10.
Model-View-ViewModel Pattern
Written by Jay Strawn
Heads up... You're reading this book for free, with parts of this chapter shown beyond this point as text.
Model-View-ViewModel (MVVM) is a structural design pattern that separates objects into three distinct groups:
- Models hold app data. They’re usually structs or simple classes.
-
Views display visual elements and controls on the screen. They’re typically subclasses of
UIView. - View models transform model information into values that can be displayed on a view. They’re usually classes, so they can be passed around as references.
Does this pattern sound familiar? Yep, it’s very similar to Model-View-Controller (MVC). Note that the class diagram at the top of this page includes a view controller; view controllers do exist in MVVM, but their role is minimized.
In this chapter, you’ll learn how to implement view models and organize your projects to include them. You’ll start with a simple example on what a view model does, then you’ll take a MVC project and refactor it into MVVM.
When should you use it?
Use this pattern when you need to transform models into another representation for a view. For example, you can use a view model to transform a Date into a date-formatted String, a Decimal into a currency-formatted String, or many other useful transformations.
This pattern compliments MVC especially well. Without view models, you’d likely put model-to-view transformation code in your view controller. However, view controllers are already doing quite a bit: handling viewDidLoad and other view lifecycle events, handling view callbacks via IBActions and several other tasks as well.
This leads to what developers jokingly refer to as “MVC: Massive View Controller”.
How can you avoid overstuffing your view controllers? It’s easy — use other patterns besides MVC! MVVM is a great way to slim down massive view controllers that require several model-to-view transformations.
Playground example
Open IntermediateDesignPatterns.xcworkspace in the Starter directory, and then open the MVVM page.
import PlaygroundSupport
import UIKit
// MARK: - Model
public class Pet {
public enum Rarity {
case common
case uncommon
case rare
case veryRare
}
public let name: String
public let birthday: Date
public let rarity: Rarity
public let image: UIImage
public init(name: String,
birthday: Date,
rarity: Rarity,
image: UIImage) {
self.name = name
self.birthday = birthday
self.rarity = rarity
self.image = image
}
}
// MARK: - ViewModel
public class PetViewModel {
// 1
private let pet: Pet
private let calendar: Calendar
public init(pet: Pet) {
self.pet = pet
self.calendar = Calendar(identifier: .gregorian)
}
// 2
public var name: String {
return pet.name
}
public var image: UIImage {
return pet.image
}
// 3
public var ageText: String {
let today = calendar.startOfDay(for: Date())
let birthday = calendar.startOfDay(for: pet.birthday)
let components = calendar.dateComponents([.year],
from: birthday,
to: today)
let age = components.year!
return "\(age) years old"
}
// 4
public var adoptionFeeText: String {
switch pet.rarity {
case .common:
return "$50.00"
case .uncommon:
return "$75.00"
case .rare:
return "$150.00"
case .veryRare:
return "$500.00"
}
}
}
// MARK: - View
public class PetView: UIView {
public let imageView: UIImageView
public let nameLabel: UILabel
public let ageLabel: UILabel
public let adoptionFeeLabel: UILabel
public override init(frame: CGRect) {
var childFrame = CGRect(x: 0,
y: 16,
width: frame.width,
height: frame.height / 2)
imageView = UIImageView(frame: childFrame)
imageView.contentMode = .scaleAspectFit
childFrame.origin.y += childFrame.height + 16
childFrame.size.height = 30
nameLabel = UILabel(frame: childFrame)
nameLabel.textAlignment = .center
childFrame.origin.y += childFrame.height
ageLabel = UILabel(frame: childFrame)
ageLabel.textAlignment = .center
childFrame.origin.y += childFrame.height
adoptionFeeLabel = UILabel(frame: childFrame)
adoptionFeeLabel.textAlignment = .center
super.init(frame: frame)
backgroundColor = .white
addSubview(imageView)
addSubview(nameLabel)
addSubview(ageLabel)
addSubview(adoptionFeeLabel)
}
@available(*, unavailable)
public required init?(coder: NSCoder) {
fatalError("init?(coder:) is not supported")
}
}
// MARK: - Example
// 1
let birthday = Date(timeIntervalSinceNow: (-2 * 86400 * 366))
let image = UIImage(named: "stuart")!
let stuart = Pet(name: "Stuart",
birthday: birthday,
rarity: .veryRare,
image: image)
// 2
let viewModel = PetViewModel(pet: stuart)
// 3
let frame = CGRect(x: 0, y: 0, width: 300, height: 420)
let view = PetView(frame: frame)
// 4
view.nameLabel.text = viewModel.name
view.imageView.image = viewModel.image
view.ageLabel.text = viewModel.ageText
view.adoptionFeeLabel.text = viewModel.adoptionFeeText
// 5
PlaygroundPage.current.liveView = view
extension PetViewModel {
public func configure(_ view: PetView) {
view.nameLabel.text = name
view.imageView.image = image
view.ageLabel.text = ageText
view.adoptionFeeLabel.text = adoptionFeeText
}
}
// 4
view.nameLabel.text = viewModel.name
view.imageView.image = viewModel.image
view.ageLabel.text = viewModel.ageText
view.adoptionFeeLabel.text = viewModel.adoptionFeeText
viewModel.configure(view)
What should you be careful about?
MVVM works well if your app requires many model-to-view transformations. However, not every object will neatly fit into the categories of model, view or view model. Instead, you should use MVVM in combination with other design patterns.
Tutorial project
Throughout this section, you’ll add functionality to an app called Coffee Quest.
import UIKit
public let image: UIImage
public let ratingDescription: String
public init(coordinate: CLLocationCoordinate2D,
name: String,
rating: Double,
image: UIImage) {
self.coordinate = coordinate
self.name = name
self.rating = rating
self.image = image
self.ratingDescription = "\(rating) stars"
}
public var subtitle: String? {
return ratingDescription
}
private func addAnnotations() {
for business in businesses {
guard let yelpCoordinate =
business.location.coordinate else {
continue
}
let coordinate = CLLocationCoordinate2D(
latitude: yelpCoordinate.latitude,
longitude: yelpCoordinate.longitude)
let name = business.name
let rating = business.rating
let image: UIImage
// 1
switch rating {
case 0.0..<3.5:
image = UIImage(named: "bad")!
case 3.5..<4.0:
image = UIImage(named: "meh")!
case 4.0..<4.75:
image = UIImage(named: "good")!
case 4.75...5.0:
image = UIImage(named: "great")!
default:
image = UIImage(named: "bad")!
}
let annotation = BusinessMapViewModel(
coordinate: coordinate,
name: name,
rating: rating,
image: image)
mapView.addAnnotation(annotation)
}
}
public func mapView(_ mapView: MKMapView,
viewFor annotation: MKAnnotation)
-> MKAnnotationView? {
guard let viewModel =
annotation as? BusinessMapViewModel else {
return nil
}
let identifier = "business"
let annotationView: MKAnnotationView
if let existingView = mapView.dequeueReusableAnnotationView(
withIdentifier: identifier) {
annotationView = existingView
} else {
annotationView = MKAnnotationView(
annotation: viewModel,
reuseIdentifier: identifier)
}
annotationView.image = viewModel.image
annotationView.canShowCallout = true
return annotationView
}
Key points
You learned about the Model-View-ViewModel (MVVM) pattern in this chapter. Here are its key points: