@Stateの挙動を調べた

FailureView.swift

import SwiftUI

struct FailureView: View {
    @State var isShowSheet = false
    @State var shareData: String?
    
    var body: some View {
        Button("Change(Fail)") {
            shareData = "Show this message"
            isShowSheet = true
        }
        .sheet(isPresented: $isShowSheet) {
            Text("Passed data: " + (shareData ?? ""))
        }
    }
}

struct FailureView_Previews: PreviewProvider {
    static var previews: some View {
        FailureView()
    }
}

SampleApp.swift

import SwiftUI

@main
struct SampleApp: App {
    var body: some Scene {
        WindowGroup {
            FailureView()
            SuccessView()
        }
    }
}

SuccessView.swift

import SwiftUI

struct SuccessView: View {
    @State var shareItem: ShareItem?
    
    var body: some View {
        Button("Change(Success)") {
            shareItem = ShareItem(data: "Show this message")
        }
        .sheet(item: $shareItem) { item in
            Text("Passed data: " + item.data)
        }
    }
}

struct ShareItem: Identifiable {
    var id = UUID()
    var data: String
}

struct SuccessView_Previews: PreviewProvider {
    static var previews: some View {
        SuccessView()
    }
}