ios - 带有内容框架更新功能的SwiftUI ScrollView

我想拥有一个ScrollView，您可以在其中知道用户滚动时内容框架的变化（类似于UIKit didScroll中的UIScrollView委托）。

这样，您便可以根据滚动行为执行布局更改。

最佳答案

通过使用“视图首选项”作为在“视图层次结构”中通知上游布局信息的方法，我设法为该问题提供了一个不错的解决方案。

有关“视图首选项”如何工作的非常详细的解释，我建议您通过3 articles series阅读关于主题的kontiki

对于我的解决方案，我实现了两个ViewModifiers：一个使用锚首选项对视图的布局进行视图更改，第二个允许View处理其子树上视图框架的更新。

为此，我们首先定义一个Struct在上游携带可识别的帧信息：

/// 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
    }
}

并且我们定义了一个符合Struct协议的PreferenceKey来保存视图树的首选项更改：

/// 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())
    }
}

现在我们可以定义我提到的ViewModifiers：

在其布局上进行视图报告更改：

这只是使用处理程序向视图添加transformAnchorPreference修饰符，该处理程序仅构造具有当前帧ViewFrame值的Anchor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，以在其子树上更改框架：

这会在视图中添加onPreferenceChange修饰符，其中框架锚更改列表在视图的坐标空间上转换为框架（CGRect），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：

我将通过一个示例来说明用法：

在这里，我将收到ScrollView中的Header View框架更改的通知。由于此标题视图位于ScrollView内容的顶部，因此，报告的帧原点上的帧更改等同于contentOffset的ScrollView更改。

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)
    }
}

关于ios - 带有内容框架更新功能的SwiftUI ScrollView，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/58416242/