Resume shimmering animation when application re-enters foreground

EssentialFeed/EssentialFeediOS/Feed UI/Views/Helpers/UIView+Shimmering.swift

@@ -15,37 +15,43 @@ extension UIView {
 		}
 
 		get {
-			return layer.mask?.animation(forKey: shimmerAnimationKey) != nil
+			layer.mask is ShimmeringLayer
 		}
 	}
 
-	private var shimmerAnimationKey: String {
-		return "shimmer"
-	}
-
 	private func startShimmering() {
-		let white = UIColor.white.cgColor
-		let alpha = UIColor.white.withAlphaComponent(0.75).cgColor
-		let width = bounds.width
-		let height = bounds.height
-
-		let gradient = CAGradientLayer()
-		gradient.colors = [alpha, white, alpha]
-		gradient.startPoint = CGPoint(x: 0.0, y: 0.4)
-		gradient.endPoint = CGPoint(x: 1.0, y: 0.6)
-		gradient.locations = [0.4, 0.5, 0.6]
-		gradient.frame = CGRect(x: -width, y: 0, width: width*3, height: height)
-		layer.mask = gradient
-
-		let animation = CABasicAnimation(keyPath: #keyPath(CAGradientLayer.locations))
-		animation.fromValue = [0.0, 0.1, 0.2]
-		animation.toValue = [0.8, 0.9, 1.0]
-		animation.duration = 1.25
-		animation.repeatCount = .infinity
-		gradient.add(animation, forKey: shimmerAnimationKey)
+		layer.mask = ShimmeringLayer(size: bounds.size)
 	}
 
 	private func stopShimmering() {
 		layer.mask = nil
 	}
+
+	private class ShimmeringLayer: CAGradientLayer {
+		private var observer: Any?
+
+		convenience init(size: CGSize) {
+			self.init()
+
+			let white = UIColor.white.cgColor
+			let alpha = UIColor.white.withAlphaComponent(0.75).cgColor
+
+			colors = [alpha, white, alpha]
+			startPoint = CGPoint(x: 0.0, y: 0.4)
+			endPoint = CGPoint(x: 1.0, y: 0.6)
+			locations = [0.4, 0.5, 0.6]
+			frame = CGRect(x: -size.width, y: 0, width: size.width*3, height: size.height)
+
+			let animation = CABasicAnimation(keyPath: #keyPath(CAGradientLayer.locations))
+			animation.fromValue = [0.0, 0.1, 0.2]
+			animation.toValue = [0.8, 0.9, 1.0]
+			animation.duration = 1.25
+			animation.repeatCount = .infinity
+			add(animation, forKey: "shimmer")
+
+			observer = NotificationCenter.default.addObserver(forName: UIApplication.willEnterForegroundNotification, object: nil, queue: nil) { [weak self] _ in
+				self?.add(animation, forKey: "shimmer")
+			}
+		}
+	}
 }