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Update navigation bar style based on SwiftUI content offset
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,109 @@ | ||
| // | ||
| // Copyright 2024 Signal Messenger, LLC | ||
| // SPDX-License-Identifier: AGPL-3.0-only | ||
| // | ||
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| public import SwiftUI | ||
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| // MARK: - Scroll Anchor | ||
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| public struct ScrollAnchor: Equatable { | ||
| public let topAnchor: Anchor<CGPoint> | ||
| public let correction: CGFloat | ||
| } | ||
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| public struct ScrollAnchorPreferenceKey: PreferenceKey { | ||
| public typealias Value = [ScrollAnchor] | ||
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| public static var defaultValue: Value = [] | ||
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| public static func reduce(value: inout Value, nextValue: () -> Value) { | ||
| // We can't determine which one anchor we want without a GeometryProxy | ||
| value.append(contentsOf: nextValue()) | ||
| } | ||
| } | ||
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| public struct ProvideScrollAnchor: ViewModifier { | ||
| var correction: CGFloat | ||
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| public func body(content: Content) -> some View { | ||
| content | ||
| .transformAnchorPreference( | ||
| key: ScrollAnchorPreferenceKey.self, | ||
| value: .top | ||
| ) { (key, anchor) in | ||
| key.append(ScrollAnchor(topAnchor: anchor, correction: correction)) | ||
| } | ||
| } | ||
| } | ||
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| extension View { | ||
| /// Apply to the top-most element in a scroll view | ||
| /// (`ScrollView`, `List`, `Form`) to which ``readScrollOffset()`` | ||
| /// has been applied to read the scroll offset. | ||
| /// | ||
| /// If this is applied to multiple subviews within a scroll view, | ||
| /// the highest value is used in `readScrollOffset`. | ||
| public func provideScrollAnchor(correction: CGFloat = 0) -> some View { | ||
| self.modifier(ProvideScrollAnchor(correction: correction)) | ||
| } | ||
| } | ||
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| // MARK: - ScrollOffset | ||
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| public struct ScrollOffsetPreferenceKey: PreferenceKey { | ||
| public static var defaultValue: CGFloat = -.infinity | ||
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| public static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) { | ||
| value = max(value, nextValue()) | ||
| } | ||
| } | ||
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| public struct ScrollOffsetReader: ViewModifier { | ||
| @State private var scrollOffset: CGFloat = 0 | ||
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| public func body(content: Content) -> some View { | ||
| GeometryReader { geometry in | ||
| content | ||
| .onPreferenceChange(ScrollAnchorPreferenceKey.self) { anchors in | ||
| scrollOffset = anchors.map { scrollAnchor in | ||
| -(geometry[scrollAnchor.topAnchor].y + scrollAnchor.correction) | ||
| }.max() ?? 0 | ||
| } | ||
| } | ||
| .preference(key: ScrollOffsetPreferenceKey.self, value: scrollOffset) | ||
| } | ||
| } | ||
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| extension View { | ||
| /// Apply to a scroll view (`ScrollView`, `List`, `Form`) to | ||
| /// have it read scroll anchors in the content applied with | ||
| /// ``provideScrollAnchor(correction:)`` and report the | ||
| /// scroll view offset in ``ScrollOffsetPreferenceKey``. | ||
| /// | ||
| /// Note that if `provideScrollAnchor` is applied to multiple views in the | ||
| /// scrolling content (as is done with ``SignalSection``), this will return | ||
| /// the highest value read. If the content is | ||
| /// loaded lazily (as is done with ``SignalList``), this will report the | ||
| /// offset of the highest currently-rendered item, which may not reflect the | ||
| /// total scroll distance. | ||
| public func readScrollOffset() -> some View { | ||
| self.modifier(ScrollOffsetReader()) | ||
| } | ||
| } | ||
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| #Preview { | ||
| NavigationView { | ||
| SignalList { | ||
| SignalSection { | ||
| ForEach(0..<50) { | ||
| Text(verbatim: "Item \($0)") | ||
| } | ||
| } | ||
| } | ||
| .navigationTitle(Text(verbatim: "Title text")) | ||
| } | ||
| .onPreferenceChange(ScrollOffsetPreferenceKey.self) { scrollOffset in | ||
| print(scrollOffset) | ||
| } | ||
| } |
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