Add animation operator to Effect. (pointfreeco#1134)

* Add animation operator to Effect.

* Define it only on Effect.

Author: mbrandonw

Sources/ComposableArchitecture/Effects/Animation.swift

@@ -0,0 +1,64 @@
+import Combine
+import SwiftUI
+
+extension Effect {
+  /// Wraps the emission of each element with SwiftUI's `withAnimation`.
+  ///
+  /// This publisher is most useful when using with ``Effect/task(priority:operation:)-2czg0``
+  ///
+  /// ```swift
+  /// case .buttonTapped:
+  ///   return .task {
+  ///     .activityResponse(await environment.apiClient.fetchActivity())
+  ///   }
+  ///   .animation()
+  /// ```
+  ///
+  /// - Parameter animation: An animation.
+  /// - Returns: A publisher.
+  public func animation(_ animation: Animation? = .default) -> Self {
+    AnimatedPublisher(upstream: self, animation: animation)
+      .eraseToEffect()
+  }
+}
+
+private struct AnimatedPublisher<Upstream: Publisher>: Publisher {
+  typealias Output = Upstream.Output
+  typealias Failure = Upstream.Failure
+
+  var upstream: Upstream
+  var animation: Animation?
+
+  func receive<S: Combine.Subscriber>(subscriber: S)
+  where S.Input == Output, S.Failure == Failure {
+    let conduit = Subscriber(downstream: subscriber, animation: self.animation)
+    self.upstream.receive(subscriber: conduit)
+  }
+
+  private class Subscriber<Downstream: Combine.Subscriber>: Combine.Subscriber {
+    typealias Input = Downstream.Input
+    typealias Failure = Downstream.Failure
+
+    let downstream: Downstream
+    let animation: Animation?
+
+    init(downstream: Downstream, animation: Animation?) {
+      self.downstream = downstream
+      self.animation = animation
+    }
+
+    func receive(subscription: Subscription) {
+      self.downstream.receive(subscription: subscription)
+    }
+
+    func receive(_ input: Input) -> Subscribers.Demand {
+      withAnimation(self.animation) {
+        self.downstream.receive(input)
+      }
+    }
+
+    func receive(completion: Subscribers.Completion<Failure>) {
+      self.downstream.receive(completion: completion)
+    }
+  }
+}

Sources/ComposableArchitecture/TestSupport/TestStore.swift

@@ -176,8 +176,8 @@
 
     /// The current state.
     ///
-    /// When read from a trailing closure assertion in ``send`` or ``receive``, it will equal the
-    /// `inout` state passed to the closure.
+    /// When read from a trailing closure assertion in ``send(_:_:file:line:)`` or
+    /// ``receive(_:_:file:line:)``, it will equal the `inout` state passed to the closure.
     public private(set) var state: State
 
     private let file: StaticString