How to fix "Use of undeclared type" error in Swift 5.1?

Question

I'm new to Swift and iOS development and try to learn by following the official SwiftUI tutorials. I got errors in section 5(https://developer.apple.com/tutorials/swiftui/handling-user-input). I followed each step meticulously and after I did section 5 step 1, Xcode popped up an error "Use of undeclared type 'UserData'". I tried to ignore and followed through. However, my LandmarkList, LandmarkDetail, SceneDelegate files warned me of the same error. I even checked the completed version of the project that I downloaded from the tutorial page. They looked exactly the same except the parts that I haven't done. Does anyone here have a similar experience as mine or can you kindly offer some suggestions? Thank you!

I'm using Xcode 11 and Swift 5.

This is the UserData class I created under the Models folder.

import Combine
import SwiftUI

final class UserData: ObservableObject {
    @Published var showFavoritesOnly = false
    @Published var landmarks = landmarkData
}

This is the rootview file that keeps prompting errors.

import SwiftUI

struct LandmarkList: View {
    @EnvironmentObject private var userData: UserData  // Use of undeclared type 'UserData'

    var body: some View {
        NavigationView {
            List { // Unable to infer complex closure return type; add explicit type to disambiguate
                Toggle(isOn: $userData.showFavoritesOnly){ // Use of unresolved identifier '$userData'
                    Text("Favorites Only")
                }

                ForEach(userData.landmarks) { landmark in
                    if !self.userData.showFavoritesOnly || landmark.isFavorite {
                        NavigationLink(destination: LandmarkDetail(landmark: landmark)
                            .environmentObject(self.userData)) {
                            LandmarkRow(landmark: landmark)
                        }
                    }
                }
            }
            .navigationBarTitle(Text("Landmarks"))
        }
    }
}

struct LandmarkList_Previews: PreviewProvider {
    static var previews: some View {
        ForEach(["iPhone SE", "iPhone XS Max"], id: \.self) { deviceName in
            LandmarkList()
                .previewDevice(PreviewDevice(rawValue: deviceName))
                .previewDisplayName(deviceName)
        }
        .environmentObject(UserData()) // Use of unresolved identifier 'UserData'
    }
}

Answer 1

You need to clean and close Xcode, then delete Derived Data, then re-open Xcode. I had been constantly having this problem while doing apple tutorials.

Answer 2

don't need to delete derived data folder or exit Xcode. just close project after tutorial section 5 step 5 then open project, allow it to index build then do step 6. this enables project to rebuild it's cache in memory.