如何在 SwiftUI 中制作下拉搜索栏

Question

我想在我的应用中使用 SwiftUI 制作下拉搜索栏，并且必须与 iOS13 兼容，所以我无法使用 ScrollViewReader。我尝试使用Introspect，但我不明白它的作用和工作原理，因为除了下拉刷新之外，文档没有解释如何为 ScrollView 做一些事情。我使用 Introspect for TextField 成为并退出 FirstResponder，它可以工作，但是当我使用 introspect for scrollview 时，该函数仅在滚动视图首次出现时运行。

@State private var willSearch = false

...

.introspectTextField{ textfield in
    textfield.returnKeyType = .done
    if willSearch {
        textfield.becomeFirstResponder()
    }else{
        textfield.resignFirstResponder()
    }
}
// this works when `willSearch` changed value

这就是我的 ScrollView 内省的样子

extension UIScrollView {
    var isBouncing: Bool {
        return (contentOffset.y + contentInset.top < 0)
    }
}

...

.introspectScrollView { scroll in
    if scroll.isDragging{
        self.willSearch = scroll.isBouncing;
    }
}
// this doesn't work

我也尝试通过关注this suggestion 来检测滚动，但效果不是很好，而且我可以听到我的 MacBook 风扇转动的声音比平时大，所以它可能会影响性能。

这大致就是我希望它的行为Pull down search bar as seen in Telegram

Answer 1

我更喜欢使用UIKit 来实现应用内导航，并且只使用SwiftUI 来构建屏幕UI。所以我使用UIHostingController + UINavigationController 并且可以轻松集成UISearchController。比如这样：

import UIKit
import SwiftUI

class ViewController: UIHostingController<Content>, UISearchResultsUpdating {

    private let viewModel = ViewModel()

    init() {
        super.init(rootView: Content(viewModel: viewModel))
        title = "Test"
    }

    @objc required dynamic init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        let searchController = UISearchController(searchResultsController: nil)
        searchController.searchResultsUpdater = self
        self.navigationItem.searchController = searchController
        navigationController?.view.setNeedsLayout() // this is needed for iOS 13 only
    }

    func updateSearchResults(for searchController: UISearchController) {
        viewModel.searchDidChange(searchController.searchBar.text ?? "")
    }

}

struct Content: View {
    @ObservedObject var viewModel: ViewModel
    var body: some View {
        ScrollView {
            VStack {
                ForEach(viewModel.items, id: \.self) { item in
                    Text(item)
                        .frame(maxWidth: .infinity)
                }
            }
        }
    }
}

class ViewModel: ObservableObject {
    @Published var items: [String]
    private let allItems: [String]
    init() {
        self.allItems = (0..<100).map { "\($0)" }
        self.items = allItems
    }

    func searchDidChange(_ text: String) {
        items = allItems.filter {
            $0.starts(with: text)
        }
    }
}

警告：您必须禁用 Main.storyboard (https://sarunw.com/posts/how-to-create-new-xcode-project-without-storyboard/) 并手动加载根控制器。比如这样：

import UIKit

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let scene = (scene as? UIWindowScene) else { return }
        let window = UIWindow(windowScene: scene)
        window.rootViewController = UINavigationController(rootViewController: ViewController())
        window.makeKeyAndVisible()
        self.window = window
    }

}