SwiftUI：展示Sheet时绑定的@State变量未及时更新问题

SwiftUI的状态更新是批量异步处理的：当你在同一个按钮动作里先后修改displayString和showSheet时，这两个状态变化会被合并到同一个更新周期中。而sheet(isPresented:)的初始化逻辑会优先于displayString的视图更新生效，导致首次展示Sheet时，它捕获的还是displayString的初始值。

后续关闭Sheet后再次点击按钮，Sheet已经完成过一次初始化，状态更新的关联逻辑已建立，因此能正确获取到最新的displayString值。

推荐使用sheet(item:)替代sheet(isPresented:)，通过绑定可选值触发Sheet展示，确保状态更新完全生效后再初始化Sheet：

import SwiftUI

struct SimpleParentView: View {
    // 用可选字符串替代Bool状态，nil表示不展示Sheet
    @State private var sheetDisplayString: String? = nil
    
    var body: some View {
        VStack {
            SimpleChildView(sheetDisplayString: $sheetDisplayString)
        }
        // 绑定item，当可选值非nil时展示Sheet
        .sheet(item: $sheetDisplayString) { string in
            VStack {
                Text(string)
                Button("Close") {
                    sheetDisplayString = nil
                }
                .padding()
            }
        }
    }
}

struct SimpleChildView: View {
    @Binding var sheetDisplayString: String?
    
    var body: some View {
        Button(action: {
            sheetDisplayString = "Updated String 1"
        }) {
            Text("Button 1 (show sheet in parent)")
        }
        Button(action: {
            sheetDisplayString = "Updated String 2"
        }) {
            Text("Button 2 (show sheet in parent)")
        }
    }
}

// 让String符合Identifiable协议，满足sheet(item:)的要求
extension String: Identifiable {
    public var id: String { self }
}

struct SimpleParentView_Previews: PreviewProvider {
    static var previews: some View {
        SimpleParentView()
    }
}

若需兼容iOS16及以下版本，也可将状态封装到Observable对象中，确保状态变化被正确追踪：

import SwiftUI

class SheetState: ObservableObject {
    @Published var showSheet = false
    @Published var displayString = "Initial String"
}

struct SimpleParentView: View {
    @StateObject private var sheetState = SheetState()
    
    var body: some View {
        VStack {
            SimpleChildView(sheetState: sheetState)
        }
        .sheet(isPresented: $sheetState.showSheet) {
            VStack {
                Text(sheetState.displayString)
                Button("Close") {
                    sheetState.showSheet = false
                }
                .padding()
            }
        }
        // 触发视图更新，确保Sheet感知displayString变化
        .onChange(of: sheetState.displayString) { _ in }
    }
}

struct SimpleChildView: View {
    @ObservedObject var sheetState: SheetState
    
    var body: some View {
        Button(action: {
            sheetState.displayString = "Updated String 1"
            sheetState.showSheet = true
        }) {
            Text("Button 1 (show sheet in parent)")
        }
        Button(action: {
            sheetState.displayString = "Updated String 2"
            sheetState.showSheet = true
        }) {
            Text("Button 2 (show sheet in parent)")
        }
    }
}

sheet(isPresented:)依赖Bool状态变化触发，但SwiftUI不会为同一更新周期内的多个状态变化单独触发多次视图更新。而sheet(item:)通过可选值变化触发，赋值时状态变化会被完整处理，Sheet初始化时能直接拿到最新值。

内容的提问来源于stack exchange，提问作者Nerdy Bunz