带有内容框架更新关闭的 SwiftUI ScrollView

Question

我想要一个ScrollView，您可以在其中了解用户滚动时内容框架的变化（类似于 UIKit 中的didScroll 委托UIScrollView）。

这样，您就可以根据滚动行为执行布局更改。

Answer 1

通过使用View Preferences作为在View Hierarchy中通知上游布局信息的方法，我设法为这个问题提供了一个很好的解决方案。

关于View Preferences如何工作的详细解释，我建议阅读3 articles serieskontiki的主题

对于我的解决方案，我实现了两个 ViewModifiers：一个使用 锚首选项 对其布局进行视图报告更改，第二个允许 View 处理对框架的更新对其子树的视图。

为此，我们首先定义一个Struct，将可识别的帧信息携带到上游：

/// Represents the `frame` of an identifiable view as an `Anchor`
struct ViewFrame: Equatable {

    /// A given identifier for the View to faciliate processing
    /// of frame updates
    let viewId : String


    /// An `Anchor` representation of the View
    let frameAnchor: Anchor<CGRect>

    // Conformace to Equatable is required for supporting
    // view udpates via `PreferenceKey`
    static func == (lhs: ViewFrame, rhs: ViewFrame) -> Bool {
        // Since we can currently not compare `Anchor<CGRect>` values
        // without a Geometry reader, we return here `false` so that on
        // every change on bounds an update is issued.
        return false
    }
}

我们定义了一个符合PreferenceKey 协议的Struct 来保存视图树首选项的变化：

/// A `PreferenceKey` to provide View frame updates in a View tree
struct FramePreferenceKey: PreferenceKey {
    typealias Value = [ViewFrame] // The list of view frame changes in a View tree.

    static var defaultValue: [ViewFrame] = []

    /// When traversing the view tree, Swift UI will use this function to collect all view frame changes.
    static func reduce(value: inout [ViewFrame], nextValue: () -> [ViewFrame]) {
        value.append(contentsOf: nextValue())
    }
}

现在我们可以定义我提到的ViewModifiers：

对其布局进行视图报告更改：

这只是向 View 添加了一个 transformAnchorPreference 修饰符，它使用一个处理程序简单地构造一个具有当前帧 Anchor 值的 ViewFrame 实例并将其附加到 FramePreferenceKey 的当前值：

/// Adds an Anchor preference to notify of frame changes
struct ProvideFrameChanges: ViewModifier {
    var viewId : String

    func body(content: Content) -> some View {
        content
            .transformAnchorPreference(key: FramePreferenceKey.self, value: .bounds) {
                $0.append(ViewFrame(viewId: self.viewId, frameAnchor: $1))
            }
    }
}

extension View {

    /// Adds an Anchor preference to notify of frame changes
    /// - Parameter viewId: A `String` identifying the View
    func provideFrameChanges(viewId : String) -> some View {
        ModifiedContent(content: self, modifier: ProvideFrameChanges(viewId: viewId))
    }
}

为视图提供更新处理程序，以便在其子树上更改帧：

这会在视图中添加onPreferenceChange 修饰符，其中帧锚点更改列表将转换为视图坐标空间上的帧（CGRect），并报告为由视图 ID 键控的帧更新字典：

typealias ViewTreeFrameChanges = [String : CGRect]

/// Provides a block to handle internal View tree frame changes
/// for views using the `ProvideFrameChanges` in own coordinate space.
struct HandleViewTreeFrameChanges: ViewModifier {
    /// The handler to process Frame changes on this views subtree.
    /// `ViewTreeFrameChanges` is a dictionary where keys are string view ids
    /// and values are the updated view frame (`CGRect`)
    var handler : (ViewTreeFrameChanges)->Void

    func body(content: Content) -> some View {
        GeometryReader { contentGeometry in
            content
                .onPreferenceChange(FramePreferenceKey.self) {
                    self._updateViewTreeLayoutChanges($0, in: contentGeometry)
                }
        }
    }

    private func _updateViewTreeLayoutChanges(_ changes : [ViewFrame], in geometry : GeometryProxy) {
        let pairs = changes.map({ ($0.viewId, geometry[$0.frameAnchor]) })
        handler(Dictionary(uniqueKeysWithValues: pairs))
    }
}

extension View {
    /// Adds an Anchor preference to notify of frame changes
    /// - Parameter viewId: A `String` identifying the View
    func handleViewTreeFrameChanges(_ handler : @escaping (ViewTreeFrameChanges)->Void) -> some View {
        ModifiedContent(content: self, modifier: HandleViewTreeFrameChanges(handler: handler))
    }
}

让我们使用它：

我会举例说明用法：

在这里，我将收到ScrollView 内的标题视图 框架更改的通知。由于这个Header View在ScrollView内容的顶部，所以在frame origin上报告的frame变化等同于ScrollView的contentOffset变化

enum TestEnum : String, CaseIterable, Identifiable {
    case one, two, three, four, five, six, seven, eight, nine, ten

    var id: String {
        rawValue
    }
}

struct TestView: View {
    private let _listHeaderViewId = "testView_ListHeader"

    var body: some View {
        ScrollView {
            // Header View
            Text("This is some Header")
                .provideFrameChanges(viewId: self._listHeaderViewId)

            // List of test values
            ForEach(TestEnum.allCases) {
                Text($0.rawValue)
                    .padding(60)
            }
        }
            .handleViewTreeFrameChanges {
                self._updateViewTreeLayoutChanges($0)
            }
    }

    private func _updateViewTreeLayoutChanges(_ changes : ViewTreeFrameChanges) {
        print(changes)
    }
}

Answer 2

这个问题有一个优雅的解决方案，Soroush Khanlou 已经发布了它的要点，所以我不会复制粘贴它。你可以找到它here，是的......可惜它还不是框架的一部分！