Merge pull request #349 from omnivore-app/experiment/prefetch-on-ios

Prefetch page content on iOS

apple/OmnivoreKit/Sources/App/Views/Home/HomeFeedViewModel.swift

@@ -76,6 +76,9 @@ final class HomeFeedViewModel: ObservableObject {
         if thisSearchIdx > 0, thisSearchIdx <= self?.receivedIdx ?? 0 {
           return
         }
+
+        dataService.prefetchPages(items: result.items)
+
         self?.items = isRefresh ? result.items : (self?.items ?? []) + result.items
         self?.isLoading = false
         self?.receivedIdx = thisSearchIdx

apple/OmnivoreKit/Sources/App/Views/WebReader/WebReaderContainer.swift

@@ -204,7 +204,7 @@ struct WebReaderContainerView: View {
               Color.systemBackground
                 .transition(.opacity)
                 .onAppear {
-                  DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(250)) {
+                  DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(100)) {
                     withAnimation(.linear(duration: 0.2)) {
                       showOverlay = false
                     }

apple/OmnivoreKit/Sources/App/Views/WebReader/WebReaderViewModel.swift

@@ -21,13 +21,20 @@ final class WebReaderViewModel: ObservableObject {
   @Published var isLoading = false
   @Published var articleContent: ArticleContent?
 
+  var slug: String?
   var subscriptions = Set<AnyCancellable>()
 
   func loadContent(dataService: DataService, slug: String) {
+    self.slug = slug
     isLoading = true
 
     guard let viewer = dataService.currentViewer else { return }
 
+    if let content = dataService.pageFromCache(slug: slug) {
+      articleContent = content
+      // continue to load from the web if possible
+    }
+
     dataService.articleContentPublisher(username: viewer.username, slug: slug).sink(
       receiveCompletion: { [weak self] completion in
         guard case .failure = completion else { return }
@@ -35,6 +42,7 @@ final class WebReaderViewModel: ObservableObject {
       },
       receiveValue: { [weak self] articleContent in
         self?.articleContent = articleContent
+        dataService.pageCache.setObject(CachedPageContent(slug, articleContent), forKey: NSString(string: slug))
       }
     )
     .store(in: &subscriptions)
@@ -167,12 +175,16 @@ final class WebReaderViewModel: ObservableObject {
 
     switch actionID {
     case "deleteHighlight":
+      dataService.invalidateCachedPage(slug: slug)
       deleteHighlight(messageBody: messageBody, replyHandler: replyHandler, dataService: dataService)
     case "createHighlight":
+      dataService.invalidateCachedPage(slug: slug)
       createHighlight(messageBody: messageBody, replyHandler: replyHandler, dataService: dataService)
     case "mergeHighlight":
+      dataService.invalidateCachedPage(slug: slug)
       mergeHighlight(messageBody: messageBody, replyHandler: replyHandler, dataService: dataService)
     case "updateHighlight":
+      dataService.invalidateCachedPage(slug: slug)
       updateHighlight(messageBody: messageBody, replyHandler: replyHandler, dataService: dataService)
     case "articleReadingProgress":
       updateReadingProgress(messageBody: messageBody, replyHandler: replyHandler, dataService: dataService)

apple/OmnivoreKit/Sources/Models/ArticleContent.swift

@@ -1,5 +1,15 @@
 import Foundation
 
+public class CachedPageContent: NSObject {
+  public let slug: String
+  public let value: ArticleContent
+
+  public init(_ slug: String, _ content: ArticleContent) {
+    self.slug = slug
+    self.value = content
+  }
+}
+
 public struct ArticleContent {
   public let htmlContent: String
   public let highlights: [Highlight]

apple/OmnivoreKit/Sources/Services/DataService/DataService.swift

@@ -1,6 +1,16 @@
+import Combine
 import Foundation
 import Models
 
+public class CacheManager: NSObject, NSCacheDelegate {
+  public func cache(_: NSCache<AnyObject, AnyObject>, willEvictObject obj: Any) {
+    // This is just used for debugging
+    if let content = obj as? CachedPageContent {
+      print("evicting page from cache", content.slug)
+    }
+  }
+}
+
 public final class DataService: ObservableObject {
   public static var registerIntercomUser: ((String) -> Void)?
   public static var showIntercomMessenger: (() -> Void)?
@@ -9,12 +19,20 @@ public final class DataService: ObservableObject {
   public internal(set) var currentViewer: Viewer?
   let networker: Networker
 
+  public let pageCache = NSCache<NSString, CachedPageContent>()
+  let pageCacheQueue = DispatchQueue.global(qos: .background)
+
   let highlightsCache = NSCache<AnyObject, CachedPDFHighlights>()
   let highlightsCacheQueue = DispatchQueue(label: "app.omnivore.highlights.cache.queue", attributes: .concurrent)
 
+  let cacheManager: CacheManager
+  var subscriptions = Set<AnyCancellable>()
+
   public init(appEnvironment: AppEnvironment, networker: Networker) {
     self.appEnvironment = appEnvironment
     self.networker = networker
+    self.cacheManager = CacheManager()
+    pageCache.delegate = cacheManager
   }
 
   public func clearHighlights() {
@@ -30,3 +48,37 @@ public final class DataService: ObservableObject {
     }
   }
 }
+
+public extension DataService {
+  func prefetchPages(items: [FeedItem]) {
+    print("prefetching pages")
+    guard let viewer = currentViewer else { return }
+
+    for item in items {
+      let slug = item.slug
+      articleContentPublisher(username: viewer.username, slug: slug).sink(
+        receiveCompletion: { _ in },
+        receiveValue: { [weak self] articleContent in
+          self?.pageCache.setObject(CachedPageContent(slug, articleContent), forKey: NSString(string: slug))
+        }
+      )
+      .store(in: &subscriptions)
+    }
+  }
+
+  func pageFromCache(slug: String) -> ArticleContent? {
+    if let content = pageCache.object(forKey: NSString(string: slug)) {
+      print("cache hit", slug)
+      return content.value
+    } else {
+      print("cache miss", slug)
+    }
+    return nil
+  }
+
+  func invalidateCachedPage(slug: String?) {
+    if let slug = slug {
+      pageCache.removeObject(forKey: NSString(string: slug))
+    }
+  }
+}