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

You may only read it in superView, but you can change it with transformPreference after you set it .

struct ContentView: View {
var body: some View {
    ScrollView {
        VStack{
        Text("Hello")
            .preference(key: WidthPreferenceKey.self, value: 20)
        }.transformPreference(WidthPreferenceKey.self, {
        $0 = 30})
    }.onPreferenceChange(WidthPreferenceKey.self) {
        print($0)
    }
}
}

The last value is 30 now. Hope it is what you want.

You can read from other layer:

  ScrollView {

        Text("Hello").preference(key: WidthPreferenceKey.self, value: CGFloat(40.0))
            .backgroundPreferenceValue(WidthPreferenceKey.self) { x -> Color in
               print(x)
                return Color.clear
        }

    }