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Merge pull request #2191 from woocommerce/issue/927-observables
Add PublishSubject observable
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‎WooCommerce/Classes/Tools/Observables/Observable.swift‎

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import Foundation
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/// Signature of a block called for every emitted Observable value.
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///
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typealias OnNext<Element> = (Element) -> ()
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/// Emits values over time.
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///
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/// Acts like `Publisher` in Combine and `Observable` in ReactiveX.
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///
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/// This class is a pseudo-abstract class. It does not do anything on its own. Use the
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/// subclasses like `PublishSubject` instead.
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///
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/// See here for more info about Observables:
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///
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/// - https://developer.apple.com/documentation/combine/publisher
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/// - http://reactivex.io/documentation/observable.html
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///
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class Observable<Element> {
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/// Subscribe to values emitted by this `Observable`.
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///
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/// The given `onNext` is called a "Observer" or "Subscriber".
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///
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/// Example:
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///
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/// ```
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/// class ViewModel {
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/// let onDataLoaded: Observable<[Item]>
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/// }
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///
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/// func viewDidLoad() {
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/// viewModel.onDataLoaded.subscribe { items in
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/// // do something with `items`
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/// tableView.reloadData()
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/// }
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/// }
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/// ```
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///
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func subscribe(_ onNext: @escaping OnNext<Element>) -> ObservationToken {
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fatalError("Abstract method. This must be implemented by subclasses.")
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}
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}

WooCommerce/Classes/Tools/ObservationToken.swift renamed to WooCommerce/Classes/Tools/Observables/ObservationToken.swift

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/// An observation token that contains a closure when the observation is cancelled.
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///
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/// This acts like `AnyCancellable` in Combine and `IDisposable` in ReactiveX.
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///
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/// Example usage can be found in `ProductImageActionHandler`.
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///
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/// See:
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///
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/// - https://developer.apple.com/documentation/combine/anycancellable
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///
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final class ObservationToken {
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private let onCancel: () -> Void
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‎WooCommerce/Classes/Tools/Observables/Observer.swift‎

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import Foundation
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/// A subscription to an `Observable`.
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///
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/// Acts like `Subscriber` in Combine and `IObserver` in ReactiveX.
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///
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/// Observers are simply the callbacks when you subscribe to an `Observable`. Consider this:
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///
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/// ```
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/// viewModel.onDataLoaded.subscribe { items in
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/// /// do something
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/// }
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/// ```
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///
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/// The block passed to the `subscribe` method with the "do something" comment is the `Observer`.
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///
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/// Currently, this `struct` is simply a container to clarify these concepts. In other frameworks,
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/// an `Observer` can have more callbacks like `onCompleted`.
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///
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/// See these for more info about Observers:
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///
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/// - https://developer.apple.com/documentation/combine/subscriber
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/// - http://introtorx.com/Content/v1.0.10621.0/02_KeyTypes.html#IObserver
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///
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struct Observer<Element> {
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private let onNext: (Element) -> ()
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init(onNext: @escaping OnNext<Element>) {
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self.onNext = onNext
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}
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/// Send the given value to the observer.
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///
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func send(_ element: Element) {
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onNext(element)
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}
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}

‎WooCommerce/Classes/Tools/Observables/PublishSubject.swift‎

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import Foundation
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/// Emits values to observers as soon as the values arrive. Only the values emitted after the
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/// subscription will be emitted.
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///
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/// Acts like `PassthroughSubject` in Combine and `PublishSubject` in ReactiveX.
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///
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/// This observable is a bridge between the imperative and reactive programming paradigms. It
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/// allows consumers to _manually_ emit values using the `send()` method.
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///
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/// Multiple observers are allowed which makes this a possible replacement for
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/// `NSNotificationCenter` observations.
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///
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/// ## Example Usage
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///
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/// In a class that you would like to emit values (or events), add the `PublishSubject` defining
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/// the value type:
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///
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/// ```
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/// class PostListViewModel {
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/// /// Calls observers/subscribers whenever the list of Post changes.
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/// private let postsSubject = PublishSubject<[Post]>()
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/// }
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/// ```
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///
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/// Since `PublishSubject` exposes `send()` which makes this a **mutable** Observable, we recommend
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/// exposing only the `Observable<[Post]>` interface:
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///
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/// ```
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/// class PostListViewModel {
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/// private let postsSubject = PublishSubject<[Post]>()
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///
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/// /// The public Observable that the ViewController will subscribe to
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/// var posts: Observable<[Post]> {
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/// postsSubject
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/// }
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/// }
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/// ```
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///
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/// The `ViewController` can then subscribe to the `posts` Observable:
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///
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/// ```
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/// func viewDidLoad() {
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/// viewModel.posts.subscribe { posts in
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/// // do something with posts
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/// tableView.reloadData()
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/// }
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/// }
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/// ```
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///
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/// Whenever the list of post changes, like after fetching from the API, the `ViewModel` can
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/// _notify_ the `ViewController` by updating `postsSubject`:
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///
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/// ```
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/// fetchFromAPI { fetchedPosts
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/// // Notify the observers (e.g. ViewController) that the list of posts have changed
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/// postsSubject.send(fetchedPosts)
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/// }
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/// ```
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///
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/// ## References
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///
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/// See here for info about similar observables in other frameworks:
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///
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/// - https://developer.apple.com/documentation/combine/passthroughsubject
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/// - http://reactivex.io/documentation/subject.html
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///
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final class PublishSubject<Element>: Observable<Element> {
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private typealias OnCancel = () -> ()
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/// The list of Observers that will be notified when a new value is sent.
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///
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private var observers = [UUID: Observer<Element>]()
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override func subscribe(_ onNext: @escaping OnNext<Element>) -> ObservationToken {
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let uuid = UUID()
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observers[uuid] = Observer(onNext: onNext)
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let onCancel: OnCancel = { [weak self] in
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self?.observers.removeValue(forKey: uuid)
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}
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return ObservationToken(onCancel: onCancel)
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}
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/// Emit a new value. All observers are immediately called with the given value.
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///
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func send(_ element: Element) {
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observers.values.forEach { observer in
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observer.send(element)
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}
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}
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}

‎WooCommerce/WooCommerce.xcodeproj/project.pbxproj‎

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@@ -1124,10 +1128,14 @@
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@@ -2338,6 +2346,25 @@
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};
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@@ -2504,6 +2531,7 @@
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@@ -2600,6 +2628,7 @@
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