SwiftUI is Apple's declarative UI framework: you describe what the UI should look like for a given state, and the framework figures out how to render and update it. You no longer manually mutate views like in UIKit — you change state and the UI re-renders automatically.
A view conforms to the protocol and returns its content from a computed property. is a pure function of the view's properties.
Viewbodybodystruct Counter: View {
@State private var count = 0 // source of truth
var body: some View { // opaque return type
VStack(spacing: 12) {
Text("Count: \(count)") // re-reads count each render
Button("Increment") {
count += 1 // mutating state triggers re-render
}
}
}
}
some View is an opaque return type: it means "some specific concrete View type, decided by the compiler." It lets you return complex nested view types without spelling them out, while keeping the type static for performance.
label.text = "Count: \(count)" — you push each change.Text("Count: \(count)") — you declare the relationship once; the framework re-invokes body when count changes and diffs the result.Views are lightweight structs, cheap to create and throw away on every state change. SwiftUI keeps the underlying render tree and only applies the differences.
The declarative model removes a whole class of UIKit bugs where the UI drifts out of sync with the data. Understanding that body is re-run on every state change — and must stay a cheap pure function — is the foundation for everything else in SwiftUI, from state to performance.
A library of IT interview questions with detailed answers — from Junior to Senior.
Donate