SplittedCallbackHell.swift

func prepareUserProfile() {
  self.downloadUser() { user in
    downloadUserMoviesAndFriendDataToPresentProfile(user: user)
  }
}

func downloadUserMoviesAndFriendDataToPresentProfile(user: User) {
  self.downloadFavouriteMovies(for: user) { movies in
    downloadRatingsAndFriendsDataThenPresentProfile(user: user, movies: movies)
  }
}

func downloadRatingsAndFriendsDataThenPresentProfile(user: User, movies: [Movie]) {
  self.downloadRatings(for: movies) { ratings in
    self.downloadFriendsDataAndPresentProfile(user: user, movies: movies, ratings: ratings)
  }
}

func downloadFriendsDataAndPresentProfile(user: User, movies: [Movie], ratings: [Rating]) {
  self.downloadFriends(for: users) { friends in
    downloadFriendsMoviesAndPresentProfile(user: user, movies: movies, ratings: ratings, friend: friends)
  }
}

func downloadFriendsMoviesAndPresentProfile(user: User, movies: [Movie], ratings: [Rating], friend: [User]) {
  self.downloadMovies(for: friends] { friendMovies in
    self.presentProfile(
      for: user, 
      movies: movies, 
      ratings: ratings, 
      friends: friends,
      friendMovies: friendMovies
    )
  }
}

// usage:
prepareUserProfile()