How do I use preference keys to pass values up the view tree in SwiftUI?

In SwiftUI, preference keys provide a way to pass values up the view hierarchy, allowing child views to communicate information back up to their parent views. This can be particularly useful for layouts or scenarios where a child view needs to report information that might influence the layout or behavior of a parent view.

Here’s how you can create a custom preference key and use it to pass values up the view tree:


        // Define a preference key
        struct HeightPreferenceKey: PreferenceKey {
            typealias Value = CGFloat
            static var defaultValue: CGFloat = 0
            
            static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
                value = max(value, nextValue())
            }
        }

        struct ChildView: View {
            var body: some View {
                Text("I have some height!")
                    .padding()
                    .background(Color.blue)
                    .foregroundColor(.white)
                    .overlay(
                        GeometryReader { geometry in
                            Color.clear.preference(key: HeightPreferenceKey.self, value: geometry.size.height)
                        }
                    )
            }
        }

        struct ParentView: View {
            @State private var childHeight: CGFloat = 0

            var body: some View {
                VStack {
                    ChildView()
                        .onPreferenceChange(HeightPreferenceKey.self) { value in
                            self.childHeight = value
                        }
                    
                    Text("Child height is: \(childHeight)")
                        .padding()
                }
            }
        }

How do I use preference keys to pass values up the view tree in SwiftUI?

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How do I use preference keys to pass values up the view tree in SwiftUI?

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