Swift - Firestore CRUD 1

See: http://iosrevisited.blogspot.co.uk/2017/10/firebase-cloud-firestore-example-project-swift.html

01. Set up Firebase Cloud Firestore To iOS Project :
pod 'Firebase/Core'
pod 'Firebase/Firestore'

02. Add google plist to project

03. Initialize Firebase in AppDelegate.
import Firebase

04. In the application:didFinishLaunchingWithOptions: method, initialize the FirebaseApp object:
FirebaseApp.configure()

05. Add the following code to your VC:
var docRef : DocumentReference!

06. Add the following code in viewDidLoad() method, where "friends" is the collection and "profile" is the document.
Note, every document needs a different name or it gets overwritten
docRef = Firestore.firestore().document("friends/profile")

07. Saving/Adding data to Cloud Firestore:
Using docRef.setData(dataToSave), you do NOT create an AutoId

Add the following code in save button action method:

@IBAction func saveButtonTapped(_ sender: UIButton) {
    guard let name = nameField.text, !name.isEmpty else { return }
    guard let profession = professionField.text, !profession.isEmpty else { return }
    
    let dataToSave : [String: Any] = ["name": name, "profession": profession]
    docRef.setData(dataToSave) { (error) in
        if let error = error {
            print("Oh no! Some error \(error.localizedDescription)")
        }else {
            print("Data has been saved")
        }
    }
}

08. Fetching/Getting data From Cloud Firestore:
Add the following code inside fetch button action method: 

@IBAction func fetchButtonTapped(_ sender: Any) {
    docRef.getDocument { (docSnapshot, error) in
        guard let docSnapshot = docSnapshot, docSnapshot.exists else { return }
        let data = docSnapshot.data()
        let name = data["name"] as? String ?? ""
        let profession = data["profession"] as? String ?? ""
        self.titleLabel.text = "\(name) is a \(profession)"
    }
}

09.Listening for RealTime in Cloud Firestore:
First add the following listener property.

var dataListener : FIRListenerRegistration!

10. Next add the viewWillAppear() method as following:
override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    dataListener =  docRef.addSnapshotListener { (docSnapshot, error) in
        guard let docSnapshot = docSnapshot, docSnapshot.exists else { return }
        let data = docSnapshot.data()
        let name = data["name"] as? String ?? ""
        let profession = data["profession"] as? String ?? ""
        self.titleLabel.text = "\(name) is a \(profession)"
    }
}

11. Above listener will keep on calling every time the data changes. So we need to remove the observer on viewWillDisappear. So that we can avoid memory problems.

Add the following method:
override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    dataListener.remove()
}

12. Next step is update data in real time. Now add some text in text field then tap save button.
On saving only we can see the title changes in real time without fetch action.