通过 Swift 泛型初始化 SwiftUI 视图

Question

我正在将 Pull To Refresh 添加到基于 SwiftUI 的应用程序中。为此，我不得不退回到 UIKit。由于我想跨多个表和数据模型（使用共享协议）使用 Pull To Refresh，因此我想使用泛型。

我传入了我的dataModel的类型，我还想传入我想在scrollView中实例化的SwiftUI View的类型。

但是，当我添加我声明支持协议“视图”的通用类型 V 时，我在代码尝试实例化视图时遇到编译错误。

类型“V”没有成员“init”

如果我放弃通用类型“V”，而是硬编码一个特定的 SwiftUI 视图类型来实例化，比如 DataView，这一切都很好 - 但这并没有给我所需的视图类型的灵活性。

请问我该如何解决这个编译错误？

import SwiftUI

struct GenericRefreshView<T, V>: UIViewRepresentable where T:ObservableObject, T:dataModel, V: View {

    @EnvironmentObject var dataModel: T

    func makeCoordinator() -> Coordinator {
        Coordinator(self, regs: dataModel)
    }

    let size: CGSize

    func makeUIView(context: Context) -> UIScrollView {
        let scrollView = UIScrollView()
        scrollView.refreshControl = UIRefreshControl()
        scrollView.refreshControl?.addTarget(context.coordinator, action: #selector(Coordinator.handleRefreshControl(sender:)), for: .valueChanged)

        // *******************************************************************************
        let refreshVC = UIHostingController(rootView: V()) // Type 'V' has no member 'init'
        // *******************************************************************************

        refreshVC.view.frame = CGRect(x: 0, y: 0, width: size.width, height: size.height)

        scrollView.addSubview(refreshVC.view)

        return scrollView
    }

    func updateUIView(_ uiView: UIScrollView, context: Context) {}

    class Coordinator: NSObject {
        var refreshScrollView: GenericRefreshView
        var dataModel: T

        init(_ refreshScrollView: GenericRefreshView, dataM: T) {
            self.refreshScrollView = refreshScrollView
            self.dataModel = dataM
        }

        @objc func handleRefreshControl (sender: UIRefreshControl) {
            self.dataModel.refresh()
            sender.endRefreshing()
        }
    }
}

Answer 1

你可以要求它在外部注入，比如

struct GenericRefreshView<T, V>: UIViewRepresentable where T:ObservableObject, T:dataModel, V: View {

    @EnvironmentObject var dataModel: T
    let contentView: V                      // here !!
    // let builder: () -> V                 // alternate, as view builder

    func makeCoordinator() -> Coordinator {
        Coordinator(self, regs: dataModel)
    }

    let size: CGSize

    func makeUIView(context: Context) -> UIScrollView {
        let scrollView = UIScrollView()
        scrollView.refreshControl = UIRefreshControl()
        scrollView.refreshControl?.addTarget(context.coordinator, action: #selector(Coordinator.handleRefreshControl(sender:)), for: .valueChanged)

        let refreshVC = UIHostingController(rootView: self.contentView)    // here !!

   ...

更新：简化以避免遗漏部分，这里是使用演示

struct DemoView: View {
    var body: some View {
        GenericRefreshView(dataModel: TestViewModel(), 
                         contentView: Text("Demo"), 
                                size: CGSize(width: 100, height: 100))
    }
}

struct GenericRefreshView<T, V>: UIViewRepresentable where T:ObservableObject, V: View {

    @ObservedObject var dataModel: T     // << EnvironmentObject is internal, so cannot be inferred from input arguments, so not usable here
    let contentView: V
    let size: CGSize