A swift autocomplete solution. I needed this to subclass a specific textview class (from the Material pod, if you're interested), but it should work fine with UITextView. I also used RxSwift to capture text changes, which can probably be done with the text view's delegate methods, though it might be harder.

AutoCompleteTextView.swift

//
//  AutoCompleteTextField.swift
//
//  Created by Phlippie Bosman on 2016/08/23.
//  Copyright © 2016 Kalido. All rights reserved.
//

import Material
import RxSwift

@objc protocol AutoCompleteProvider {
    // for async fetching
    func provideSuggestionsForAutoCompleteTextField(textField: AutoCompleteTextField, forString string: String, toCallback callback: ([String]) -> Void)
    // for dismissing async fetch
    optional func disposeFetchForAutoCompleteTextField(textField: AutoCompleteTextField)
}

class AutoCompleteTextField: ErrorTextField, UITableViewDelegate, UITableViewDataSource {
    
    // public interface
    var autocompleteDelay: NSTimeInterval = 0.5
    var autoCompleteProvider: AutoCompleteProvider?
    var autoCompleteFont: UIFont = RobotoFont.lightWithSize(12)
    var autoCompleteTextColor: UIColor = MaterialColor.black
    var autoCompleteBackgroundColor: UIColor = MaterialColor.white
    var autoCompleteSeparatorsEnabled: Bool = false
    var autoCompleteMaxHeight: CGFloat = 500
    
    // b/c my tableview got stuck behind other subviews of this textview's superview
    // set this to the textview's superview (might work with getter property?)
    var viewToBringAutoCompleteToFront: UIView?
    
    
    // ui
    private let completionsTableView = UITableView(forAutoLayout: ())
    private var tableViewHeightConstraint: NSLayoutConstraint?
    
    // data
    private var autoCompleteSuggestionStrings: [String] = []
    private let reuseIdentifier = "cell"
    private let disposeBag = DisposeBag()
    
    // call this! in like viewDidLoad or something
    func setupAutoComplete() {
        completionsTableView.registerClass(UITableViewCell.self, forCellReuseIdentifier: reuseIdentifier)
        completionsTableView.separatorStyle = autoCompleteSeparatorsEnabled ? .SingleLine : .None
        completionsTableView.backgroundColor = autoCompleteBackgroundColor
        completionsTableView.delegate = self
        completionsTableView.dataSource = self
        self.rx_text
            .skip(1) // an event always fires at startup and it is useless
            .doOnNext { [weak self] (text) in
                if text == "" {
                    self?.dismissAutoCompleteSuggestions()
                }
                
                if let this = self,
                    let provider = this.autoCompleteProvider,
                    let dispose = provider.disposeFetchForAutoCompleteTextField {
                    dispose(this)
                }
            }
            .throttle(autocompleteDelay, scheduler: MainScheduler.instance)
            .subscribeNext { [weak self] (text) in
                if text == "" {
                    self?.dismissAutoCompleteSuggestions()
                } else if let this = self
                    where this.isFirstResponder(),
                    let provider = this.autoCompleteProvider {
                        provider.provideSuggestionsForAutoCompleteTextField(this, forString: text, toCallback: this.presentAutoCompleteSuggestions)
                }
        }
        .addDisposableTo(disposeBag)
        
        addSubview(completionsTableView)
        tableViewHeightConstraint = completionsTableView.autoSetDimension(.Height, toSize: 20)
        completionsTableView.autoMatchDimension(.Width, toDimension: .Width, ofView: self)
        completionsTableView.autoPinEdge(.Top, toEdge: .Bottom, ofView: self, withOffset: 8)
        completionsTableView.autoPinEdgeToSuperviewEdge(.Left, withInset: 0)
        completionsTableView.userInteractionEnabled = true
        adjustTableviewHeight()
    }
    
    func numberOfSectionsInTableView(tableView: UITableView) -> Int {
        return 1
    }
    
    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return autoCompleteSuggestionStrings.count
    }
    
    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = UITableViewCell(style: .Default, reuseIdentifier: reuseIdentifier)
        cell.backgroundColor = autoCompleteBackgroundColor
        let suggestion = autoCompleteSuggestionStrings[indexPath.row]
        if let label = cell.textLabel {
            label.text = suggestion
            label.font = autoCompleteFont
            label.textColor = autoCompleteTextColor
        }
        cell.addGestureRecognizer(
            UITapGestureRecognizer(target: self, action: #selector(didSelectSuggestion(_:))))
        cell.userInteractionEnabled = true
        return cell
    }
    
    func presentAutoCompleteSuggestions(suggestions: [String]) {
        autoCompleteSuggestionStrings = suggestions
        completionsTableView.reloadData()
        adjustTableviewHeight()
        viewToBringAutoCompleteToFront?.bringSubviewToFront(self)
    }
    
    private func adjustTableviewHeight() {
        if let heightConstraint = tableViewHeightConstraint {
            let h = min(completionsTableView.contentSize.height, autoCompleteMaxHeight)
            UIView.animateWithDuration(0.3) {
                heightConstraint.constant = h
                self.setNeedsUpdateConstraints()
            }
        }
    }
    
    func didSelectSuggestion(sender: AnyObject!) {
        guard let tap = sender as? UIGestureRecognizer,
            let cell = tap.view as? UITableViewCell,
            let cellLabel = cell.textLabel,
            let suggestion = cellLabel.text
            else { return }
        text = suggestion
        dismissAutoCompleteSuggestions()
    }
    
    func dismissAutoCompleteSuggestions() {
        autoCompleteSuggestionStrings = []
        completionsTableView.reloadData()
        adjustTableviewHeight()
    }
    
    /*
     This allows the suggestion cells to capture touch events
     even though the table view is entirely outside the text view's frame
    */
    override func pointInside(point: CGPoint, withEvent event: UIEvent?) -> Bool {
        return super.pointInside(point, withEvent: event) ||
            completionsTableView.pointInside(
                convertPoint(point, toView: completionsTableView),
                withEvent: event)
    }
    
    override func resignFirstResponder() -> Bool {
        dismissAutoCompleteSuggestions()
        return super.resignFirstResponder()
    }
}

Pods used:
Material
RxSwift