How to show a loading animation during firebase query in SwiftUI

Question

I am building an app with SwiftUI, and have an ObservableObject for querying my Firestore database. My documents are relatively large and I often need to query a lot of them, so I wanted to incorporate some sort of loading indicator while the query is downloading the data.

This is an example of the ObservableObject I have created:

import FirebaseFirestore
import SwiftUI

struct Document: Identifiable, Equatable {
    var id: String
    var content: String
}

class Fetch: ObservableObject {
    init(loading: Binding<Bool>) {
        self._loading = loading
        readDocuments()
    }
    
    @Published var documents: [Document] = []
    @Binding var loading: Bool
    
    var collection: CollectionReference = Firestore.firestore().collection("DOCUMENTS")
    
    func newDocument(content: String) {
        let id = self.collection.document().documentID
        self.collection.document(id).setData(["id": id, "content": content]) { (error) in handleError(error: error, message: "\(id) CREATED") }
    }
    
    func deleteDocument(document: Document) {
        if self.documents.contains(document) {
            self.collection.document(document.id).delete() { (error) in handleError(error: error, message: "\(document.id) DELETED") }
        } else { print("\(document.id) NOT FOUND") }
    }
    
    func updateDocument(document: Document, update: [String : Any]) {
        if self.documents.contains(document) {
            self.collection.document(document.id).updateData(update) { (error) in handleError(error: error, message: "\(document.id) UPDATED") }
        } else { print("\(document.id) NOT FOUND") }
    }
    
    func readDocuments() {
        self.collection.addSnapshotListener { (snapshot, error) in
            handleError(error: error, message: "READ DOCUMENTS")
            snapshot?.documentChanges.forEach({ (change) in
                if change.type == .added {
                    self.loading = true
                    self.documents.append(Document(id: change.document.get("id") as? String ?? "FAILED TO READ",
                                                   content: change.document.get("content") as? String ?? "FAILED TO READ"))
                    self.loading = false
                }
                if change.type == .modified {
                    self.loading = true
                    self.documents = self.documents.map { (document) -> Document in
                        if document.id == change.document.documentID {
                            let modifiedDocument = Document(id: change.document.get("id") as? String ?? "FAILED TO READ",
                                                 content: change.document.get("content") as? String ?? "FAILED TO READ")
                            return modifiedDocument
                        } else {
                            return document
                        }
                    }
                    self.loading = false
                }
                if change.type == .removed {
                    self.loading = true
                    self.documents.removeAll(where: { $0.id == change.document.documentID })
                    self.loading = false
                }
                
            })
        }
    }
    
}

func handleError(error: Error?, message: String) {
    if error != nil { print((error?.localizedDescription)!); return } else { print(message) }
}

This is the example view I have created:

struct ContentView: View {
    @State var loading: Bool = false
    var body: some View {
        NavigationView {
            if loading {
                Color.blue.overlay(Text("Loading View"))
            } else {
                Subview(fetch: Fetch(loading: self.$loading))
            }
        }
    }
}

struct Subview: View {
    @ObservedObject var fetch: Fetch
    @State var newDocumentContent: String = ""
    
    var body: some View {
        VStack(spacing: 0.0) {
            List {
                ForEach(self.fetch.documents) { document in
                    NavigationLink(destination:
                    UpdateDocument(fetch: self.fetch, documentID: document.id)) {
                        Text(document.content)
                    }
                }.onDelete { indexSet in
                    self.deleteDocument(indexSet: indexSet)
                }
            }
            
            Divider()
            
            NewDocument(fetch: self.fetch, newDocumentContent: self.$newDocumentContent)
        }.navigationBarTitle("CRUD", displayMode: .inline)
    }
    
    func deleteDocument(indexSet: IndexSet) {
        self.fetch.deleteDocument(document: self.fetch.documents[indexSet.first!])
    }
}

Keep in mind for this example the data is not nearly as big to need a loading view, this is pretty much instant, but for my application this code is split into loads of different files and scenarios so I thought I'd create this example.

I have tried adding a binding boolean and toggling it when the readData() function is loading, but SwiftUI gets map and I get an error.

'Modifying state during view update, this will cause undefined behavior.'

Answer 1

You need to use @Published in the @ObservableObject (not @Binding).

Here is a possible demo:

class Fetch: ObservableObject {
    @Published var loading = false

    func longTask() {
        self.loading = true
        // simulates a long asynchronous task (eg. fetching data)
        DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
            self.loading = false
        }
    }
}

struct ContentView: View {
    @ObservedObject private var fetch = Fetch()

    var body: some View {
        ZStack {
            Text("Main view")
            if fetch.loading {
                LoadingView() // can be replaced with any other loading indicator/view
            }
        }
        .onAppear {
            self.fetch.longTask()
        }
    }
}

struct LoadingView: View {
    var body: some View {
        ZStack {
            Color.black
                .opacity(0.5)
                .edgesIgnoringSafeArea(.all)
            Text("Loading")
        }
    }
}