SwiftUI使用户不能下滑关闭Sheet

在某些情况下，我们不能直接让用户自主关闭sheet，比如说登陆界面等

Full Screen

SwiftUI提供一种全新的Sheet（请在Xcode12中尝试），让你以全屏的方式弹出sheet而不是卡片，这种方式下如果不提供关闭按钮，则用户无法自主关闭。

struct FullScreenCoverPresentedOnDismiss: View {
    @State private var isPresenting = false
    var body: some View {
        Button("Present Full-Screen Cover") {
            isPresenting.toggle()
        }
        .fullScreenCover(isPresented: $isPresenting,
                         onDismiss: didDismiss) {
            VStack {
                Text("A full-screen modal view.")
                    .font(.title)
                Text("Tap to Dismiss")
            }
            .onTapGesture {
                isPresenting.toggle()
            }
            .foregroundColor(.white)
            .frame(maxWidth: .infinity,
                   maxHeight: .infinity)
            .background(Color.blue)
            .ignoresSafeArea(edges: .all)
        }
    }

    func didDismiss() {
        // Handle the dismissing action.
    }
}

这种方式固然不错，但是这有点不美观(至少我是这么认为的)，并且似乎不像2021的东西。

Sheet

在UIKit当中我们可以轻松控制用户是否可以滑动关闭试图，由于SwiftUI还不太成熟所以这功能没有，但是我们可以通过构建UIkit桥梁完成。

首先加入以下代码

struct MbModalHackView: UIViewControllerRepresentable {
    var dismissable: () -> Bool = { false }
    
    func makeUIViewController(context: UIViewControllerRepresentableContext<MbModalHackView>) -> UIViewController {
        MbModalViewController(dismissable: self.dismissable)
    }
    
    func updateUIViewController(_ uiViewController: UIViewController, context: Context) {
        
    }
}

extension MbModalHackView {
    private final class MbModalViewController: UIViewController, UIAdaptivePresentationControllerDelegate {
        let dismissable: () -> Bool
        
        init(dismissable: @escaping () -> Bool) {
            self.dismissable = dismissable
            super.init(nibName: nil, bundle: nil)
        }
        
        required init?(coder: NSCoder) {
            fatalError("init(coder:) has not been implemented")
        }
        
        override func didMove(toParent parent: UIViewController?) {
            super.didMove(toParent: parent)
            
            setup()
        }
        
        func presentationControllerShouldDismiss(_ presentationController: UIPresentationController) -> Bool {
            dismissable()
        }
        
        // set delegate to the presentation of the root parent
        private func setup() {
            guard let rootPresentationViewController = self.rootParent.presentationController, rootPresentationViewController.delegate == nil else { return }
            rootPresentationViewController.delegate = self
        }
    }
}

extension UIViewController {
    fileprivate var rootParent: UIViewController {
        if let parent = self.parent {
            return parent.rootParent
        }
        else {
            return self
        }
    }
}

extension View {
    /// Control if allow to dismiss the sheet by the user actions
    public func allowAutoDismiss(_ dismissable: @escaping () -> Bool) -> some View {
        self
            .background(MbModalHackView(dismissable: dismissable))
    }
    
    /// Control if allow to dismiss the sheet by the user actions
    public func allowAutoDismiss(_ dismissable: Bool) -> some View {
        self
            .background(MbModalHackView(dismissable: { dismissable }))
    }
}

添加这些代码后在sheet上做一些修改

.sheet(isPresented: self.$showGuide, content: {
            GuidePageView(canHidGuide: self.$canHidGuide, hand: self.$hand)
                .allowAutoDismiss { self.canHidGuide }
            
        })

注意，我们添加了一个.allowAutoDismiss { self.canHidGuide }这个需要传入一个Bool类型变量，代表是否支持用户直接下拉关闭Sheet，你可以传入一个State变量控制这个参数。