async I/O for images and local state

ImageViewer/ViewController.swift

@@ -1,9 +1,10 @@
 //TODO: Implement support for bonus chapters
 //TODO: Implement scrolling support
-//TODO: Actually implement caching and do I/O on background thread
 
 import UIKit
 
+let preloadCount = 10
+
 enum PageTurn {
     case next
     case previous
@@ -59,9 +60,6 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
     var readerView = UIView()
 
     var imageView = UIImageView()
-    var previousImage: UIImage?
-    var currentImage: UIImage?
-    var nextImage: UIImage?
     var mode = PageTurnMode.leftToRight
     var metadata: Metadata!
     var currentPage: Int! = nil
@@ -92,6 +90,10 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
     var comics: [Comic] = []
     var collectionView: UICollectionView!
 
+    let imageLoader = ImageLoader.init()
+
+    let localStateQueue = DispatchQueue(label: "state.local.queue", qos: .background)
+
     override func viewDidLoad() {
         super.viewDidLoad()
 
@@ -223,16 +225,19 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
     }
 
     func saveLocalState() {
-        do {
-            try JSONEncoder().encode(
-                LocalState(
-                    chapter: currentChapter, page: currentPage, turnMode: mode,
-                    backgroundColor: convertColorToString(
-                        color: readerView.backgroundColor ?? .white))
-            ).write(
-                to: currentPath.appendingPathComponent("state.json"))
-        } catch {
-            print("failed to save local state")
+        let color = self.readerView.backgroundColor ?? .white
+        localStateQueue.async {
+            do {
+                try JSONEncoder().encode(
+                    LocalState(
+                        chapter: self.currentChapter, page: self.currentPage, turnMode: self.mode,
+                        backgroundColor: self.convertColorToString(
+                            color: color))
+                ).write(
+                    to: self.currentPath.appendingPathComponent("state.json"))
+            } catch {
+                print("failed to save local state")
+            }
         }
     }
 
@@ -275,10 +280,10 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
             saveGlobalState()
             loadLocalState()
             setImages(path: path, metadata: metadata)
-            imageView.image = currentImage
             info.text = """
+                \(metadata.title)
                 \(metadata.chapters[currentChapter - 1].title)
-                Volume \(Int(metadata.chapters[currentChapter - 1].volume))
+                Volume \(Int(metadata.chapters[currentChapter - 1].volume)) of \(Int(metadata.chapters.last!.volume))
                 Chapter \(currentChapter!) of \(Int(metadata.chapters.last!.chapter))
                 Page \(currentPage!) of \(metadata.chapters[currentChapter - 1].pages)
                 """
@@ -363,8 +368,7 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
 
     func setupBottomBarInfo() {
         info.textAlignment = .center
-        info.numberOfLines = 0
-        info.text = ""
+        info.numberOfLines = 5
         info.translatesAutoresizingMaskIntoConstraints = false
         info.textColor = .white
         bottomBarView.addSubview(info)
@@ -399,7 +403,7 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
         readerView.isHidden = true
         homeView.isHidden = false
         globalState.comicName = nil
-        hideTopBar()
+        hideBar()
 
         setNeedsStatusBarAppearanceUpdate()
         saveGlobalState()
@@ -585,7 +589,7 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
         return false
     }
 
-    func toggleTopBar() {
+    func toggleBar() {
         topBarView.isHidden.toggle()
         bottomBarView.isHidden.toggle()
 
@@ -595,14 +599,15 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
         }
     }
 
-    func hideTopBar() {
+    func hideBar() {
         topBarView.isHidden = true
+        bottomBarView.isHidden = true
         pageTurnDropdownView.isHidden = true
         backgroundColorDropdownView.isHidden = true
     }
 
     @objc func handleTopTap() {
-        toggleTopBar()
+        toggleBar()
     }
 
     @objc func handleLeftTap() {
@@ -655,52 +660,61 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
     }
 
     func changeImage(turn: PageTurn) {
-        let (chapter, page) = getChapterAndPageFromTurn(
+        var (chapter, page) = getChapterAndPageFromTurn(
             chapter: currentChapter, page: currentPage, turn: turn)
         if (chapter, page) == (currentChapter, currentPage) { return }
-        let (c, p) = getChapterAndPageFromTurn(chapter: chapter, page: page, turn: turn)
-        let vol = Int(metadata.chapters[currentChapter - 1].volume)
-        switch turn {
-        case .next:
-            if nextImage == nil { return }
-            previousImage = currentImage
-            currentImage = nextImage
-            nextImage = getImage(chapter: c, volume: vol, page: p, path: self.currentPath)
-        case .previous:
-            if previousImage == nil { return }
-            nextImage = currentImage
-            currentImage = previousImage
-            previousImage = getImage(chapter: c, volume: vol, page: p, path: self.currentPath)
+        var vol = Int(metadata.chapters[chapter - 1].volume)
+        if let path = getImagePath(chapter: chapter, volume: vol, page: page, path: currentPath) {
+            imageLoader.loadImage(at: path) {
+                [weak self] image in
+                self?.imageView.image = image
+            }
+        } else {
+            return
         }
         currentPage = page
         currentChapter = chapter
-        imageView.image = currentImage
+
+        for _ in 0...preloadCount {
+            (chapter, page) = getChapterAndPageFromTurn(
+                chapter: chapter, page: page, turn: .next)
+            vol = Int(metadata.chapters[chapter - 1].volume)
+            if let path = getImagePath(
+                chapter: chapter, volume: vol, page: page, path: currentPath)
+            {
+                imageLoader.preloadImage(at: path)
+            } else {
+                print("could not preload image")
+            }
+        }
+
         saveLocalState()
         info.text = """
+            \(metadata.title)
             \(metadata.chapters[currentChapter - 1].title)
-            Volume \(Int(metadata.chapters[currentChapter - 1].volume))
+            Volume \(Int(metadata.chapters[currentChapter - 1].volume)) of \(Int(metadata.chapters.last!.volume))
             Chapter \(currentChapter!) of \(Int(metadata.chapters.last!.chapter))
             Page \(currentPage!) of \(metadata.chapters[currentChapter - 1].pages)
             """
     }
 
-    func getImage(chapter: Int, volume: Int, page: Int, path: URL) -> UIImage! {
+    func getImagePath(chapter: Int, volume: Int, page: Int, path: URL) -> URL! {
         let newPath = path.appendingPathComponent(String(format: "volume_%04d", volume))
             .appendingPathComponent(
                 // TODO: it should not be necessary to sub 1
                 String(format: "chapter_%04d_image_%04d.png", chapter, page - 1))
-        if !fileManager.fileExists(atPath: newPath.path) {
-            //TODO: Remove this
-            let alternatePath = path.appendingPathComponent(String(format: "volume_%04d", volume))
-                .appendingPathComponent(
-                    String(format: "chapter%03d_image_%03d.png", chapter, page - 1))
-            if !fileManager.fileExists(atPath: alternatePath.path) {
-                print("Did not find image at path")
-                return nil
-            }
-            return UIImage(contentsOfFile: alternatePath.path)!
+        if fileManager.fileExists(atPath: newPath.path) {
+            return newPath
         }
-        return UIImage(contentsOfFile: newPath.path)!
+        //TODO: Remove this
+        let alternatePath = path.appendingPathComponent(String(format: "volume_%04d", volume))
+            .appendingPathComponent(
+                String(format: "chapter%03d_image_%03d.png", chapter, page - 1))
+        if !fileManager.fileExists(atPath: alternatePath.path) {
+            print("Did not find image at path")
+            return nil
+        }
+        return alternatePath
     }
 
     func getChapterAndPageFromTurn(chapter: Int, page: Int, turn: PageTurn) -> (Int, Int) {
@@ -748,21 +762,31 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
     func setImages(path: URL, metadata: Metadata) {
         var directories: [URL] = []
         if currentPage != nil && currentChapter != nil {
-            currentImage = getImage(
-                chapter: currentChapter, volume: Int(metadata.chapters[currentChapter - 1].volume),
-                page: currentPage, path: currentPath)
+            var vol = Int(metadata.chapters[currentChapter - 1].volume)
+            imageLoader.loadImage(
+                at: getImagePath(
+                    chapter: currentChapter, volume: vol, page: currentPage, path: currentPath
+                )
+            ) {
+                [weak self] image in
+                self?.imageView.image = image
+            }
 
-            let (nc, np) = getChapterAndPageFromTurn(
+            var (chapter, page) = getChapterAndPageFromTurn(
                 chapter: currentChapter, page: currentPage, turn: .next)
-            nextImage = getImage(
-                chapter: nc, volume: Int(metadata.chapters[nc - 1].volume),
-                page: np, path: currentPath)
+            for _ in 0...preloadCount {
+                vol = Int(metadata.chapters[chapter - 1].volume)
+                if let path = getImagePath(
+                    chapter: chapter, volume: vol, page: page, path: currentPath)
+                {
+                    imageLoader.preloadImage(at: path)
+                } else {
+                    print("could not preload image")
+                }
+                (chapter, page) = getChapterAndPageFromTurn(
+                    chapter: chapter, page: page, turn: .next)
+            }
 
-            let (pc, pp) = getChapterAndPageFromTurn(
-                chapter: currentChapter, page: currentPage, turn: .previous)
-            previousImage = getImage(
-                chapter: pc, volume: Int(metadata.chapters[pc - 1].volume),
-                page: pp, path: currentPath)
             return
         }
 
@@ -781,9 +805,8 @@ class ViewController: UIViewController, UIGestureRecognizerDelegate {
             let dir: URL = directories[0]
             let ps: [String] = try fileManager.contentsOfDirectory(atPath: dir.path).sorted()
             let current = dir.appendingPathComponent(ps[0])
-            let next = dir.appendingPathComponent(ps[1])
-            currentImage = UIImage(contentsOfFile: current.path)
-            nextImage = UIImage(contentsOfFile: next.path)
+            imageView.image = UIImage(contentsOfFile: current.path)
+
             let path_meta = current.lastPathComponent.components(separatedBy: "_")
             assert(path_meta[0] == "chapter")
             currentChapter = Int(path_meta[1])!
@@ -888,3 +911,54 @@ class ImageCell: UICollectionViewCell {
         fatalError("init(coder:) has not been implemented")
     }
 }
+
+class ImageLoader {
+    private let cache = NSCache<NSString, UIImage>()
+    private var loadingTasks: [String: [((UIImage?) -> Void)]] = [:]
+    private let queue = DispatchQueue(label: "image.loading.queue", qos: .userInitiated)
+
+    init() {
+        cache.totalCostLimit = 50 * 1024 * 1024
+    }
+
+    func preloadImage(at path: URL) {
+        loadImage(at: path, completion: nil)
+    }
+
+    func loadImage(at path: URL, completion: ((UIImage?) -> Void)?) {
+        if let cached = cache.object(forKey: path.path as NSString) {
+            completion?(cached)
+            return
+        }
+
+        if loadingTasks[path.path] != nil {
+            if let completion = completion {
+                loadingTasks[path.path]?.append(completion)
+            }
+            return
+        }
+
+        loadingTasks[path.path] = completion != nil ? [completion!] : []
+
+        queue.async { [weak self] in
+            guard let self = self else { return }
+
+            let image = UIImage(contentsOfFile: path.path)
+
+            DispatchQueue.main.async {
+                if let image = image, let data = image.pngData() {
+                    self.cache.setObject(image, forKey: path.path as NSString, cost: data.count)
+                } else {
+                    print("no image to cache found")
+                }
+
+                self.loadingTasks[path.path]?.forEach { $0(image) }
+                self.loadingTasks.removeValue(forKey: path.path)
+            }
+        }
+    }
+
+    func imageIfLoaded(at path: String) -> UIImage? {
+        return cache.object(forKey: path as NSString)
+    }
+}