Implementing a Mutable Collection Property for ReactiveCocoa

14 October 2015

I've been recently playing a lot with Reactive, especially ReactiveCocoa. Since they launched the version for Swift I can say I'm like a baby using it in my project. There's something in particular which I use a lot in a MVVM pattern which are properties.

What's a property?

For those who don't know what a Property is in ReactiveCocoa 3/4 it's a custom generic type that encapsulates a variable internally. Why? Because this new variable exposes a SignalProducer that reports changes on this variable as events. That way you can know when the variable value changes and subscribe to these changes using ReactiveCocoa concepts.

Here's an example of a property:

import ReactiveCocoa
let myProperty: MutableProperty<String> = MutableProperty("")
myProperty.producer.startWithNext {(newValue) in print("This is my new value \(newValue)")}
myProperty.value = "yai!"

As you can see we in the example above the property has a producer that we can subscribe to, and it sends new values of that property when the value changes. In that case we're updating the value to yai! and then the subscriber is printing that value.

Types of properties

ReactiveCocoa offers currently three types of Properties that cover most of the cases where we'll need to this pattern and all of them conform the same protocol:

public protocol PropertyType {
typealias Value
var value: Value { get }
var producer: SignalProducer<Value, NoError> { get }
  • ConstantProperty: Kind of property that doesn't mutate its value once it's initialized. The main advantage (in my opinion) on using this kind of property is the fact that you can connect it with other ReactiveCocoa components.

  • PropertyOf: It's a kind of property that once created doesn't allow the modification of it's value externaly. You can only subscribe to the changes of this property that are sent from another Signal/SignalProducer or even another property. Consequently this kind of property can be initialized using these three components:

public init<P: PropertyType where P.Value == T>(_ property: P)
public init(initialValue: T, producer: SignalProducer<T, NoError>)
public init(initialValue: T, signal: Signal<T, NoError>)
  • MutableProperty: Compared with the previous property in this case you can update the property value after being initialized and in the same way all these changes will be propagated to the producer subscribers.

Properties in MVVM pattern

Properties are very useful in the MVVM pattern because it allows us to detect changes int hese properties values and then update the view according to these changes. For example, imagine the following situation:

class ProfileView: UIView {
// MARK: - Attributes
let avatarView: UIImageView = UIImageView()
let viewModel: ProfileViewModel
// MARK: - Constructors
init(viewModel: ProfileViewModel) {
self.viewModel = viewModel
// MARK: - Setup
private func setupObservers() {
self.viewModel.avatarImage.startWithNext { [weak self] (avatarImage) in
self?.avatarView.image = avatarImage
class ProfileViewModel {
// MARK: - Attributes
let avatarImage: MutableProperty<UIImage> = MutableProperty(UIImage())

We want to update the avatar image in the ProfileView when we get the image from somewhere, no matter the source. Then we would define our Profile view ViewModel that includes that MutableProperty of type UIImage. From the view we subscribe to that property and when there's a new image we just set it to the UIImageView. Data source and its use in the layout is fully decouple. The view doesn't know where the data comes from, the image might come from a local cache, from a web request, from the camera... It just has to know how to set that image in the view. Great right? You can extend that to more views around the app and for more types of properties and then you can have your views "synchronizes" with the data source.

There's also a great avantage when working with ReactiveCocoa properties and is the fact tat you can use Reactive concepts like for example applying functional operators and combine multiple properties in a single one.

In the example above, we could for example define a map function with the following format:

func addBadge(badgeConfig: BadgeConfig)(image: UIImage) -> UIImage {
// Add badge logic

Then have a new property in the view model

lazy var avatarImageWithBadge: MutableProperty<UIImage> = {
PropertyOf(avatarImage.value, producer: avatarImage.producer |> map(addBadge(myBadgeConfig)))

Collection property

When you work with collections in your view it's very complicated to have granularity with these properties deteting what really changed in the collection. I noticed I ended up calling reloadData() method in the table/collection view and forcing a relayout of all the elements in the view. Not good performance right? Components like the NSFetchedResultsController were designed to avoid this things but in this case the component is extremly coupled to CoreData, if you want to use it for example with your custom collections you have to look for a custom implementation (I don't have any in mind right now) that proxies collections operations and notifies different observers about these operations like insertion, deletion, update, passing the index back where these operations where executed.

What if we had this approach based on ReactiveCocoa, using Properties? Let's try to develop a MutableCollectionProperty

Note: I've created a repository where this new component has been implemented, You can clone the repository and try it on your environment


MutableCollectionProperty is a ReactiveCocoa property that notifies about the changes that are produced in an internal collection. It exposes Swift array methods to modify collections in order to redirect these changes to the attached subscribers as shown in the example below:

let property: MutableCollectionProperty<String> = MutableCollectionProperty(["test1", "test2"])
property.changesProducer.startWithNext { [weak self] next in
case .StartChange:
case .Insertion(let index, let element):
self?.tableView.insertRowsAtIndexPaths([NSIndexPath(row: index, section: 0)], withRowAnimation: .Automatic)
case .EndChange:
default: break

Every sequence of changes is preceded by an event StartChange and ends with a EndChange. It allows multiple changes together and set the view which is going to reflect these changes in an "update" state. The methods exposed by the property are:

public func removeFirst()
public func removeLast()
public func removeAll()
public func removeAtIndex(index: Int)
public func append(element: T)
public func appendContentsOf(elements: [T])
public func insert(newElement: T, atIndex index: Int)
public func replace(subRange: Range<Int>, with elements: [T])

You can get this component from here and use it with ReactiveCocoa on your projects. I've already proposed the feature to the ReactiveCocoa team on this PR still waiting for response :).

If you're interested on Reactive paradigms and you want to keep learning, I'm currently writing about about the use of Reactive in Swift apps using ReactiveCocoa. You follow the status here

If you found any bug or you would like to comment something about Reactive or this port in particular, feel free to drop me a line, [email protected] We're using this an another Reactive concepts on GitDo

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