Simple animatable bindings using RxSwift + RxCocoa

Rx+Animated.swift

// Our binder will work universally for every UIView subclass.
extension Reactive where Base: UIView {

    /// Provides an observer that will animate incoming values.
    ///
    /// - Parameters:
    ///   - keyPath: keyPath to the property that will be mutated and animated.
    ///   - animator: the animator instance that will be used to animate changes.
    /// - Returns: a Binder instance.
    public func animated<Value>(
        // using keyPaths will allow us to have a single function that will work
        // on every property of the Base class.
        // Otherwise we would have to write one for every property that we might
        // want to animate later on (alpha, frame and so on).
        // Of course, it is your responsibility to use it only with animatable properties.
        _ keyPath: ReferenceWritableKeyPath<Base, Value>,
        // I am a big fan of UIViewPropertyAnimator and I think it works wonderfully
        // for the current use case. Alternatively, we can have an UIView.AnimationOptions
        // parameter here or some other sort of a configuration that will later on be used
        // for performing the animation.
        with animator: UIViewPropertyAnimator
    ) -> Binder<Value> {
        return Binder(self.base) { view, newValue in
            // This is what essentially your ordinary Binder looks like.
            // Here we're just setting the newValue inside an animation closure.
            animator.addAnimations {
                view[keyPath: keyPath] = newValue
            }
            // And then triggering the animator.
            animator.startAnimation()
        }
    }

}

extension UIViewPropertyAnimator {

    // If you use the same animation style all over the app, you can save yourself some code
    // by making a static property that represents that animation.
    // As an example, I like fading out / in views with a short, ease both curve animation,
    // as it looks smoother than doing it instantly, and also is short enough to not be
    // distracting or annoying when it happens all the time.
    static let short = UIViewPropertyAnimator(duration: 0.2, curve: .easeInOut, animations: nil)

}

// Using all of this is super clean and simple.

// Instead of the usual
itemsCount.map { ($0 == 0) ? 0.4 : 1.0 }.drive(itemBadge.rx.alpha).disposed(by: bag)

// we now use our new binder function
itemsCount.map { ($0 == 0) ? 0.4 : 1.0 }.drive(itemBadge.rx.animated(\.alpha, with: .short)).disposed(by: bag)