Swift 2.0: Protocol-Oriented MVVM

Posted on August 17th, 2015

UPDATE: Read this instead for the most up-to-date implementation.

Ever since the mind-blowing Protocol-Oriented Programming in Swift WWDC Session, I’ve been thinking a lot about using protocols. But in reality, I haven’t really been using them as much. I’m still digesting what protocol-oriented programming means, and where in my code I should be using it instead of my other go-to programming patterns.

So I was very excited where a HUGE use-case came to mind. MVVM! I already use MVVM – see my earlier MVVM blog post if you’d like to learn more. And adding in protocol-oriented just clicks here!

I’m going to use a very simple example. A Settings screen that currently only has one settings – put your app in Minion Mode!, but you can of course extrapolate to multiple settings:

Simulator Screen Shot Aug 17, 2015, 8.26.21 AM

The View Cell

A cell that has a label and switch button is very generic. You can use this same cell in multiple places – a “Remember Me” setting on a Sign-In screen comes to mind for example. So you want to keep it as a generic view.

A Big Configure

Usually, I use a configure method in my cell that keeps track of all the possible settings different parts of my app that use that cell need. So it would look something like this:

class SwitchWithTextTableViewCell: UITableViewCell {
    
    @IBOutlet private weak var label: UILabel!
    @IBOutlet private weak var switchToggle: UISwitch!
    
    typealias onSwitchToggleHandlerType = (switchOn: Bool) -> Void
    private var onSwitchToggleHandler: onSwitchToggleHandlerType?
    
    override func awakeFromNib() {
        super.awakeFromNib()
    }
    
    func configure(withTitle title: String,
        switchOn: Bool,
        onSwitchToggleHandler: onSwitchToggleHandlerType? = nil)
    {
        label.text = title
        switchToggle.on = switchOn
        
        self.onSwitchToggleHandler = onSwitchToggleHandler
    }
    
    @IBAction func onSwitchToggle(sender: UISwitch) {
        onSwitchToggleHandler?(switchOn: sender.on)
    }
}

class SwitchWithTextTableViewCell: UITableViewCell {

@IBOutlet private weak var label: UILabel!

@IBOutlet private weak var switchToggle: UISwitch!

typealias onSwitchToggleHandlerType = (switchOn: Bool) -> Void

private var onSwitchToggleHandler: onSwitchToggleHandlerType?

override func awakeFromNib() {

super.awakeFromNib()

}

func configure(withTitle title: String,

switchOn: Bool,

onSwitchToggleHandler: onSwitchToggleHandlerType? = nil)

{

label.text = title

switchToggle.on = switchOn

self.onSwitchToggleHandler = onSwitchToggleHandler

}

@IBAction func onSwitchToggle(sender: UISwitch) {

onSwitchToggleHandler?(switchOn: sender.on)

}

With Swift’s default parameters, adding additional settings to the configure method without breaking all the other places in your code that uses is super simple. For example, when a designer comes and says that the switch color should be different, I just add that as a default parameter:

    func configure(withTitle title: String,
        switchOn: Bool,
        switchColor: UIColor = .purpleColor(),
        onSwitchToggleHandler: onSwitchToggleHandlerType? = nil)
    {
        label.text = title
        switchToggle.on = switchOn
        // color option added!
        switchToggle.onTintColor = switchColor
        
        self.onSwitchToggleHandler = onSwitchToggleHandler
    }

func configure(withTitle title: String,

switchOn: Bool,

switchColor: UIColor = .purpleColor(),

onSwitchToggleHandler: onSwitchToggleHandlerType? = nil)

{

label.text = title

switchToggle.on = switchOn

// color option added!

switchToggle.onTintColor = switchColor

self.onSwitchToggleHandler = onSwitchToggleHandler

}

While this might not seem like a big deal in this case, in reality, my configure methods become super long and complicated over time! This is where the cool Protocol-Oriented way comes into play…

The Protocol-Oriented Way

protocol SwitchWithTextCellProtocol {
    var title: String { get }
    var switchOn: Bool { get }
    var switchColor: UIColor { get }
    
    func onSwitchTogleOn(on: Bool)
}

class SwitchWithTextTableViewCell: UITableViewCell {

    @IBOutlet private weak var label: UILabel!
    @IBOutlet private weak var switchToggle: UISwitch!

    private var delegate: SwitchWithTextCellProtocol?
    
    override func awakeFromNib() {
        super.awakeFromNib()
    }
    
    func configure(withDelegate delegate: SwitchWithTextCellProtocol) {
        self.delegate = delegate
        
        label.text = delegate.title
        switchToggle.on = delegate.switchOn
    }

    @IBAction func onSwitchToggle(sender: UISwitch) {
        delegate?.onSwitchTogleOn(sender.on)
    }
}

protocol SwitchWithTextCellProtocol {

var title: String { get }

var switchOn: Bool { get }

var switchColor: UIColor { get }

func onSwitchTogleOn(on: Bool)

}

class SwitchWithTextTableViewCell: UITableViewCell {

@IBOutlet private weak var label: UILabel!

@IBOutlet private weak var switchToggle: UISwitch!

private var delegate: SwitchWithTextCellProtocol?

override func awakeFromNib() {

super.awakeFromNib()

}

func configure(withDelegate delegate: SwitchWithTextCellProtocol) {

self.delegate = delegate

label.text = delegate.title

switchToggle.on = delegate.switchOn

}

@IBAction func onSwitchToggle(sender: UISwitch) {

delegate?.onSwitchTogleOn(sender.on)

}

And what happens when the designer wants to add the ability to change the default color? This is where the magic of protocol extensions comes into play!

extension SwitchWithTextCellProtocol {
    
    // set the default color here!
    var switchColor: UIColor {
        return .purpleColor()
    }
}

class SwitchWithTextTableViewCell: UITableViewCell {
    
    // truncated, see above 

    func configure(withDelegate delegate: SwitchWithTextCellProtocol) {
        self.delegate = delegate
        
        label.text = delegate.title
        switchToggle.on = delegate.switchOn
        // color option added!
        switchToggle.onTintColor = delegate.switchColor()
    }
}

extension SwitchWithTextCellProtocol {

// set the default color here!

var switchColor: UIColor {

return .purpleColor()

}

class SwitchWithTextTableViewCell: UITableViewCell {

// truncated, see above

func configure(withDelegate delegate: SwitchWithTextCellProtocol) {

self.delegate = delegate

label.text = delegate.title

switchToggle.on = delegate.switchOn

// color option added!

switchToggle.onTintColor = delegate.switchColor()

}

The protocol extension implements the default switchColor option, so anyone who has already implemented this protocol or doesn’t care about setting a color, doesn’t need to worry about it. Only the new cell that has the one different color can implement it.

The ViewModel

So now, the rest is easy. I’m simply going to have a ViewModel for my MinionMode Setting cell:

import UIKit

struct MinionModeViewModel: SwitchWithTextCellProtocol {
    var title = "Minion Mode!!!"
    var switchOn = true
    
    var switchColor: UIColor {
        return .yellowColor()
    }

    func onSwitchTogleOn(on: Bool) {
        if on {
            print("The Minions are here to stay!")
        } else {
            print("The Minions went out to play!")
        }
    }
}

import UIKit

struct MinionModeViewModel: SwitchWithTextCellProtocol {

var title = "Minion Mode!!!"

var switchOn = true

var switchColor: UIColor {

return .yellowColor()

}

func onSwitchTogleOn(on: Bool) {

if on {

print("The Minions are here to stay!")

} else {

print("The Minions went out to play!")

}

The ViewController

The final step is to pass my view model to the cell when configuring the cell in the ViewController:

import UIKit

class SettingsViewController: UITableViewController {

    enum Setting: Int {
        case MinionMode
        // other settings here
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }

    // MARK: - Table view data source

    override func tableView(tableView: UITableView,
        numberOfRowsInSection section: Int) -> Int
    {
        return 1
    }

    override func tableView(tableView: UITableView,
        cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell
    {
        if let setting = Setting(rawValue: indexPath.row) {
            switch setting {
            case .MinionMode:
                let cell = tableView.dequeueReusableCellWithIdentifier("SwitchWithTextTableViewCell", forIndexPath: indexPath) as! SwitchWithTextTableViewCell
                
                // this is where the magic happens!
                cell.configure(withDelegate: MinionModeViewModel())
                return cell
            }
        }
        
        return tableView.dequeueReusableCellWithIdentifier("defaultCell", forIndexPath: indexPath)
    }

}

import UIKit

class SettingsViewController: UITableViewController {

enum Setting: Int {

case MinionMode

// other settings here

}

override func viewDidLoad() {

super.viewDidLoad()

}

// MARK: - Table view data source

override func tableView(tableView: UITableView,

numberOfRowsInSection section: Int) -> Int

{

return 1

}

override func tableView(tableView: UITableView,

cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell

{

if let setting = Setting(rawValue: indexPath.row) {

switch setting {

case .MinionMode:

let cell = tableView.dequeueReusableCellWithIdentifier("SwitchWithTextTableViewCell", forIndexPath: indexPath) as! SwitchWithTextTableViewCell

// this is where the magic happens!

cell.configure(withDelegate: MinionModeViewModel())

return cell

}

return tableView.dequeueReusableCellWithIdentifier("defaultCell", forIndexPath: indexPath)

}

With protocol extensions, protocol-oriented programming is starting to make a lot of sense and I’m looking forward to figuring out ways to use it more! You can get the full code sample on Github here.

UPDATE: Separating the Data Source and the Delegate

In the comments below, Marc Baldwin suggested separating out the cell’s data source and delegate into two protocols, just like UITableView does, and I LOVE that idea. Here is what that would look like:

The View Cell

The cell would have two protocols, and it could be configured with both:

import UIKit

protocol SwitchWithTextCellDataSource {
    var title: String { get }
    var switchOn: Bool { get }
}

protocol SwitchWithTextCellDelegate {
    func onSwitchTogleOn(on: Bool)
    
    var switchColor: UIColor { get }
    var textColor: UIColor { get }
    var font: UIFont { get }
}

extension SwitchWithTextCellDelegate {
    
    var switchColor: UIColor {
        return .purpleColor()
    }
    
    var textColor: UIColor {
        return .blackColor()
    }
    
    var font: UIFont {
        return .systemFontOfSize(17)
    }
}

class SwitchWithTextTableViewCell: UITableViewCell {

    @IBOutlet private weak var label: UILabel!
    @IBOutlet private weak var switchToggle: UISwitch!

    private var dataSource: SwitchWithTextCellDataSource?
    private var delegate: SwitchWithTextCellDelegate?
    
    override func awakeFromNib() {
        super.awakeFromNib()
    }
    
    func configure(withDataSource dataSource: SwitchWithTextCellDataSource, delegate: SwitchWithTextCellDelegate?) {
        self.dataSource = dataSource
        self.delegate = delegate
        
        label.text = dataSource.title
        switchToggle.on = dataSource.switchOn
        // color option added!
        switchToggle.onTintColor = delegate?.switchColor
    }

    @IBAction func onSwitchToggle(sender: UISwitch) {
        delegate?.onSwitchTogleOn(sender.on)
    }
}

import UIKit

protocol SwitchWithTextCellDataSource {

var title: String { get }

var switchOn: Bool { get }

}

protocol SwitchWithTextCellDelegate {

func onSwitchTogleOn(on: Bool)

var switchColor: UIColor { get }

var textColor: UIColor { get }

var font: UIFont { get }

}

extension SwitchWithTextCellDelegate {

var switchColor: UIColor {

return .purpleColor()

}

var textColor: UIColor {

return .blackColor()

}

var font: UIFont {

return .systemFontOfSize(17)

}

class SwitchWithTextTableViewCell: UITableViewCell {

@IBOutlet private weak var label: UILabel!

@IBOutlet private weak var switchToggle: UISwitch!

private var dataSource: SwitchWithTextCellDataSource?

private var delegate: SwitchWithTextCellDelegate?

override func awakeFromNib() {

super.awakeFromNib()

}

func configure(withDataSource dataSource: SwitchWithTextCellDataSource, delegate: SwitchWithTextCellDelegate?) {

self.dataSource = dataSource

self.delegate = delegate

label.text = dataSource.title

switchToggle.on = dataSource.switchOn

// color option added!

switchToggle.onTintColor = delegate?.switchColor

}

@IBAction func onSwitchToggle(sender: UISwitch) {

delegate?.onSwitchTogleOn(sender.on)

}

The ViewModel

You can now separate out the data source vs delegate logic in an extension:

import UIKit

struct MinionModeViewModel: SwitchWithTextCellDataSource {
    var title = "Minion Mode!!!"
    var switchOn = true
}

extension MinionModeViewModel: SwitchWithTextCellDelegate {
    
    func onSwitchTogleOn(on: Bool) {
        if on {
            print("The Minions are here to stay!")
        } else {
            print("The Minions went out to play!")
        }
    }
    
    var switchColor: UIColor {
        return .yellowColor()
    }
}

import UIKit

struct MinionModeViewModel: SwitchWithTextCellDataSource {

var title = "Minion Mode!!!"

var switchOn = true

}

extension MinionModeViewModel: SwitchWithTextCellDelegate {

func onSwitchTogleOn(on: Bool) {

if on {

print("The Minions are here to stay!")

} else {

print("The Minions went out to play!")

}

var switchColor: UIColor {

return .yellowColor()

}

The ViewController

This is the part I’m not as sure about – the ViewController will now pass in the viewModel twice:

override func tableView(tableView: UITableView,
        cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell
    {
        if let setting = Setting(rawValue: indexPath.row) {
            switch setting {
            case .MinionMode:
                let cell = tableView.dequeueReusableCellWithIdentifier("SwitchWithTextTableViewCell", forIndexPath: indexPath) as! SwitchWithTextTableViewCell
                
                // this is where the magic happens!
                let viewModel = MinionModeViewModel()
                cell.configure(withDataSource: viewModel, delegate: viewModel)
                return cell
            }
        }
        
        return tableView.dequeueReusableCellWithIdentifier("defaultCell", forIndexPath: indexPath)
    }

override func tableView(tableView: UITableView,

cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell

{

if let setting = Setting(rawValue: indexPath.row) {

switch setting {

case .MinionMode:

let cell = tableView.dequeueReusableCellWithIdentifier("SwitchWithTextTableViewCell", forIndexPath: indexPath) as! SwitchWithTextTableViewCell

// this is where the magic happens!

let viewModel = MinionModeViewModel()

cell.configure(withDataSource: viewModel, delegate: viewModel)

return cell

}

return tableView.dequeueReusableCellWithIdentifier("defaultCell", forIndexPath: indexPath)

}

I updated the code on Github here!

Enjoy the article? Join over 20,000+ Swift developers and enthusiasts who get my weekly updates.

Samuel Beek

I was wondering wether the name “delegate” makes sense in this context? since you populate the cell with data through the delegate as well. I always think of delegates as pointers to functions, but in this case its more than that. What are your thoughts on that?
- Samuel Beek
  
  I feel the term data source makes more sense in this piece of code: label.text = delegate.title
- Marc Baldwin
  
  I’d probably suggest following the approach used by UITableView and use two protocols; one for the source of data i.e. SwitchCellDataSource and one for the action i.e. SwitchCellDelegate. This practise is known as the Interface Segregation Principle and is the I in SOLID. See https://en.wikipedia.org/wiki/Interface_segregation_principle
  - Natasha Murashev
    
    Good idea! I really like that.
  - Samuel Beek
    
    Cool, thanks :).
  - Natasha Murashev
    
    I updated the blog post with this as an example.
    - Steve Riggins
      
      Presentation in the delegate always gets me. UITableViews do this, but what if you want to have your presentation be A/B testable outside of what happens when a switch is selected? Does your delegate have its own delegate?
      - Christian Tietze
        
        If you call it XYZDelegate, it sure sounds ridiculous — but deferring the decision of which A/B testing value should be used in a request sure is the concern of an object different from the delegate.

Bob

First thank you for this wonderful article. Is it ok to make the MinionModeViewModel’s init like the following init(controller:UIViewController) {…} ? if I need a reference for my controller in the ViewModel.

Natasha Murashev

You should not have to reference your controller in your view model. That’s a big code smell. What are you trying to do?

Bob

What if I want to push a view controller after responding to an event. For example : in your code let’s say you want to push a controller if the switch is on. How can we deal with that ?

Natasha Murashev

The ViewModel should send out an NSNotification – “viewModelDidChange” – the view controller will listen to the notification and act on it (in your case, by pushing another view controller).

Bob

Thank you for the clarification:)

Christian Tietze

Srdan Rasic proposed an interesting technique using didSet property observers as an alternative to full-blown notification posting which I’ve written about here, if Natasha doesn’t mind linking to my own blog (sample at the bottom): http://christiantietze.de/posts/2015/01/mvvm-cocoa/

I have forked Natasha’s repo and incorporated the changes:

https://github.com/DivineDominion/ProtocolOrientedMVVMExperimentSwift

Using that technique, changes to the view model will reflect in the view immediately if you want. (Or, in this case, trigger pushing another scene.) That’s coupling the view model to the view instead of the controller, or: it results in a (live) model of the view.

Now that’s odd since MinionModeViewModel already has onSwitchTogleOn(_:) as callback. In a real application, I’d implement the SwitchWithTextCellDelegate in a separate object, an event handler, so to speak, which would perform changes on the view model in memory. The view model object would then not implement both protocols.

I propose something along the lines of:

protocol SwitchWithTextCellDataSource {

    var title: String { get }

    var switchOn: Dynamic<Bool> { get }

}

// Use a default style with protocol extensions

protocol SwitchWithTextCellStyle {

    var switchColor: UIColor { get }

    var textColor: UIColor { get }

    var font: UIFont { get }

}

protocol SwitchWithTextCellDelegate {

    func onSwitchTogleOn(on: Bool)

}

protocol SwitchWithTextCellDataSource {

var title: String { get }

var switchOn: Dynamic<Bool> { get }

}

// Use a default style with protocol extensions

protocol SwitchWithTextCellStyle {

var switchColor: UIColor { get }

var textColor: UIColor { get }

var font: UIFont { get }

}

protocol SwitchWithTextCellDelegate {

func onSwitchTogleOn(on: Bool)

}

The delegate does respond to actions now, and that’s it. I think it shouldn’t know about colors and fonts. The MinionModeViewModel may, instead, because its minion-esque appearance depends on these properties as well.

Rusty

Using NSNotifications to communicate from a model that a viewController owns is even more code stink than back referencing. delegation is the solution! Delegation is used to great effect all over this article.

Simply define a delegate protocol

protocol ModelViewChangedDelegate { func modelView(mv:MinionModelView, switchDidChangeValue switchIsOn:Bool) }

then add a weak var delegate: ModelViewChangedDelegate property to your MinionViewModel and, when you need to respond to events inside your vm, you call delegate.modelView(self, switchDidChangeValue:on)

Then your viewController can respond to the change and the viewModel doesn’t know anything about the vc.

NSNotifications are wonderful when you have system wide, asynchronous, disconnected, non-deterministic events. Using them for general control flow creates a troubleshooting nightmare.

Mark Oruba

Hi, great article..

I have read many times regarding MVVM that the ViewModel never should have any reference to the view aka (import UIKit) I know in the sample it is being used just for the color but it is correct?
loveforgeter

very clear!
JB GonnaGo

Is there an ‘in a nutshell’ few sentences way to differentiate protocol oriented programming from ordinary OO polymorphism?
finneycanhelp

Nice article
Bikshapathi K

Hi Natasha,
It is Nice article. But I observed small bug in the above code. when we come to this screen, initially the switch is ON, If we made the switch OFF and scroll up till the row is invisible and the switch is ON. Here the datasource is not being maintained properly, It is always true in the above code.
Boris

Hi Natasha, big fan of your blog. Shouldn’t

switchToggle.on = delegate.switchOn

be

switchToggle.on = delegate?.switchOn

Also, what happens if the delegate is nil ? Does optional chaining default to false when encountering nil along the way in this call chain?
- Boris
  
  Ah, never mind. Hadn’t noticed that delegate is not the property but is the passed argument (which isn’t of an optional type).
Michael Wang

the best one
manwey

Hi Natasha, I had an issue with yours first approach:

I don´t know how to call the parameter onSwitchToggleHandler in the function configure inside tableView:cellForRowAtIndexPath

What is the way to pass the Closure as a parameter?

Thanks in advance

import UIKit
import XCPlayground

class SwitchWithTextTableViewCell: UITableViewCell {
var label: UILabel!
var switchControl: UISwitch!

typealias onSwitchTogglerHandlerType = (switchOn: Bool) -> Void

private var onSwitchToggleHandler: onSwitchTogglerHandlerType?

required init(coder aDecoder: NSCoder) {
fatalError(“init(coder:)”)
}

override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
super.init(style: style, reuseIdentifier: reuseIdentifier)

let gap : CGFloat = 10
let labelHeight: CGFloat = 30
let labelWidth: CGFloat = 150
let lineGap : CGFloat = 5
let switchControlY : CGFloat = gap + labelHeight + lineGap

label = UILabel()
label.frame = CGRectMake(gap, gap, labelWidth, labelHeight)
label.textColor = UIColor.blackColor()
contentView.addSubview(label)

switchControl = UISwitch(frame:CGRectMake(gap, switchControlY, labelWidth, labelHeight))
switchControl.onTintColor = UIColor.blackColor()
accessoryView = switchControl
}

func configure(
title: String,
titleFont: UIFont,
titleColor: UIColor,
switchOn: Bool,
switchColor: UIColor = .purpleColor(),
onSwitchToggleHandler: onSwitchTogglerHandlerType? = nil){

label.text = title
label.font = titleFont
label.textColor = titleColor

switchControl.on = switchOn
switchControl.onTintColor = switchColor
self.onSwitchToggleHandler = onSwitchToggleHandler
switchControl.addTarget(self, action: #selector(SwitchWithTextTableViewCell.onSwitchToggle(_:)), forControlEvents: UIControlEvents.ValueChanged)
}

func onSwitchToggle(sender: UISwitch) {
onSwitchToggleHandler?(switchOn: sender.on)
}
}

class TableViewController: UITableViewController {
override func viewDidLoad() {
super.viewDidLoad()
self.tableView.registerClass(SwitchWithTextTableViewCell.self, forCellReuseIdentifier: “CellIdentifier”)
}

override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
return 1
}

override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return 1
}

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCellWithIdentifier(“CellIdentifier”, forIndexPath: indexPath) as! SwitchWithTextTableViewCell

cell.configure(“Test”, titleFont: UIFont.systemFontOfSize(18.0), titleColor: UIColor.greenColor(), switchOn: false, switchColor: UIColor.redColor(), onSwitchToggleHandler:nil)
return cell
}
}

let tableViewController = TableViewController(style: UITableViewStyle.Plain)
tableViewController.tableView.frame = CGRect(x: 0, y: 0, width: 300, height: 100)
XCPlaygroundPage.currentPage.liveView = tableViewController.view
- manwey
  
  I answered to my self:
  
  cell.configure(“Test”, titleFont: UIFont.systemFontOfSize(18.0), titleColor: UIColor.greenColor(), switchOn: false, switchColor: UIColor.redColor(), onSwitchToggleHandler:{[unowned self] (boolValue:Bool) in
  self.switchControlIsOn = boolValue
  
  }
  
  )
Rajesh

Hello Natasha,
Thanks for sharing.

but one question: when cell.configure(withDataSource: viewModel, delegate: nil)

When I pass nil in delegate, switch color is set to green, it hasn’t taken default purple color? why so?
VuTuan Tran

can you explain: you are passing arguments of type ViewModel to configure method. However, in SwitchWithTextTableViewCell, the types are SwitchWithTextCellDataSource and SwitchWithTextCellDataSource
Joao Paulo

Hello Natasha, thanks for this post. I has been helping me a lot in understanding protocol-oriented-mvvm. One question, where would I put the outlets, If I were using a UIView? Would it go in the ViewController?