Some times, only the preview of SwiftUI view crashes. Both the simulator and the real device don't crash. So we need to do extra works or don't run some methods if the target is a preview.

We can't identify preview from debug, so we add a isPreview variable under debug, and that won't leave the variable in release version.

struct SomeView : View {
    #if DEBUG
    var isPreview = false
    #endif
    var body: some View {
        Text("Hello World!")
            .onAppear {
                #if DEBUG
                if isPreview {
                    return
                }
                #endif
                
                foo()
            }
    }
    
    private func foo() {
        ...
    }
}

struct SomeView_Previews: PreviewProvider {
    static var previews: some View {
        Group {
            SomeView(isPreview: true)
                .previewDevice(PreviewDevice(rawValue: "iPhone SE 2"))
                .previewDisplayName("iPhone SE 2")
            
            SomeView(isPreview: true)
                .previewDevice(PreviewDevice(rawValue: "iPhone 14 Pro"))
                .previewDisplayName("iPhone 14 Pro")
        }
    }
}

In my recent project, a @State variable was not set after set.

        self.project?.currentTransUnit?.isVerified = true
        self.project?.update() // debug point. 
        // po "self.project?.currentTransUnit?.isVerified" prints false.

So I created another project to narrow the issue.

//
//  Model_SampleApp.swift
//  Model Sample
//
//  Created by zhaoxin on 2022/8/28.
//

import SwiftUI

@main
struct Model_SampleApp: App {
    @State private var foo:Foo? = Foo(bar: Bar(item: Item(name: "Johnny")))
    
    var body: some Scene {
        WindowGroup {
            ContentView(foo: $foo)
        }
    }
}

//
//  ContentView.swift
//  Model Sample
//
//  Created by zhaoxin on 2022/8/28.
//

import SwiftUI

struct ContentView: View {
    @Binding var foo:Foo?
    
    var body: some View {
        VStack {
            Text(foo?.bar.item.name ?? "nil")
            
            HStack {
                TextField("name", text: Binding(get: {
                    foo?.bar.item.name ?? "nil"
                }, set: { newValue in
                    foo?.bar.item.name = newValue
                }))
            }
        }
        .padding()
        .onChange(of: foo) { newValue in
            print(newValue)
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView(foo: .constant(nil))
    }
}

//
//  Model.swift
//  Model Sample
//
//  Created by zhaoxin on 2022/8/28.
//

import Foundation

struct Foo:Equatable {
    var bar:Bar
}

struct Bar:Equatable {
    var item:Item
}

struct Item:Equatable {
    var name:String
    let id = UUID()
}

All things went fine. I noticed that onChange required Equatable. So I did a test.

func onChange<V>(of value: V, perform action: @escaping (V) -> Void) -> some View where V : Equatable

struct Item:Equatable {
    var name:String
    let id = UUID()
    
    static func == (lhs: Item, rhs: Item) -> Bool {
        return lhs.id == rhs.id
    }
}

Now the issue happened again. So I guess for optimize performance. When set to a @State variable, SwiftUI compared the previous and current value by Equatable. If they were the same, SwiftUI thought they were identical and didn't replace that current with the previous value. So if we implemented the Equatable's method with part implementation on purpose, the issue appeared on the values which were not mentioned in Equatable's method.

A Simplest TabView Sample

ContentView

struct ContentView: View {
    var body: some View {
        TabView {
            ForEach(1..<4) { id in
                TabItemView(id: id)
            }
        }
    }
}

TabItemView

struct TabItemView: View {
    @State var id:Int
    
    var body: some View {
        Text("Tab \(id)")
            .tabItem {
                Text(String(id))
            }
    }
}

Life Cycle of Tab Item View

TabItemView

struct TabItemView: View {
    @State var id:Int
    
    var body: some View {
        Text("Tab \(id)")
            .tabItem {
                Text(String(id))
            }
            .onAppear(perform: {
                print("I am tab \(id)")
            })
            .onDisappear {
                print("Tab \(id) am quit!")
            }
    }
}

Runs.

I am tab 1
Tab 1 am quit!
I am tab 2
Tab 2 am quit!
I am tab 3
Tab 3 am quit!
I am tab 1

Is the tab item view just hidden or actually quit when you change a tab?

TabItemView

struct TabItemView: View {
    @State var id:Int
    @State private var title = ""
    
    var body: some View {
        HStack {
            Text("Tab \(id)")
                .onAppear(perform: {
                    print("I am tab \(id)")
                })
                .onDisappear {
                    print("Tab \(id) am quit!")
            }
            
            TextField("Title", text: $title)
        }
        .tabItem {
            Text(String(id))
        }
        .padding()
    }
}

Input "Test" in tab 1, and change to other tabs, then change back to tab 1. You will see the "Test" is still there. That means the tab item view was just hidden, not quit.

Change Tab Name

TabItemView

struct TabItemView: View {
    @State var id:Int
    @State private var title = ""
    
    var body: some View {
        HStack {
            Text("Tab \(id)")
                .onAppear(perform: {
                    print("I am tab \(id)")
                })
                .onDisappear {
                    print("Tab \(id) am quit!")
            }
            
            TextField("Title", text: $title)
        }
        .tabItem {
            Text(getTabName())
        }
        .padding()
    }
    
    private func getTabName() -> String {
        if title.isEmpty {
            return String(id)
        }
        
        return title
    }
}

Put Indicators in Window's Title Bar

ContentView

@main
struct TabView_SampleApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .windowToolbarStyle(.unifiedCompact(showsTitle: false))
    }
}

The indicators worked. But the tab title no longer worked.

Fix Tab Title

TabItemView

struct TabItemView: View {
    @Binding var tabContent:TabContent
    
    var body: some View {
        HStack {
            Text("Tab \(tabContent.id)")
            
            TextField("Title", text: $tabContent.title)
                .frame(minWidth: 300)
        }
        .tabItem {
            Text(getTabName())
        }
        .padding()
    }
    
    private func getTabName() -> String {
        if tabContent.title.isEmpty {
            return String(tabContent.id)
        }
        
        return tabContent.title
    }
}

struct TabContent:Identifiable, Equatable, Hashable {
    var id = 0
    var title = ""
}

ContentView

struct ContentView: View {
    @State private var tabContents:[TabContent] = {
        (1..<4).map { TabContent(id: $0) }
    }()
    
    @State private var currentTabContent = TabContent(id: 1)
    
    var body: some View {
        ForEach($tabContents) { $tabContent in
            if currentTabContent.id == tabContent.id {
                TabItemView(tabContent: $tabContent)
            }
        }
        .toolbar {
            HStack {
                Picker(String(currentTabContent.title), selection: $currentTabContent) {
                    ForEach(tabContents) { tabContent in
                        Text(tabContent.title.isEmpty ? String(tabContent.id) : tabContent.title).tag(tabContent)
                    }
                }
                .pickerStyle(.segmented)
                .onChange(of: currentTabContent) { newValue in
                    if let index = tabContents.map({$0.id}).firstIndex(of: newValue.id) {
                        tabContents[index] = newValue
                    }
                }
                
                Spacer()
            }
        }
        .frame(width: 400, height: 50)
    }
}