Handling asynchronous Firestore data reading in tableView - iOS

Question

I would like to retrieve data from my simple Firestore database

I have this database:

then I have a model class where I have a method responsible for retrieving a data which looks like this:

func getDataFromDatabase() -> [String] {
    var notes: [String] = []
    collectionRef = Firestore.firestore().collection("Notes")

    collectionRef.addSnapshotListener { querySnapshot, error in
            guard let documents = querySnapshot?.documents else {
                print("Error fetching documents: \(error!)")
                return
            }
        notes = documents.map { $0["text"]! } as! [String] // text is a field saved in document
            print("inside notes: \(notes)")
    }
    print("outside notes: \(notes)")
    return notes
}

and as a UI representation I have tableViewController. Let's take one of the methods, for example

override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    print("tableview numberOfRowsInSection called")
    return model.getDataFromDatabase().count
}

Then numberOfRows is 0 and the output in the console is:

and I am ending up with no cells in tableView. I added a breakpoint and it doesn't jump inside the listener.

And even though I have 3 of them, they are kinda "late"? They are loaded afterwards. And then the tableView doesn't show anything but console says (later) that there are 3 cells.

If needed, there is also my method for showing the cells names:

 override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    print("Cells")
    let cell = tableView.dequeueReusableCell(withIdentifier: "firstCell", for: indexPath)

    cell.textLabel!.text = String(model.getDataFromDatabase()[indexPath.row].prefix(30))

    return cell
}

but this method is not even loaded (no print in the console) and this method is written below the method with numberOfRowsInSection.

I have also 2 errors (I don't know why each line is written twice) and these are:

but I don't think it has something to do with the problem.

Thank you for your help!

Firestore is async, so notes is returning as an empty array before the data has been downloaded from the sever. Either populate a viewController variable and add a didSet observer or add completionHandler to your `getDataFromDatabase` method — Galo Torres Sevilla, Apr 03 '19 at 21:50
@GaloTorresSevilla could you, please, provide code solution? At least some smaller one as this is the first time I am implementing this. I understood your comment but I am not able to implement. Thank you. — Michal Moravik, Apr 03 '19 at 22:07
@MapeSVK didn’t really have a computer in hand. That’s why I didn’t post an answer. But you already have a good one with @Bhaumik’s answer — Galo Torres Sevilla, Apr 03 '19 at 22:25
http://www.programmingios.net/returning-a-value-from-asynchronous-code/ — matt, Apr 07 '19 at 19:07

score 2 · Accepted Answer · answered Apr 03 '19 at 22:10

As @Galo Torres Sevilla mentioned, addSnapshotListener method is async and you need to add completion handler to your getDataFromDatabase() function.

Make following changes in your code:

Declare Global variable for notes.
```
var list_notes = [String]()
```

Add completion handler to getDataFromDatabase() method.

func getDataFromDatabase(callback: @escaping([String]) -> Void) {
    var notes: [String] = []
    collectionRef = Firestore.firestore().collection("Notes")

    collectionRef.addSnapshotListener { querySnapshot, error in
        guard let documents = querySnapshot?.documents else {
            print("Error fetching documents: \(error!)")
            return
        }
        notes = documents.map { $0["text"]! } as! [String] // text is a field saved in document
        print("inside notes: \(notes)")

        callback(notes)
    }
}

Lastly, call function on appropriate location where you want to fetch notes and assign retrieved notes to your global variable and reload TableView like below:
```
self.getDataFromDatabase { (list_notes) in
    self.list_notes = list_notes
    DispatchQueue.main.async {
        self.tableView.reloadData()
    }
}
```

Changes in TableView:

override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    print("tableview numberOfRowsInSection called")
    return self.list_notes.count
}

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    print("Cells")
    let cell = tableView.dequeueReusableCell(withIdentifier: "firstCell", for: indexPath)

    cell.textLabel!.text = String(self.list_notes[indexPath.row].prefix(30))

    return cell
}

thank you mate, it works! Just one question. Why is this so complicated? Isn't this fundamental stuff? Probably I am doing it wrong way, right? — Michal Moravik, Apr 03 '19 at 22:23
Glad that it helped! Only issue with your code is, you were not waiting for Firestore to download your data. So, `cellForRowAt:indexPath:` method was executing before download was complete. By adding completion handler, it solves that issue. — Bhaumik, Apr 03 '19 at 22:30

score 0 · Answer 2 · answered Apr 03 '19 at 22:04

0

All you need to do is refresh the table cell every time you retrieve the data. Put this code after you set your data inside the array.

self.tableView.reloadData()

answered Apr 03 '19 at 22:04

Dan Hadzami

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Handling asynchronous Firestore data reading in tableView - iOS

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