TabbedView using SwiftUI in Xcode11Beta (11M336w)

问题

I am following along with the session from WWDC2019 here :

https://developer.apple.com/videos/play/wwdc2019/216/

I have the following code working for creating a TabbedView using SwiftUI :

//Section1 | ContentView (mine)---------------------------

import SwiftUI

struct ContentView : View {
    var body: some View {
        NavigationView {
            TabbedView(selection: .constant(1)) {
                PlaceForm().tabItemLabel(Text("Tab1")).tag(1)
                FavoritesForm().tabItemLabel(Text("Tab2")).tag(2)
            }
        }

    }
}

#if DEBUG
struct ContentView_Previews : PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}
#endif

//--------------------------- The above produces the following tabbed view :

However, in the WWDC2019 session, the following code is used :

//Section2 | ContentView (Apple's)---------------------------

import SwiftUI

struct ContentView : View {
    var body: some View {
        NavigationView {
            TabbedView(selection: .constant(1)) {
                PlaceForm().tabItemLabel {
                    Image(systemName: "square.and.pencil")
                    Text("Tab1")
                }
                FavoritesForm().tabItemLabel {
                    Image(systemName: "clock.fill")
                    Text("Tab2")
                }
            }
        }

    }
}

#if DEBUG
struct ContentView_Previews : PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}
#endif

//---------------------------

However, on Xcode11Beta, this results in the following compiler error being thrown by Xcode11Beta

Cannot convert value of type 'TabbedView<Int, 

TupleView<(_ModifiedContent<PlaceForm, _TraitWritingModifier<AnyView?>>, 

_ModifiedContent<FavoritesForm, _TraitWritingModifier<AnyView?>>)>>' to 

closure result type '_'

as seen in the following screenshots

and

//---------------------------

What is the reason that the code demonstrated in the WWDC2019 slides don't result in the images showing up in the tabs of the tabbed view as should be expected if the information in the WWDC2019 presentation is correct?

Also, with the code in section1, switching tabs to tab2 shows a blank view as described in the following question :

SwiftUI TabbedView only shows first tab's content

Please note that the contents of PlaceForm and FavoritesForm are as reproduced below

//Section3 | PlaceForm---------------------------

import SwiftUI

struct PlaceForm : View {
    var body: some View {
        List {
            VStack {
                MapView()
                    .edgesIgnoringSafeArea(.top)
                    .frame(height: 300)

                CircleImage()
                    .offset(y: -130)
                    .padding(.bottom, -130)
                VStack {
                    VStack {
                        Text("Turtle Rock")
                            .font(.title)
                            .color(.black)
                    }
                    HStack {
                        Text("Joshua Tree National Park")
                            .font(.subheadline)
                        Spacer()
                        Text("California")
                            .font(.subheadline)
                    }
                    }
                    .padding()
            }
        }.listStyle(.grouped)

    }
}

#if DEBUG
struct PlaceForm_Previews : PreviewProvider {
    static var previews: some View {
        PlaceForm()
    }
}
#endif

//Section4 | FavoritesForm---------------------------

import SwiftUI

struct FavoritesForm : View {
    var body: some View {
        List {
            VStack {
                MapView()
                    .edgesIgnoringSafeArea(.top)
                    .frame(height: 300)

                CircleImage()
                    .offset(y: -130)
                    .padding(.bottom, -130)
                VStack {
                    VStack {
                        Text("Ninja Rock")
                            .font(.title)
                            .color(.black)
                    }
                    HStack {
                        Text("Joshua Tree National Park")
                            .font(.subheadline)
                        Spacer()
                        Text("California")
                            .font(.subheadline)
                    }
                    }
                    .padding()
            }
            }.listStyle(.grouped)

    }
}

#if DEBUG
struct FavoritesForm_Previews : PreviewProvider {
    static var previews: some View {
        FavoritesForm()
    }
}
#endif

回答1:

This issue was fixed with Xcode 11 beta 3. From iOS & iPadOS 13 Beta 3 Release Notes:

The tabItemLabel(:) modifier — now named tabItem(:) — now accepts @ViewBuilder closures. (51502668)

Example:

myView()
    .tabItem {
        Image(systemName: "circle")
        Text("Tab1")
    }

回答2:

I had success in Beta 2 by wrapping the 2 controls inside tabItemLabel with a VStack:

.tabItemLabel(VStack {
                    Image(systemName: "list.bullet")
                    Text("Foo").font(.title)
            })

回答3:

After some playing around, it looks like tabbed views don't accept system images yet. This code compiled for me. I'm running Xcode 11.0 beta (11M336w) on Catalina 10.15 Beta (19A487m).

struct TabView : View { 
    var body: some View {
        TabbedView {
            HomeFeedUIV().tabItemLabel(Image(systemName: "house")) // doesn't work

            DatabaseHomeUIV().tabItemLabel(Image("database.unselected")) // works

            NewPostUIV().tabItemLabel(Image(systemName: "square.and.pencil")) // doesn't work
        }
    }
}

I tried using a VStack for the tabItemLabels (Image and Text) but the debugger said tabItemLabels don't accept VStacks, only Images and text. I have yet to find out how to make text and and image appear, it seems to only accept one or the other. I've tried using parenthesis, brackets, curly braces, VStacks, none of them work. Looks like it's one or the other for now.

来源：https://stackoverflow.com/questions/56604914/tabbedview-using-swiftui-in-xcode11beta-11m336w