How does SwiftUI work under the hood?

SwiftUI operates under a declarative paradigm, which means you describe what the UI should look like for a given state, and SwiftUI takes care of rendering it. Here are some key points about how SwiftUI works under the hood:

1. Declarative Syntax: SwiftUI views are descriptions of the current state of the UI. They are not long-lived object instances that receive imperative commands over time. This is why SwiftUI views are value types defined using structs instead of classes. SwiftUI takes these descriptions and creates an efficient data structure to represent them (SwiftUI essentials).

2. State-Driven Updates: SwiftUI views are state-driven. When your view state changes, SwiftUI automatically updates the UI, eliminating boilerplate code and reducing update bugs. SwiftUI maintains a representation of the user interface behind the scenes. As data changes, new view values are created and given to SwiftUI, which then updates its outputs accordingly (SwiftUI essentials).

3. Composition and Performance: Because views are just declarative descriptions, breaking up one view into multiple doesn't hurt the performance of your app. This allows you to organize your code in a way that makes sense without compromising performance. Composition is a fundamental aspect of SwiftUI (SwiftUI essentials).

4. Interoperability with UIKit and AppKit: SwiftUI provides seamless interoperability with UIKit and AppKit. You can create a UIViewRepresentable to use UIKit views within SwiftUI, and similarly, you can embed SwiftUI views into UIKit or AppKit view hierarchies using classes like HostingViewController (SwiftUI essentials).

5. Platform-Specific APIs: While SwiftUI enables code sharing across platforms, it is not a "write once, run everywhere" solution. It provides a common set of high and low-level components across platforms but also includes specialized APIs for each platform to accommodate their unique designs and usage patterns (SwiftUI essentials).

6. Integration with Other Frameworks: SwiftUI is designed to work seamlessly with other frameworks within the platform's SDK. This allows you to leverage the capabilities of other frameworks easily within your SwiftUI app (SwiftUI essentials).

For a deeper dive into these concepts, you can refer to the SwiftUI essentials session from WWDC 2024.