How I do MVVM Bindings with ReactiveCocoa 3

20 Jul 2015

The first Release Candidate for the ReactiveCocoa 3 (aka The Swift Edition) has just been announced, so I thought this was a good time to share how I have been doing MVVM bindings with the new version of the framework.

Unfortunately (or fortunately, depending on the point of view), Swift does not play well with KVO, which is a very Objective-C way of doing things. KVO is great, but it was also the main source of RAC’s “magic”.
For this reason MutableProperty, a more formal and explicit way to do bindings, was introduced in the Swift version of RAC.

The most common scenario in iOS is having a bunch of UITableViewCells or UICollectionViewCells and keep their content up to date with the latest ViewModel. What I like to do is having a MutableProperty wrapping the ViewModel and describe the bindings once during the initialization of the cell, letting the framework stream the new values whenever the ViewModel changes.

In code this translates to something like this:

protocol ViewModelCell {
    typealias T
    var viewModel: MutableProperty<T?> { get }

class ExerciseSetCell: UITableViewCell, ViewModelCell {
    @IBOutlet weak var numberLabel: UILabel!
    @IBOutlet weak var currentWeightTextField: UITextField!
    @IBOutlet weak var repsTextField: UITextField!
    var viewModel: MutableProperty<ExerciseSetViewModel?> = MutableProperty(nil)
    override func awakeFromNib() {
     func setupBindings() {
        numberLabel.rac_text <~ viewModel.producer |> ignoreNil |> map { $0.number }
        currentWeightTextField.rac_text <~ viewModel.producer |> ignoreNil |> map { $0.currentWeight }
        repsTextField.rac_text <~ viewModel.producer |> ignoreNil |> map { $0.reps }

Note that to accomplish this I used the UIKit extension rac_text. Unfortunately those extensions are not present in the framework yet, so you’d have to create them yourself for now. Colin Eberhardt’s post is a good starting point.

Since I don’t really know what I’m doing, I’m open to suggestion. Please reach me out on Twitter if you have some!