Animated assign subscriber

Sources/AnimatedAssignSubscriber.swift

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+//
+//  AnimatedAssignSubscriber.swift
+//
+//  Created by Marin Todorov on 5/3/20.
+//
+
+import Foundation
+import Combine
+
+#if canImport(UIKit)
+import UIKit
+
+/// A list of animations that can be used with `Publisher.assign(to:on:animation:)`
+public enum AssignTransition {
+    public enum Direction {
+        case top, bottom, left, right
+    }
+
+    /// Flip from either bottom, top, left, or right.
+    case flip(direction: Direction, duration: TimeInterval)
+
+    /// Cross fade with previous value.
+    case fade(duration: TimeInterval)
+
+    /// A custom animation. Do not include your own code to update the target of the assign subscriber.
+    case animation(duration: TimeInterval, options: UIView.AnimationOptions, animations: () -> Void, completion: ((Bool) -> Void)?)
+}
+
+extension Publisher where Self.Failure == Never {
+
+    /// Behaves identically to `Publisher.assign(to:on:)` except that it allows the user to
+    /// "wrap" emitting output in an animation transition.
+    ///
+    /// For example if you assign values to a
+    /// `UILabel` on screen you can make it flip over when each new value is set:
+    /// ```
+    /// myPublisher
+    ///   .assign(to: \.text, on: myLabel, animation: .flip(direction: .bottom, duration: 0.33))
+    /// ```
+    /// Or you can make an image view crossfade any new images it displays:
+    /// ```
+    /// myImagePublisher
+    ///   .assign(to: \.image, on: myImageView, animation: .fade(duration: 0.5))
+    /// ```
+    /// Finally you can set any custom animation including choice side effects as so. For example
+    /// this is how you can move the label each time you set a new value via `assign(to:on:animation:)`:
+    /// ```
+    /// myPublisher
+    ///   .assign(to: \.text, on: myLabel, animation: .animation(duration: 0.33, options: .curveEaseIn, animations: { _ in
+    ///     myLabel.center.x += 10.0
+    ///   }, completion: nil))
+    /// ```
+    public func assign<Root>(to keyPath: ReferenceWritableKeyPath<Root, Self.Output>, on object: Root, animation: AssignTransition) -> AnyCancellable {
+        guard let view = object as? UIView else {
+            return assign(to: keyPath, on: object)
+        }
+
+        var transition: UIView.AnimationOptions
+        var duration: TimeInterval
+
+        switch animation {
+        case .fade(let interval):
+            duration = interval
+            transition = .transitionCrossDissolve
+        case .flip(let dir, let interval):
+            duration = interval
+            switch dir {
+            case .bottom: transition = .transitionFlipFromBottom
+            case .top: transition    = .transitionFlipFromTop
+            case .left: transition   = .transitionFlipFromLeft
+            case .right: transition  = .transitionFlipFromRight
+            }
+        case .animation(let interval, let options, let animations, let completion):
+            return self
+                .handleEvents(receiveOutput: { value in
+                    UIView.animate(withDuration: interval, delay: 0, options: options, animations: {
+                        object[keyPath: keyPath] = value
+                        animations()
+                    }, completion: completion)
+                })
+                .assign(to: keyPath, on: object)
+        }
+
+        return self
+            .handleEvents(receiveOutput: { value in
+                UIView.transition(with: view, duration: duration, options: transition, animations: {
+                    object[keyPath: keyPath] = value
+                }, completion: nil)
+            })
+            .assign(to: keyPath, on: object)
+    }
+}
+#endif