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This ScrollView has a modifier called `onScroll`, which is updated when scrolls occur.
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| struct ContentView: View { | |
| @State var scrollOffset: CGPoint = .zero | |
| var body: some View { | |
| ObservableScrollView { | |
| Text("Hello, world!") | |
| .foregroundColor(self.scrollOffset.y == 0 ? .blue : .red) | |
| } | |
| .onScroll { self.scrollOffset = $0 } | |
| } | |
| } |
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| struct ScrollOffsetPreferenceKey: PreferenceKey { | |
| typealias Value = CGPoint | |
| static var defaultValue = CGPoint.zero | |
| static func reduce(value: inout CGPoint, nextValue: () -> CGPoint) { | |
| value = nextValue() | |
| } | |
| } | |
| struct ObservableScrollView<Content> : View where Content : View { | |
| var content: Content | |
| var axes: Axis.Set | |
| var showsIndicators: Bool | |
| init(_ axes: Axis.Set = .vertical, showsIndicators: Bool = true, @ViewBuilder content: () -> Content) { | |
| self.content = content() | |
| self.axes = axes | |
| self.showsIndicators = showsIndicators | |
| } | |
| var body: some View { | |
| GeometryReader { outerGeometry in | |
| ScrollView(self.axes, showsIndicators: self.showsIndicators) { | |
| ZStack(alignment: self.axes == .vertical ? .top : .leading) { | |
| GeometryReader { innerGeometry in | |
| Color.clear | |
| .preference(key: ScrollOffsetPreferenceKey.self, value: CGPoint(x: (outerGeometry.frame(in: .global).minX - innerGeometry.frame(in: .global).minX), y: (outerGeometry.frame(in: .global).minY - innerGeometry.frame(in: .global).minY))) | |
| } | |
| VStack { | |
| self.content | |
| } | |
| } | |
| } | |
| } | |
| } | |
| } | |
| extension ObservableScrollView { | |
| func onScroll(_ onScroll: @escaping (CGPoint) -> ()) -> some View { | |
| self.onPreferenceChange(ScrollOffsetPreferenceKey.self, perform: onScroll) | |
| } | |
| } |
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Wrong implementation of reduce func. Closure will not called.