Twitter accounts picker, Garmin Connect devs need this shit.

ViewController.swift

//
//  ViewController.swift
//  TwitterAccountsDemo
//

import UIKit
import Accounts

class ViewController: UIViewController {
  var twitterAccount: ACAccount? = nil {
    didSet {
      print("Twitter username = \(twitterAccount?.username)")
    }
  }

  override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    loadTwitterAccounts()
  }

  override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
  }

  func loadTwitterAccounts() {
    let accountStore = ACAccountStore()
    let twitterAccountType = accountStore.accountTypeWithAccountTypeIdentifier(ACAccountTypeIdentifierTwitter)

    accountStore.requestAccessToAccountsWithType(twitterAccountType, options: nil) { (granted, _) -> Void in
      if (granted) {
        let accounts = accountStore.accountsWithAccountType(twitterAccountType)

        switch accounts.count {
        case 0:
          break
        case 1:
          guard let account = accounts.first as? ACAccount else { return }
          self.twitterAccount = account
        default:
          self.chooseAccount(accounts)
        }
      } else {
        // User declined permission
      }
    }
  }

  func chooseAccount(accounts: [AnyObject]) {
    let alert = UIAlertController.init(title: "You have multiple Twitter accounts", message: "Please pick one", preferredStyle: .ActionSheet)

    for account in accounts {
      guard let account = account as? ACAccount else { return }

      let action = UIAlertAction.init(title: account.username, style: .Default, handler: { (action) -> Void in
        self.twitterAccount = account

        dispatch_async(dispatch_get_main_queue(), { () -> Void in
          self.dismissViewControllerAnimated(true, completion: nil)
        })
      })

      alert.addAction(action)
    }

    alert.addAction(UIAlertAction.init(title: "Cancel", style: .Cancel, handler: { (action) -> Void in
      dispatch_async(dispatch_get_main_queue(), { () -> Void in
        self.dismissViewControllerAnimated(true, completion: nil)
      })
    }))

    dispatch_async(dispatch_get_main_queue(), { () -> Void in
      self.showViewController(alert, sender: self)
    })
  }
}