AccountsViewController.h

//
//  AccountsViewController.h
//
//  Created by Nico Hämäläinen on 28/03/14.
//

#import <UIKit/UIKit.h>

@interface AccountsViewController : UIViewController

@end

AccountsViewController.m

//
//  AccountsViewController.m
//
//  Created by Nico Hämäläinen on 28/03/14.
//

#import "AccountsViewController.h"
#import <Accounts/Accounts.h>

@interface AccountsViewController () <UIActionSheetDelegate>

@property (strong, nonatomic) ACAccountStore *accountStore;
@property (strong, nonatomic) ACAccountType  *accountType;
@property (strong, nonatomic) NSArray        *availableAccounts;
@property (strong, nonatomic) NSMutableArray *availableAccountNames;
@property (strong, nonatomic) ACAccount      *chosenAccount;

@end

@implementation AccountsViewController

- (void)viewDidLoad {
    // Needed to access the accounts on the device
    _accountStore = [[ACAccountStore alloc] init];

    // Needed to filter only Twitter accounts
    _accountType = [_accountStore accountTypeWithAccountTypeIdentifier:ACAccountTypeIdentifierTwitter];

    // Request access to the Twitter accounts on the device
    // NOTE: This pops up the alert to ask for permission
    [_accountStore requestAccessToAccountsWithType:_accountType options:nil completion:
     ^(BOOL granted, NSError *error) {
         // We can only continue if the user
         if (granted == YES) {
             // Get the list of accounts we found
             _availableAccounts = [_accountStore accountsWithAccountType:_accountType];
             // If we have any, we'll save them and let the user pick the correct one
             if ([_availableAccounts count] > 0) {
                 // Pop up dat picker sheet!
                 [self displayAccountSelector];
             }
             else {
                 // Show an alert
                 [self displayAlertView];
             }
         }
    }];

    [super viewDidLoad];
}

- (void)displayAccountSelector {
    // Localized button titles
    NSString *pickerTitle = NSLocalizedString(@"Pick an Account", nil);
    NSString *cancelTitle = NSLocalizedString(@"Cancel", nil);

    // Create an empty action sheet with our title
    UIActionSheet *actionSheet = [[UIActionSheet alloc] initWithTitle:pickerTitle
                                                             delegate:self
                                                    cancelButtonTitle:nil
                                               destructiveButtonTitle:nil
                                                    otherButtonTitles:nil];

    // Enumerate account names to display in the picker
    for (ACAccount *account in _availableAccounts) {
        [actionSheet addButtonWithTitle:[account username]];
    }

    // Add cancel button
    [actionSheet addButtonWithTitle:cancelTitle];
    [actionSheet setCancelButtonIndex:[_availableAccounts count]];

    // Display
    [actionSheet showInView:self.view];
}

- (void)displayAlertView {
    NSString *alertTitle   = NSLocalizedString(@"Error", nil);
    NSString *alertMessage = NSLocalizedString(@"No Twitter accounts found.", nil);

    UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:alertTitle
                                                        message:alertMessage
                                                       delegate:nil
                                              cancelButtonTitle:nil
                                              otherButtonTitles:nil];
    [alertView show];
}

#pragma mark - UIActionSheetDelegate

- (void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex {
    ACAccount *chosenAccount = _availableAccounts[buttonIndex];
    if (chosenAccount != nil) {
        _chosenAccount = chosenAccount;
    }

    // Do whatever you want here.
}

@end