UDFExample.swift

/**

Here's an example of how data flows from UI to/from the data model
with the current implementation.

*/

class ActionBar : UIView {
  
    var handle: Any?
    weak var delegate: ActionBarDelegate?
    weak var dataSource: ActionBarDataSource? {
        didSet {
            reloadData()
        }
    }

    func reloadData() {
        if self.dataSource == nil {
            // render view with some 'empty' state
        } else {
            let numberOfReplies = self.dataSource.numberOfReplies(for: self)
            let numberOfReactions = self.dataSource.numberOfReactions(for: self)
            // render view
        }
    }

    private func replyButtonWasTapped() {
        self.delegate?.actionBarDidPressReply(self)
        reloadData()
    }

    private func reactionButtonWasTapped() {
        self.delegate?.actionBar(self, didSelectReaction: self.reactionButton.currentReaction)
        reloadData()
    }
}


class ActionBarDataSource: NSObject {

    private let postController: PostController

    init(postController: PostController) {
        self.postController = postController
    }

    func numberOfReplies(for actionBar: ActionBar) -> Int {
        let postId = actionBar.handle as? Int
        if postId == nil {
            return 0
        }
        let post = self.postController.getPostWithId(postId!)
        if post == nil {
            return 0
        }
        return post.commentCount.intValue
    }

    func numberOfReactions(for actionBar: ActionBar) -> Int {
        let postId = actionBar.handle as? Int
        if postId == nil {
            return 0
        }
        let post = self.postController.getPostWithId(postId!)
        if post == nil {
            return 0
        }
        return post.numberOfReactions
    }
}

class ActionBarDelegate {

    private let postController: PostController
    private weak var presentingViewController: UIViewController?

    init(postController: PostController, presentingViewController: UIViewController?) {
        self.postController = postController
        self.presentingViewController = presentingViewController
    }

    func actionBarDidPressReply(_ actionBar: ActionBar) {
        let postId = actionBar.handle as? Int
        if postId == nil {
            return
        }
        let post = self.postController.getPostWithId(postId)
        if post == nil {
            return
        }
        let replyViewController = StoryDetailVC(storyId: post.entityID)
        self.presentingViewController?.navigationController?.pushViewController(replyViewController, animated: true)
    }

    func actionBar(_ actionBar: ActionBar, didSelectReaction: Reaction) {
        let postId = actionBar.handle as? Int
        if postId == nil {
            return
        }
        let post = self.postController.getPostWithId(postId)
        if post == nil {
            return
        }
        self.postController.addReaction(reaction, to: post) {
            actionBar.reloadData()
        }
        actionBar.reloadData()
    }
}


class PostController {

    func getPostWithId(_ id: Int) -> Post? {
        // fetch post by id
    }

    func addReaction(_ reaction: Reaction, to post: Post, completion: () -> Void) {
        // propegate through to server and local datamodel
    }
}

class NewsfeedViewController {

    private let actionBarDelegate: ActionBarDelegate
    private let actionBarDataSource: ActionBarDataSource

    init(actionBarDelegate: ActionBarDelegate, actionBarDataSource: ActionBarDataSource) {
        self.actionBarDelegate = actionBarDelegate
        self.actionBarDataSource = actionBarDataSource
    }

    func createCellForRow(_ row: Int) {
        let cell = // dequeue cell
        let post: Post = // find post for row
        cell.actionBar.handle = post.entityID
        cell.actionBar.delegate = actionBarDelegate
        cell.actionBar.dataSource = actionBarDataSource
    }
}

/**

Here's an example of that I believe improves on the current implementation

 Key differences:

 - no `handle` property. Since the dataSource & delegate
    (renamed to state & eventHandler due to apple conventions of delegate/dataSource being weak)
    are now always 1 to 1 with a view, there's no need for `handle` to mark what data this view is displaying.
 - `reloadData` is private. It only gets triggered from new state emitted from the observable
 - state/eventHandler functions that take the view as a param is the exception rather than the rule.
    This is ok because it's no longer necessary to call `reloadData` on the view.

 */

class ActionBar : UIView {

    var eventHandler: ActionBarEventHandler
    var state: Observable<ActionBarState> {
        didSet {
            state.subscribe(onNext: self.reloadData)
        }
    }

    // Notice how reloadData is now private. It's now clear this is only called internally
    private func reloadData(from state: ActionBarState) {
        let numberOfReplies = state.numberOfReplies
        let numberOfReactions = state.numberOfReactions
        // render view
    }

    private func replyButtonWasTapped() {
        self.eventHandler.onReply()
    }

    private func reactionButtonWasTapped() {
        self.eventHandler.onReactionSelected(self.reactionButton.currentReaction)
    }
}

class ActionBarState {
  
    private let post: Post

    init(post: Post) {
        self.post = post
    }

    var numberOfReplies: Int {
        return self.post.commentCount.intValue
    }

    var numberOfReactions: Int {
        return self.post.numberOfReactions
    }
}

class ActionBarEventHandler {

    private let post: Post
    private let postController: PostController
    private weak var presentingViewController: UIViewController?

    init(post: Post, postController: PostController, presentingViewController: UIViewController?) {
        self.post = post
        self.postController = postController
        self.presentingViewController = presentingViewController
    }

    func onReply() {
        let replyViewController = StoryDetailVC(storyId: self.post.entityID)
        self.presentingViewController?.navigationController?.pushViewController(replyViewController, animated: true)
    }

    func onReactionSelected(_ reaction: Reaction) {
        self.postController.addReaction(reaction, to: self.post)
    }
}


class PostController {

    func addReaction(_ reaction: Reaction, to post: Post, completion: () -> Void) {
        // propegate through to server and local datamodel
    }
}

class NewsfeedViewController {

    private let postController: PostController

    init(postController: PostController) {
        self.postController = postController
    }

    func createCellForRow(_ row: Int) {
        let cell = // dequeue cell
        let post: Post = // find post for row
        let postObservable: Observable<Post> = // post observable for row
        cell.actionBar.state = postObservable.map { ActionBarState($0) }
        cell.actionBar.eventHandler = ActionBarEventHandler(post: post,
                                                            postController: postController,
                                                            presentingViewController: self)
    }
}

Sample comment