How to create autocomplete text field in Swift

Using Trie like structure will be a better option here. Based on entered string, trie will return top keywords (lets say 10) starting with the entered string. Implementing this trie on server side is better. When UI makes http call, calculations will be done on server side and server will send top results to UI. Then, UI will update TableView with new data.

You can also do this with hashmap/dictionary but performance will be worse. Using trie/prefix tree approach will give you the best performance when you have thousands or millions of strings to check.

I did something like this in my app for looking up contacts. I will pseudo code this out for you to understand the concept:

1) Capture the characters entered into the textfield by the enduser

2) At some character count entered decide to query the server to return all entries that match - choose the character count you are comfortable with (I chose around 3-4 characters). Fewer returns more, more returns less obviously...up to you, perf and UX considerations.

3) Put the results of this server query into an array on the client. This will be your superset from which you will offer the suggestions to the user.

4) After each subsequent character entered into the text field you will now filter the array (array.filter()) by character string entered to this point. 5) tableView.reloadData() against the filtered array at each character entered.

6) I use a dataFlag variable to determine what datasource to show in the tableview depending on what the user is doing.

Note: You only query the server once to minimize perf impact

// this function is called automatically when the search control get user focus
func updateSearchResults(for searchController: UISearchController) {
  let searchBar = searchController.searchBar
  if searchBar.text?.range(of: "@") != nil {
    self.getUserByEmail(searchBar.text!)
  }
  if searchController.searchBar.text?.characters.count == 0 && dataFlag != "showParticipants" {
    dataFlag = "showInitSearchData"
    self.contacts.removeAll()
    self.participantTableView.reloadData()
  }
  if dataFlag == "showInitSearchData" && searchController.searchBar.text?.characters.count == 2 {
    self.loadInitialDataSet() {
      self.dataFlag = "showFilteredSearchData"
    }
  }
  if dataFlag == "showFilteredSearchData" {
    self.filterDataForSearchString()
  }

}

// filter results by textfield string
func filterDataForSearchString() {
  let searchString = searchController.searchBar.text
  self.filteredContacts =  self.contacts.filter({
    (contact) -> Bool in
    let contactText: NSString = "\(contact.givenName) \(contact.familyName)" as NSString

  return (contactText.range(of: searchString!, options: NSString.CompareOptions.caseInsensitive).location) != NSNotFound
  })

  DispatchQueue.main.async {
    self.participantTableView.reloadData()
  }  
}