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func createPeerConnection() {
    let config = RTCConfiguration()
    config.iceServers = [RTCIceServer(urlStrings: iceServers)]
    
    // Unified plan is more superior than planB
    config.sdpSemantics = .unifiedPlan
    
    // gatherContinually will let WebRTC to listen to any network changes and send any new candidates to the other client
    config.continualGatheringPolicy = .gatherContinually
    

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override func viewDidLoad() {
    super.viewDidLoad()
    
    tableView.prefetchDataSource = self
  }
  
// MARK: - Table View Data Source Prefetching
extension UsersTableViewController: UITableViewDataSourcePrefetching {
  func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) {
    print("prefetch rows at index paths: \(indexPaths)")

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  @IBAction func fetch() {
    guard !isFetchInProgress else {
      return
    }
    
    isFetchInProgress = true
  
    fetchTotalCountIfNeeded { (totalCount, err) in
      guard err == nil else {
        print("Error when get total count: \(err!)")

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  private func fetchUsers(completed: @escaping (QuerySnapshot?, Error?)->Void) {
    let usersDB = Firestore.firestore().collection("users")
    var query: Query
    
    if self.users.count == 0 {
      query = usersDB
        .order(by: "firstName")
        .limit(to: countPerPage)
    }
    else {

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  func fetchTotalCountIfNeeded(completed: @escaping (Int?, Error?)->Void) {
    guard totalCount == 0 else {
      completed(totalCount, nil)
      return
    }
    
    let usersCountRef = Firestore.firestore().collection("counters").document("users")
    usersCountRef.getDocument { (doc, error) in
      if let error = error {
        completed(nil, error)

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@IBAction func populate() {
    createUsersCounterIfNeeded { (error) in
      if let error = error {
        print("Error when creating users counter: \(error)")
      }
      else {
        self.populateUsers(addedCount: self.addedCount)
      }
    }
  }

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// Create users and update users counter
  private func populateUsers(addedCount: Int) {
    let db = Firestore.firestore()
    db.runTransaction({ (transaction, errorPointer) -> Any? in
      // Update user counter to added users amount
      let usersCounterRef = db.collection("counters").document("users")
      let usersCounterDoc: DocumentSnapshot
      
      do {
        try usersCounterDoc = transaction.getDocument(usersCounterRef)

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  private func createUsersCounterIfNeeded(completed: @escaping (_ error: Error?) -> Void) {
    let usersCounter = Firestore.firestore().collection("counters").document("users")
    usersCounter.getDocument { (doc, error) in
      guard let doc = doc else {
        completed(error)
        return
      }
      
      if doc.exists {
        completed(nil)

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class ViewController: UIViewController {

    @IBOutlet weak var imageView: UIImageView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        
        let renderer = UIGraphicsImageRenderer(size: CGSize(width: 200, height: 200))
        let image = renderer.image { (context) in

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class PushButton: UIButton {
    override func draw(_ rect: CGRect) {
        let path = UIBezierPath(ovalIn: rect)
        UIColor.green.setFill()
        path.fill()
    }
}