Breaking down responsibilities

ViewController.swift

import UIKit

// MARK: - User

class User: Equatable, Hashable
{
  var id: String
  var email: String
  var hashValue: Int {
    return id.hashValue
  }
  
  init(id: String, email: String)
  {
    self.id = id
    self.email = email
  }
}

func ==(lhs: User, rhs: User) -> Bool
{
  return lhs.email == rhs.email
}

// MARK: - Post

struct Post: Comparable
{
  var timestamp = NSDate()
}

func <(lhs: Post, rhs: Post) -> Bool
{
  return lhs.timestamp.compare(rhs.timestamp) == .OrderedAscending
}

func ==(lhs: Post, rhs: Post) -> Bool
{
  return lhs.timestamp == rhs.timestamp
}

// MARK: - API

class UserManager
{
  func loggedInUser() -> User?
  {
    return User(id: "a", email: "a@a.com")
  }
  
  func followersForUser(user: User) -> [User]
  {
    return [User(id: "b", email: "b@b.com"), User(id: "c", email: "c@c.com")]
  }
}

class PostManager
{
  func fetchPostsForUser(user: User, completionHandler: (posts: [Post]?, error: ErrorType?) -> ())
  {
    // ...
  }
}

// MARK: - View Controller

class ViewController: UITableViewController
{
  let userManager = UserManager()
  let postManager = PostManager()
  
  var recentPosts = [Post]()
  
  @IBOutlet weak var loginButton: UIButton!
  
  override func viewDidLoad()
  {
    super.viewDidLoad()
    displayFollowerPosts()
  }
  
  // MARK: - Steps to follow
  
  func displayFollowerPosts()
  {
    if let currentUser = userManager.loggedInUser() {
      let followers = userManager.followersForUser(currentUser)
      fetchPostsByAllFollowers(followers, completionHandler: { (posts: [Post]?, error: ErrorType?) -> () in
        if let error = error {
          self.showAlert(error)
        } else if let posts = posts {
          self.updateFollowerPosts(posts)
        }
      })
      hideLoginButton()
    } else {
      showLoginButton()
    }
  }
  
  func updateFollowerPosts(posts: [Post])
  {
    recentPosts = Array(posts.sort { $0 > $1 }[0...4])
    refreshFollowerPosts()
    hideLoginButton()
  }
  
  // MARK: - Fetch posts
  
  func fetchPostsByAllFollowers(followers: [User], completionHandler: (posts: [Post]?, error: ErrorType?) -> ())
  {
    var allFollowerPosts = [User: [Post]]()
    for follower in followers {
      fetchPostsByFollower(follower) { (posts: [Post]?, error: ErrorType?) -> () in
        if let error = error {
          completionHandler(posts: nil, error: error)
        } else if let posts = posts {
          allFollowerPosts[follower] = posts
          if allFollowerPosts.count == followers.count {
            completionHandler(posts: Array(allFollowerPosts.values.flatten()), error: nil)
          }
        }
      }
    }
  }
  
  func fetchPostsByFollower(follower: User, completionHandler: (posts: [Post]?, error: ErrorType?) -> ())
  {
    postManager.fetchPostsForUser(follower) { (posts: [Post]?, error: ErrorType?) -> () in
      if let error = error {
        completionHandler(posts: nil, error: error)
      } else if let posts = posts {
        completionHandler(posts: posts, error: nil)
      }
    }
  }
  
  // MARK: - UI
  
  func refreshFollowerPosts()
  {
    tableView.reloadData()
  }
  
  func showLoginButton()
  {
    loginButton.hidden = false
  }
  
  func hideLoginButton()
  {
    loginButton.hidden = true
  }
  
  func showAlert(error: ErrorType)
  {
    // ...
  }
}