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/WebReaderViewModel.swift

@@ -28,6 +28,15 @@ final class WebReaderViewModel: ObservableObject {
 
     guard let viewer = dataService.currentViewer else { return }
 
+    if let content = dataService.pageCache.object(forKey: NSString(string: slug)) {
+      print("RETRIEVED FORM CACHE", slug)
+      isLoading = false
+      articleContent = content.value
+      return
+    } else {
+      print("MISSED CACHE", slug)
+    }
+
     dataService.articleContentPublisher(username: viewer.username, slug: slug).sink(
       receiveCompletion: { [weak self] completion in
         guard case .failure = completion else { return }
@@ -35,6 +44,7 @@ final class WebReaderViewModel: ObservableObject {
       },
       receiveValue: { [weak self] articleContent in
         self?.articleContent = articleContent
+        dataService.pageCache.setObject(CachedPageContent(articleContent), forKey: NSString(string: slug))
       }
     )
     .store(in: &subscriptions)

apple/OmnivoreKit/Sources/Models/ArticleContent.swift

@@ -1,5 +1,13 @@
 import Foundation
 
+public class CachedPageContent: NSObject {
+  public let value: ArticleContent
+
+  public init(_ content: ArticleContent) {
+    self.value = content
+  }
+}
+
 public struct ArticleContent {
   public let htmlContent: String
   public let highlights: [Highlight]

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

@@ -1,6 +1,13 @@
+import Combine
 import Foundation
 import Models
 
+public class CacheManager: NSObject, NSCacheDelegate {
+  public func cache(_: NSCache<AnyObject, AnyObject>, willEvictObject obj: Any) {
+    print("evicting object", obj)
+  }
+}
+
 public final class DataService: ObservableObject {
   public static var registerIntercomUser: ((String) -> Void)?
   public static var showIntercomMessenger: (() -> Void)?
@@ -9,12 +16,37 @@ 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 prefetchPages(items: [FeedItem]) {
+    print("prefetching items", items, "cost limit", pageCache.countLimit)
+
+    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(articleContent), forKey: NSString(string: slug))
+        }
+      )
+      .store(in: &subscriptions)
+    }
   }
 
   public func clearHighlights() {