SwiftUI is Apple's framework for building user interfaces in the Swift programming language. One set of tools and APIs covers all Apple platforms: iOS, iPadOS, macOS, tvOS, visionOS and watchOS.
SwiftUI uses a declarative syntax: the code states what the interface should show, for example a list of items made of text fields with their alignment, font and color, and the system works out how to draw and update it. Each view is a Swift structure. Animation follows the same style: a few lines add an effect, and at runtime the system handles the movement, even when the state changes in the middle of it. SwiftUI works alongside UIKit and AppKit, Apple's other interface frameworks, so an existing app can adopt it one part at a time. In Xcode, Apple's development environment, previews show each view as the code changes, also interactively as on a device, in Dark Mode or with larger text; selecting a control in the preview highlights its line of code.
SwiftUI comes with Apple's developer tools. A free Apple developer account allows testing on one's own devices; distributing apps requires the paid Apple Developer Program.