Chat mentions solution made with Rx

mentions.swift

    var mention:Observable<[FRep]>!
    var rightSideTrimmedTextLastWord = String()
    
    var lastTypedMention = String()
    var lastTypedMentionRange:Range<String.Index>?
    
    
    func observeMessageText(){
        
         mention = messageText
            .flatMap{ [weak self] inputText -> Observable<[FRep]> in
                if inputText.isEmpty {
                    return .just([])
                }
                
                    //get's text up to the textinput cursor
                    let textWithRemovedRightSide = inputText[inputText.utf16.startIndex..<inputText.utf16.index(inputText.utf16.startIndex, offsetBy: self!.textview.selectedRange.upperBound)]
                
                    //first @ from the left side to the last index - searching to the left
                    if let lastTypedMentionRange = String(textWithRemovedRightSide)
                            .range(of: "@", options: String.CompareOptions.backwards, range: nil, locale: nil){

                        let range = lastTypedMentionRange.lowerBound..<textWithRemovedRightSide.endIndex
                        self?.lastTypedMentionRange = range
                        
                        //Check if there is a character before @ and hide mention suggestion if there is
                        if range.lowerBound > inputText.utf16.startIndex{
                            let characterBeforeTheMentionStart = inputText[inputText.utf16.index(range.lowerBound, offsetBy: -1)]
                            if characterBeforeTheMentionStart != " " { return .just([]) }
                        }
                        
                        self?.lastTypedMention = String(inputText[range])
                        
                        if (self?.chat.members
                            .filter{ $0.hashedEmail != FirebaseConstants.myUserId }
                            .compactMap{$0.name}
                            .contains(self?.lastTypedMention.replacingOccurrences(of: "@", with: "")))!{
                                self?.toggleMentionsView(toState: .closed)
                                self?.filteredUsers.removeAll()
                                return .just([])
                        }
                        
                        self?.currentMention = String(inputText[range])
                        self?.filteredUsers = (self?.chatMembersValidForMentions
                            .compactMap{$0}
                            .filter{$0.name != nil}
                            .filter{($0.name!.lowercased()
                                .hasPrefix((self?.lastTypedMention.lowercased()
                                    .replacingOccurrences(of: "@", with: ""))!))})!
                    }else{
                        return Observable.just([])
                    }
                
                
                return Observable.just(self!.filteredUsers)
            }
        
        
        mention
            .subscribe(onNext:{ [weak self] in
                    !$0.isEmpty ? self?.toggleMentionsView(toState: .open) : self?.toggleMentionsView(toState: .closed)
            })
            .disposed(by: disposeBag)
    }
    
    enum MentionsViewState{
        case open, closed
    }
    
    func toggleMentionsView(toState state:MentionsViewState){
        
        switch state {
        case .closed:
            mentionsViewHeightContraint?.constant = CGFloat(0.0)
            return
        case .open:
            DispatchQueue.main.async {[weak self] in
                
                //4.5 because the agreement is to show 4.5 rows so the user knows they can scroll
                guard let mentionsRowHeight = self?.mentionsRowHeight,
                    let numberOfUsers = self?.filteredUsers.count else { return }
                let mentionsViewExpandedHeight = min(Double(numberOfUsers)*mentionsRowHeight,4.5*mentionsRowHeight)

                UIView.animate(withDuration: 0.22, delay: 0.1, usingSpringWithDamping: 2.5, initialSpringVelocity: 7, options: .curveEaseInOut, animations: {
                    
                    self?.mentionsViewHeightContraint?.constant = CGFloat(mentionsViewExpandedHeight)
                    self?.view.layoutIfNeeded()
                    self?.didShowMentionsView = true
                }, completion: nil)
            return
            }
        }
        
  
    }

Solution where you only need to provide two datasources

1. Text from the inputfield
2. Set of chat participants

What is great about this solution is that your names can contain emojis, zalgo or any other imaginable combination, it'll work.

Any improvements suggestion is greatly appreciated :)