Preload audio files when the user opens the app

This kicks off generation and download of audio files. Pending
files are put in a queue and re-attempted. In the UI if a user
hits the pending state they just get some UI now.

apple/OmnivoreKit/Sources/App/Views/Home/Components/FeedCardNavigationLink.swift

@@ -5,6 +5,7 @@ import Views
 
 struct FeedCardNavigationLink: View {
   @EnvironmentObject var dataService: DataService
+  @EnvironmentObject var audioSession: AudioSession
 
   let item: LinkedItem
 
@@ -33,7 +34,7 @@ struct FeedCardNavigationLink: View {
       .opacity(0)
       .buttonStyle(PlainButtonStyle())
       .onAppear {
-        Task { await viewModel.itemAppeared(item: item, dataService: dataService) }
+        Task { await viewModel.itemAppeared(item: item, dataService: dataService, audioSession: audioSession) }
       }
       FeedCard(item: item) {
         viewModel.selectedLinkItem = item.objectID
@@ -44,6 +45,7 @@ struct FeedCardNavigationLink: View {
 
 struct GridCardNavigationLink: View {
   @EnvironmentObject var dataService: DataService
+  @EnvironmentObject var audioSession: AudioSession
 
   @State private var scale = 1.0
 
@@ -86,7 +88,7 @@ struct GridCardNavigationLink: View {
         withAnimation { tapAction() }
       })
         .onAppear {
-          Task { await viewModel.itemAppeared(item: item, dataService: dataService) }
+          Task { await viewModel.itemAppeared(item: item, dataService: dataService, audioSession: audioSession) }
         }
     }
     .aspectRatio(1.8, contentMode: .fill)

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

@@ -11,11 +11,13 @@ import Views
 
   struct HomeFeedContainerView: View {
     @EnvironmentObject var dataService: DataService
+    @EnvironmentObject var audioSession: AudioSession
+
     @AppStorage(UserDefaultKey.homeFeedlayoutPreference.rawValue) var prefersListLayout = false
     @ObservedObject var viewModel: HomeFeedViewModel
 
     func loadItems(isRefresh: Bool) {
-      Task { await viewModel.loadItems(dataService: dataService, isRefresh: isRefresh) }
+      Task { await viewModel.loadItems(dataService: dataService, audioSession: audioSession, isRefresh: isRefresh) }
     }
 
     var body: some View {
@@ -339,6 +341,7 @@ import Views
 
   struct HomeFeedGridView: View {
     @EnvironmentObject var dataService: DataService
+    @EnvironmentObject var audioSession: AudioSession
 
     @State private var itemToRemove: LinkedItem?
     @State private var confirmationShown = false
@@ -361,7 +364,7 @@ import Views
     }
 
     func loadItems(isRefresh: Bool) {
-      Task { await viewModel.loadItems(dataService: dataService, isRefresh: isRefresh) }
+      Task { await viewModel.loadItems(dataService: dataService, audioSession: audioSession, isRefresh: isRefresh) }
     }
 
     var body: some View {

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

@@ -40,14 +40,14 @@ import Views
   var searchIdx = 0
   var receivedIdx = 0
 
-  func itemAppeared(item: LinkedItem, dataService: DataService) async {
+  func itemAppeared(item: LinkedItem, dataService: DataService, audioSession: AudioSession) async {
     if isLoading { return }
     let itemIndex = items.firstIndex(where: { $0.id == item.id })
     let thresholdIndex = items.index(items.endIndex, offsetBy: -5)
 
     // Check if user has scrolled to the last five items in the list
     if let itemIndex = itemIndex, itemIndex > thresholdIndex, items.count < thresholdIndex + 10 {
-      await loadItems(dataService: dataService, isRefresh: false)
+      await loadItems(dataService: dataService, audioSession: audioSession, isRefresh: false)
     }
   }
 
@@ -55,7 +55,7 @@ import Views
     items.insert(item, at: 0)
   }
 
-  func loadItems(dataService: DataService, isRefresh: Bool) async {
+  func loadItems(dataService: DataService, audioSession: AudioSession, isRefresh: Bool) async {
     let syncStartTime = Date()
     let thisSearchIdx = searchIdx
     searchIdx += 1
@@ -123,6 +123,7 @@ import Views
       cursor = queryResult.cursor
       if let username = dataService.currentViewer?.username {
         await dataService.prefetchPages(itemIDs: newItems.map(\.unwrappedID), username: username)
+        await audioSession.preload(itemIDs: newItems.map(\.unwrappedID))
       }
     } else {
       updateFetchController(dataService: dataService)

apple/OmnivoreKit/Sources/Services/AudioSession/AudioSession.swift

@@ -66,6 +66,54 @@ public class AudioSession: NSObject, ObservableObject, AVAudioPlayerDelegate {
     downloadTask?.cancel()
   }
 
+  public func preload(itemIDs: [String], retryCount: Int = 0) async {
+    var pendingList = [String]()
+
+    for pageId in itemIDs {
+      let permFile = pathForAudioFile(pageId: pageId)
+      if FileManager.default.fileExists(atPath: permFile.path) {
+        print("audio file already downloaded: ", permFile)
+        continue
+      }
+
+      // Attempt to fetch the file if not downloaded already
+      let result = try? await downloadAudioFile(pageId: pageId)
+      if result == nil {
+        print("audio file had error downloading: ", pageId)
+        pendingList.append(pageId)
+      }
+
+      if let result = result, result.pending {
+        print("audio file is pending download: ", pageId)
+        pendingList.append(pageId)
+      }
+    }
+
+    print("audio files pending download: ", pendingList)
+    if pendingList.isEmpty {
+      return
+    }
+
+    if retryCount > 5 {
+      print("reached max preload depth, stopping preloading")
+      return
+    }
+
+    let retryDelayInNanoSeconds = UInt64(retryCount * 2 * 1_000_000_000)
+    try? await Task.sleep(nanoseconds: retryDelayInNanoSeconds)
+
+    await preload(itemIDs: pendingList, retryCount: retryCount + 1)
+  }
+
+  public var localAudioUrl: URL? {
+    if let pageId = item?.id {
+      return FileManager.default
+        .urls(for: .documentDirectory, in: .userDomainMask)[0]
+        .appendingPathComponent(pageId + ".mp3")
+    }
+    return nil
+  }
+
   public var scrubState: PlayerScrubState = .reset {
     didSet {
       switch scrubState {
@@ -103,6 +151,16 @@ public class AudioSession: NSObject, ObservableObject, AVAudioPlayerDelegate {
     }
   }
 
+  public func fileNameForAudioFile(_ pageId: String) -> String {
+    pageId + "-" + currentVoice + ".mp3"
+  }
+
+  public func pathForAudioFile(pageId: String) -> URL {
+    FileManager.default
+      .urls(for: .documentDirectory, in: .userDomainMask)[0]
+      .appendingPathComponent(fileNameForAudioFile(pageId))
+  }
+
   public func startAudio() {
     state = .loading
     setupNotifications()
@@ -110,19 +168,24 @@ public class AudioSession: NSObject, ObservableObject, AVAudioPlayerDelegate {
     let pageId = item!.unwrappedID
 
     downloadTask = Task {
-      do {
-        _ = try await downloadAudioFile(pageId: pageId)
-        if Task.isCancelled { return }
-        DispatchQueue.main.async {
-          self.startDownloadedAudioFile(pageId: pageId)
-        }
-      } catch {
-        // TODO: display a failure toast here
+      let result = try? await downloadAudioFile(pageId: pageId)
+      if Task.isCancelled { return }
+
+      if result == nil {
         DispatchQueue.main.async {
+          NSNotification.operationSuccess(message: "Error generating audio.")
           self.stop()
         }
-        print("FAILED TO DOWNLOAD AUDIO URL")
-        print(error)
+      }
+
+      if let result = result, result.pending {
+        DispatchQueue.main.async {
+          NSNotification.operationSuccess(message: "Your audio is being generated.")
+        }
+      }
+
+      DispatchQueue.main.async {
+        self.startDownloadedAudioFile(pageId: pageId)
       }
     }
   }
@@ -136,10 +199,7 @@ public class AudioSession: NSObject, ObservableObject, AVAudioPlayerDelegate {
 
     // TODO: Maybe check if app is active so it doesn't end up playing later?
 
-    let audioUrl = FileManager.default
-      .urls(for: .documentDirectory, in: .userDomainMask)[0]
-      .appendingPathComponent(pageId + ".mp3")
-
+    let audioUrl = pathForAudioFile(pageId: pageId)
     if !FileManager.default.fileExists(atPath: audioUrl.path) {
       stop()
       return
@@ -300,16 +360,12 @@ public class AudioSession: NSObject, ObservableObject, AVAudioPlayerDelegate {
     }
   }
 
-  func downloadAudioFile(pageId: String) async throws -> URL? {
-    let audioUrl = FileManager.default
-      .urls(for: .documentDirectory, in: .userDomainMask)[0]
-      .appendingPathComponent(pageId + ".mp3")
+  func downloadAudioFile(pageId: String) async throws -> (pending: Bool, url: URL?) {
+    let audioUrl = pathForAudioFile(pageId: pageId)
 
-//    if FileManager.default.fileExists(atPath: audioUrl.path) {
-//      // Prevent re-download
-//      // TODO: We aren't doing this very safely, we should be verifying a checksum
-//      return audioUrl
-//    }
+    if FileManager.default.fileExists(atPath: audioUrl.path) {
+      return (pending: false, url: audioUrl)
+    }
 
     guard let url = URL(string: "/api/article/\(pageId)/mp3/\(currentVoice)", relativeTo: appEnvironment.serverBaseURL) else {
       throw BasicError.message(messageText: "Invalid audio URL")
@@ -328,10 +384,7 @@ public class AudioSession: NSObject, ObservableObject, AVAudioPlayerDelegate {
     }
     print("httpResponse: ", httpResponse)
     if let httpResponse = result?.1 as? HTTPURLResponse, httpResponse.statusCode == 202 {
-      print("Tell the user the download has been queued")
-      DispatchQueue.main.async {
-        NSNotification.operationSuccess(message: "Your audio is being created.")
-      }
+      return (pending: true, nil)
     }
 
     guard let data = result?.0 else {
@@ -360,7 +413,7 @@ public class AudioSession: NSObject, ObservableObject, AVAudioPlayerDelegate {
       throw BasicError.message(messageText: errorMessage)
     }
 
-    return audioUrl
+    return (pending: false, url: audioUrl)
   }
 
   public func audioPlayerDidFinishPlaying(_ player: AVAudioPlayer, successfully _: Bool) {