you often need to make platform-specific adjustments to fully utilize the capabilities of each device.

For example, on watchOS, you might use the digital crown for navigation, which is not applicable on other platforms. Similarly, macOS might require different scene types and windowing models compared to iOS or visionOS. This platform-specific customization is necessary to provide the best user experience on each device.

Additionally, SwiftUI's interoperability with other frameworks like UIKit and AppKit is crucial for integrating new features into existing apps. This deep integration with platform-specific frameworks further complicates backward compatibility.

For more details, you can refer to the SwiftUI essentials session.