带有内容框架更新关闭的 SwiftUI ScrollView

Question

我想要一个ScrollView你可以在用户滚动时知道内容框架变化的地方（类似于didScrollUIKit 中的委托UIScrollView）。

这样，您就可以根据滚动行为执行布局更改。

Answer 1

通过使用视图首选项作为通知视图层次结构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。

有关“查看首选项”如何工作的非常详细的说明，我建议您阅读kontiki撰写的关于该主题的3 篇系列文章

对于我的解决方案，我实现了两个ViewModifiers：一个使用锚点首选项在其布局上更改视图报告，第二个允许 aView处理其子树上视图上的帧的更新。

为此，我们首先定义一个Struct来携带可识别的帧信息上行：

/// 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Struct符合PreferenceKey协议来保存视图树首选项更改：

/// 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ViewModifiers我提到的：

在其布局上更改视图报告：

这只是transformAnchorPreference通过一个处理程序向 View添加一个修饰符，该处理程序简单地构造一个ViewFrame具有当前帧Anchor值的实例并将其附加到 的当前值FramePreferenceKey：

/// 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))
    }
}

为视图的子树上的帧更改提供更新处理程序：

这会onPreferenceChange向视图添加一个修饰符，其中框架锚点更改列表转换为CGRect视图坐标空间上的框架（），并报告为由视图 ID 键控的框架更新字典：

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：

我将通过一个例子来说明用法：

在这里，我会得到的通知头查看内部框架的变化ScrollView。由于这个Header View是在ScrollView内容的顶部，所以上报的框架原点上的框架contentOffset变化相当于ScrollView

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)
    }
}