视图中的 Firestore 方法 model 未在 SwiftUI onAppear 方法中调用

Question

I want to implement a Text field that displays the current user's existing score in the DB (Firestore). Because of the nature of async in Firebase query, I also need to do some adjustment in my codes. However, it seems that completion() handler does not work well:

// ViewModel.swift

import Foundation
import Firebase
import FirebaseFirestore

class UserViewModel: ObservableObject {
    let current_user_id = Auth.auth().currentUser!.uid
    private var db = Firestore.firestore()
    @Published var xp:Int?
    
    func fetchData(completion: @escaping () -> Void) {
        let docRef = db.collection("users").document(current_user_id)
        
        docRef.getDocument { snapshot, error in
              print(error ?? "No error.")
              self.xp = 0
              guard let snapshot = snapshot else {
                  completion()
                  return
              }
            self.xp = (snapshot.data()!["xp"] as! Int)
            completion()
        }
    }
}

// View.swift

import SwiftUI
import CoreData
import Firebase

{
    @ObservedObject private var users = UserViewModel()
    var body: some View {
        VStack {
            HStack {
                // ...
                Text("xp: \(users.xp ?? 0)")
//                    Text("xp: 1500")
                    .fontWeight(.bold)
                    .padding(.horizontal)
                    .foregroundColor(Color.white)
                    .background(Color("Black"))
                    .clipShape(CustomCorner(corners: [.bottomLeft, .bottomRight, .topRight, .topLeft], size: 3))
                    .padding(.trailing)
            }
            .padding(.top)
            .onAppear() {
                self.users.fetchData()
            }
// ...
    }
}

My result kept showing 0 in Text("xp: \(users.xp?? 0)") , which represents that the step is yet to be async'ed. So what can I do to resolve it?

Answer 1

I would first check to make sure the data is valid in the Firestore console before debugging further. That said, you can do away with the completion handler if you're using observable objects and you should unwrap the data safely. Errors can always happen over network calls so always safely unwrap anything that comes across them. Also, make use of the idiomatic get() method in the Firestore API, it makes code easier to read.

That also said, the problem is your call to fetch data manually in the horizontal stack's onAppear method. This pattern can produce unsavory results in SwiftUI, so simply remove the call to manually fetch data in the view and perform it automatically in the view model's initializer.

class UserViewModel: ObservableObject {
    @Published var xp: Int?
    
    init() {
        guard let uid = Auth.auth().currentUser?.uid else {
            return
        }
        let docRef = Firestore.firestore().collection("users").document(uid)

        docRef.getDocument { (snapshot, error) in
            if let doc = snapshot,
               let xp = doc.get("xp") as? Int {
                self.xp = xp
            } else if let error = error {
                print(error)
            }
        }
    }
}

struct ContentView: View {
    @ObservedObject var users = UserViewModel()

    var body: some View {
        VStack {
            HStack {
                Text("xp: \(users.xp ?? 0)")
            }
        }
    }
}

SwiftUI View - viewDidLoad()? is the problem you ultimately want to solve.