using alamofireimage for transitions when loading thumbnails and graphs

It was already being used in the Today Extension view but not for
the graphs and graph thumbnails elsewhere in the app.
Using it simplifies the code and allows for easily adding transitions.
When scrolling the gallery or loading a goal, the user experience
is enhanced by using smooth rather than jarring animations and transitions.

BeeSwift/Components/GoalImageView.swift

@@ -9,17 +9,18 @@ import BeeKit
 /// Shows the current graph for a goal
 /// Handles placeholders for loading and queued states, and automatically updates when the goal changes
 class GoalImageView : UIView {
-    private static let downloader = ImageDownloader(imageCache: AutoPurgingImageCache())
     private let logger = Logger(subsystem: "com.beeminder.com", category: "GoalImageView")
 
     private let imageView = UIImageView()
     private let beeLemniscateView = BeeLemniscateView()
 
     private var currentlyShowingGraph = false
-    private var inProgressDownload: RequestReceipt? = nil
-    private var currentDownloadToken: UUID? = nil
 
     public let isThumbnail: Bool
+
+    private enum Constant {
+        static var placeholderImage: UIImage { UIImage(named: "GraphPlaceholder")! }
+    }
 
     public var goal: Goal? {
         didSet {
@@ -48,13 +49,13 @@ class GoalImageView : UIView {
         imageView.snp.makeConstraints { (make) in
             make.edges.equalToSuperview()
         }
-        self.imageView.image = UIImage(named: "GraphPlaceholder")
+        self.imageView.image = Constant.placeholderImage
 
         self.addSubview(beeLemniscateView)
         beeLemniscateView.snp.makeConstraints { (make) in
             make.edges.equalToSuperview()
         }
-        beeLemniscateView.isHidden = true
+        beeLemniscateView.isHidden = false
 
         NotificationCenter.default.addObserver(
             forName: NSNotification.Name(rawValue: GoalManager.goalsUpdatedNotificationName),
@@ -68,78 +69,40 @@ class GoalImageView : UIView {
     }
 
     private func clearGoalGraph() {
-        imageView.image = UIImage(named: "GraphPlaceholder")
+        imageView.af.cancelImageRequest()
+
+        imageView.af.run(.noTransition, with: Constant.placeholderImage)
         currentlyShowingGraph = false
         beeLemniscateView.isHidden = true
     }
 
-    private func showGraphImage(image: UIImage) {
-        imageView.image = image
-        currentlyShowingGraph = true
-        beeLemniscateView.isHidden = !(goal?.queued ?? false)
-    }
-
     private func refresh() {
-        // Invalidate the download token, meaning that any queued download callbacks
-        // will no-op. This avoids race conditions with downloads finishing out of order.
-        let newDownloadToken = UUID()
-        self.currentDownloadToken = newDownloadToken
-
-        if let downloadReceipt = inProgressDownload {
-            GoalImageView.downloader.cancelRequest(with: downloadReceipt)
-            inProgressDownload = nil
-        }
-
         //  - Deadbeat: Placeholder, no animation
-        if ServiceLocator.currentUserManager.isDeadbeat() {
+        guard !ServiceLocator.currentUserManager.isDeadbeat() else {
             clearGoalGraph()
             return
         }
 
         //  No Goal: Placeholder, no animation
-        guard let goal = self.goal else {
+        guard let goal else {
             clearGoalGraph()
             return
         }
 
+        // Load the appropriate image for the goal
+        let urlString = isThumbnail ? goal.cacheBustingThumbUrl : goal.cacheBustingGraphUrl
+        let request = URLRequest(url: URL(string: urlString)!)
+
         // When queued, we should show a loading indicator over any existing graph,
         // but not over the placeholder image.
-        if goal.queued {
-            beeLemniscateView.isHidden = !currentlyShowingGraph
-        }
-
-        // Load the approppriate iamge for the goal
-        let urlString = if isThumbnail {
-            goal.cacheBustingThumbUrl
-        } else {
-            goal.cacheBustingGraphUrl
-        }
-        let request = URLRequest(url: URL(string: urlString)!)
-
-        // Explicitly check the cache to see if the image is already present, and if so set it directly
-        // This avoids flicker when showing images from the cache, which will otherwise briefly show
-        // the placeholder while waiting for the async download callback
-        if let image = GoalImageView.downloader.imageCache?.image(for: request, withIdentifier: nil) {
-            showGraphImage(image: image)
-            return
-        }
-
-        // Download the image and show it once downloaded
-        inProgressDownload = GoalImageView.downloader.download(request, completion: { response in
-            if newDownloadToken != self.currentDownloadToken {
-                // Another refresh has happend since we were enqueued. Skip performing any updates
-                return
-            }
-
-            switch(response.result) {
-            case .success(let image):
-                // Image downloaded. Show it, and have loading indicator match queued state
-                self.showGraphImage(image: image)
-                break;
-            case .failure(let error):
-                self.logger.error("Error downloading goal graph: \(error)")
-                self.clearGoalGraph()
-            }
-        })
+        beeLemniscateView.isHidden = !currentlyShowingGraph
+
+        imageView.af.setImage(withURLRequest: request,
+                              cacheKey: urlString,
+                              placeholderImage: imageView.image == nil ? Constant.placeholderImage : nil,
+                              filter: RoundedCornersFilter(radius: isThumbnail ? 2 : 4),
+                              progressQueue: DispatchQueue.global(qos: .background),
+                              imageTransition: .crossDissolve(0.4),
+                              runImageTransitionIfCached: false)
     }
 }