Why isn't onPreferenceChange being called if it's inside a ScrollView in SwiftUI?

Question

I\'ve been seeing some strange behavior for preference keys with ScrollView. If I put the onPreferenceChange inside the ScrollView it won\'t be cal

Answer 1

It seems like the issue is not necessarily with ScrollView, but with your usage of PreferenceKey. For instance, here is a sample struct in which a PreferenceKey is set according to the width of a Rectangle, and then printed using .onPreferenceChange(), all inside of a ScrollView. As you drag the Slider to change the width, the key is updated and the print closure is executed.

struct ContentView: View {
    @State private var width: CGFloat = 100

    var body: some View {
        VStack {
            Slider(value: $width, in: 100...200)

            ScrollView(.vertical) {
                Rectangle()
                    .background(WidthPreferenceKeyReader())
                    .onPreferenceChange(WidthPreferenceKey.self) {
                        print($0)
                }
            }
            .frame(width: self.width)
        }
    }
}

struct WidthPreferenceKeyReader: View {
    var body: some View {
        GeometryReader { geometry in
            Rectangle()
                .fill(Color.clear)
                .preference(key: WidthPreferenceKey.self, value: geometry.size.width)
        }
    }
}

As you noted, the first time the key tries to set, the console prints "Bound preference WidthPreferenceKey tried to update multiple times per frame," but a real value is immediately set afterward, and it continues to update dynamically.

What value are you actually trying to set, and what are you trying to do in .onPreferenceChange()?