Add CachingDiskStorage

Tusker.xcodeproj/project.pbxproj

@@ -113,6 +113,7 @@
 		D62D2426217ABF63005076CC /* UserActivityType.swift in Sources */ = {isa = PBXBuildFile; fileRef = D62D2425217ABF63005076CC /* UserActivityType.swift */; };
 		D62FF04823D7CDD700909D6E /* AttributedStringHelperTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = D62FF04723D7CDD700909D6E /* AttributedStringHelperTests.swift */; };
 		D6311C5025B3765B00B27539 /* ImageDataCache.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6311C4F25B3765B00B27539 /* ImageDataCache.swift */; };
+		D6311C5625B4CEA000B27539 /* CachingDiskStorage.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6311C5525B4CEA000B27539 /* CachingDiskStorage.swift */; };
 		D6333B372137838300CE884A /* AttributedString+Helpers.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6333B362137838300CE884A /* AttributedString+Helpers.swift */; };
 		D6333B792139AEFD00CE884A /* Date+TimeAgo.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6333B782139AEFD00CE884A /* Date+TimeAgo.swift */; };
 		D63569E023908A8D003DD353 /* StatusState.swift in Sources */ = {isa = PBXBuildFile; fileRef = D60A4FFB238B726A008AC647 /* StatusState.swift */; };
@@ -470,6 +471,7 @@
 		D62D2425217ABF63005076CC /* UserActivityType.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = UserActivityType.swift; sourceTree = "<group>"; };
 		D62FF04723D7CDD700909D6E /* AttributedStringHelperTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AttributedStringHelperTests.swift; sourceTree = "<group>"; };
 		D6311C4F25B3765B00B27539 /* ImageDataCache.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ImageDataCache.swift; sourceTree = "<group>"; };
+		D6311C5525B4CEA000B27539 /* CachingDiskStorage.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CachingDiskStorage.swift; sourceTree = "<group>"; };
 		D6333B362137838300CE884A /* AttributedString+Helpers.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "AttributedString+Helpers.swift"; sourceTree = "<group>"; };
 		D6333B782139AEFD00CE884A /* Date+TimeAgo.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "Date+TimeAgo.swift"; sourceTree = "<group>"; };
 		D63661BF2381C144004B9E16 /* PreferencesNavigationController.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = PreferencesNavigationController.swift; sourceTree = "<group>"; };
@@ -1496,6 +1498,7 @@
 				D6F1F84C2193B56E00F5FE67 /* Cache.swift */,
 				04DACE8D212CC7CC009840C4 /* ImageCache.swift */,
 				D6311C4F25B3765B00B27539 /* ImageDataCache.swift */,
+				D6311C5525B4CEA000B27539 /* CachingDiskStorage.swift */,
 			);
 			path = Caching;
 			sourceTree = "<group>";
@@ -1874,6 +1877,7 @@
 				D6A3BC8A2321F79B00FD64D5 /* AccountTableViewCell.swift in Sources */,
 				D66A77BB233838DC0058F1EC /* UIFont+Traits.swift in Sources */,
 				D68FEC4F232C5BC300C84F23 /* SegmentedPageViewController.swift in Sources */,
+				D6311C5625B4CEA000B27539 /* CachingDiskStorage.swift in Sources */,
 				D62275AA24F1E01C00B82A16 /* ComposeTextView.swift in Sources */,
 				0450531F22B0097E00100BA2 /* Timline+UI.swift in Sources */,
 				D667E5F52135BCD50057A976 /* ConversationTableViewController.swift in Sources */,

Tusker/Caching/CachingDiskStorage.swift

@@ -0,0 +1,71 @@
+//
+//  CachingDiskStorage.swift
+//  Tusker
+//
+//  Created by Shadowfacts on 1/17/21.
+//  Copyright © 2021 Shadowfacts. All rights reserved.
+//
+
+import Foundation
+import Cache
+
+/// This class wraps a `DiskStorage` and maintains an in-memory cache of whether objects
+/// exist on disk to avoid unnecessary disk I/O when calling synchronous methods like
+/// `existsObject(forKey:)`.
+class CachingDiskStorage<T>: StorageAware {
+    
+    private let storage: DiskStorage<T>
+    private var files = [String: FileState]()
+    
+    init(config: DiskConfig, transformer: Transformer<T>) throws {
+        storage = try DiskStorage(config: config, transformer: transformer)
+    }
+    
+    private func state(for key: String) -> FileState {
+        return files[key] ?? .unknown
+    }
+    
+    func entry(forKey key: String) throws -> Entry<T> {
+        if state(for: key) == .doesNotExist {
+            throw StorageError.notFound
+        }
+        return try storage.entry(forKey: key)
+    }
+    
+    func existsObject(forKey key: String) throws -> Bool {
+        switch state(for: key) {
+        case .unknown:
+            return try storage.existsObject(forKey: key)
+        case .exists:
+            return true
+        case .doesNotExist:
+            return false
+        }
+    }
+    
+    func removeObject(forKey key: String) throws {
+        try storage.removeObject(forKey: key)
+        files[key] = .doesNotExist
+    }
+    
+    func setObject(_ object: T, forKey key: String, expiry: Expiry? = nil) throws {
+        try storage.setObject(object, forKey: key, expiry: expiry)
+        files[key] = .exists
+    }
+    
+    func removeAll() throws {
+        try storage.removeAll()
+        files.removeAll()
+    }
+    
+    func removeExpiredObjects() throws {
+        try storage.removeExpiredObjects()
+    }
+    
+}
+
+extension CachingDiskStorage {
+    private enum FileState {
+        case unknown, exists, doesNotExist
+    }
+}

Tusker/Caching/ImageCache.swift

@@ -37,10 +37,6 @@ class ImageCache {
     func get(_ url: URL, completion: ((Data?, UIImage?) -> Void)?) -> Request? {
         let key = url.absoluteString
         if !ImageCache.disableCaching,
-           // todo: calling object(forKey: key) does disk I/O and this method is often called from the main thread
-           // in performance sensitive paths. a nice optimization to DiskStorage would be adding an internal cache
-           // of the state (unknown/exists/does not exist) of whether or not objects exist on disk so that the slow, disk I/O
-           // path can be avoided most of the time
            let entry = try? cache.get(key) {
             if let completion = completion {
                 backgroundQueue.async {

Tusker/Caching/ImageDataCache.swift

@@ -12,7 +12,7 @@ import Cache
 class ImageDataCache {
     
     private let memory: MemoryStorage<Entry>
-    private let disk: DiskStorage<Data>?
+    private let disk: CachingDiskStorage<Data>?
     
     private let storeOriginalDataInMemory: Bool
     private let desiredPixelSize: CGSize?
@@ -23,7 +23,7 @@ class ImageDataCache {
         
         if let diskExpiry = diskExpiry {
             let diskConfig = DiskConfig(name: name, expiry: diskExpiry)
-            self.disk = try! DiskStorage(config: diskConfig, transformer: TransformerFactory.forData())
+            self.disk = try! CachingDiskStorage(config: diskConfig, transformer: TransformerFactory.forData())
         } else {
             self.disk = nil
         }