The 3 RxSwift Building Blocks of UI Components

So here’s what I learned so far about the building blocks of reactive UI components from peeking at the RxCocoa source.

The 3 Building Blocks

UI components in general can have properties (read/write), input ports (read), and output ports (write). Classic UIKit/AppKit output ports would be delegate calls; classic input ports would be commands like display(banana:) that you probably write every day or so.

Translated to the world of RxSwift:

  • Observable is the basic output sequence
  • Observer is the basic consumer of input event sequences
  • a combination of both fits mutable properties, the like Variable type, for example

Then there are special “traits” for UI bindings which guarantee to work on the main queue.

  • ControlEvent is an Observable trait
  • UIBindingObserver is an Observer trait
  • ControlProperty has both traits’s attributes and is an Observable sequence as well as an Observer

Translating Known UIControl/NSControl Properties to RxSwift

For example, NSTextField and UITextField expose .rx.text which is a ControlProperty<String>. That means it’s read-write. You can bind other sequences to this property and have the text field update its content; and you can observe user-generated changes as well.

The alpha value or translucency of a view component is a UIBindingObserver; similarly, isEnabled is a UIBindingObserver. Look at UIControl+Rx of RxCocoa for details.

The reasoning I came up with: you usually want to change the alpha value or enabled state in reaction to some event, so it needs to be an Observer (or “sink”) of sorts. You don’t expect any view component to generate changes to these on its own terms. These things are toggles that a controller usually manipulates.

So even though the underlying property of the UIKit/AppKit component is a readwrite (or var) property, it does not make too much sense to expose a ControlProperty in these cases.

That doesn’t mean you couldn’t, thanks to KVO. Here’s a working but conceptually bad example of how to expose a view property in a reactive way that I wrote:

public extension Reactive where Base: NSScrollView {
    public var backgroundColor: ControlProperty<NSColor> {
        let source = self.observeWeakly(NSColor.self, "backgroundColor", options: [.initial, .new])
            // Skip nil values (which in practice does not happen, but KVO observe returns Optional)
            .filter { $0 != nil }.map { $0! }

        // `base` is a property of `Reactive` and, here, of type `NSScrollView`
        let observer = UIBindingObserver(UIElement: base) { (scrollView, newColor: NSColor) in
            scrollView.backgroundColor = newColor

        return ControlProperty(values: source, valueSink: observer)

As you see, the ControlProperty is composed of a UIBindingObserver and a regular Observable sequence. But there’s a catch: ControlProperty requires your source sequence to adhere to a few conventional criteria. These criteria are not enforced through the types in this case. ControlProperty takes responsibility of adhering to certain characteristics but it’s your job to make them happen. Let’s talk about these rules in detail.

The Rules of Using These 3 RxCocoa Types Correctly


The implementer (you!) has to provide an Observable that’s safe and auto-completes on deallocation of the object. ControlEvent itself takes care of main queue scheduling if needed.

Quoting the Swift source of RxCocoa v3.5, ControlEvent

  • it never fails
  • it won’t send any initial value on subscription
  • it will Complete sequence on control being deallocated
  • it never errors out
  • it delivers events on MainScheduler.instance

Most Observable sequences you build don’t send initial values, but there are some that have replay behavior you should be aware of.

For example, a Variable("initial") sends .next("initial") when subscription starts. A BehaviorSubject<String>("inital") does, too. But a PublishSubject<String>() doesn’t send anything upon subscription. You can see that it doesn’t require an initial value. Observable.from(["initial"]) will start right away, too.


The requirements of ControlProperty are similar to these of ControlEvent, but an initial value is expected, so the source has to shareReplay(1).

Again, ControlProperty itself takes care of main queue scheduling.

  • it never fails
  • shareReplay(1) behavior
    • it’s stateful, upon subscription (calling subscribe) last element is immediately replayed if it was produced
  • it will Complete sequence on control being deallocated
  • it never errors out
  • it delivers events on MainScheduler.instance


The “soft” attributes (that is, not enforced through the typing system) of UIBindingObserver are simpler: it does not bind errors (errors will merely be logged). And if the incoming sequence is not on the main queue already, UIBindingObserver dispatches to the main queue asynchronously.

From the call site, you don’t have to do anything.

It’s pretty easy to use:

public extension Reactive where Base: BananaView {
    public var size: UIBindingObserver<BananaView, Size> {
        // `base` is a property of `Reactive` and of type `BananaView`
        return UIBindingObserver(UIElement: base) { (bananaView, newSize) in
            bananaView.growVisibleBanana(to: newSize)

You pass in an object to UIBindingObserver.init that you want to change for incoming events. UIBindingObserver does the weak–strong-dance for you. It keeps a weak reference to the element only, so you don’t end up with a retain cycle. When a .next event reaches the observer, and if the weakly referenced element still exists, a reference to the element is passed back into the closure where you can mutate the underlying property.

If you create your objects using Variable type properties, you don’t need much of this stuff, of course. But if you want to provide a reactive extension to a regular view component, this is how you can do.