From 3b87916d9a26610dc010c4dff7483c138606cc93 Mon Sep 17 00:00:00 2001
From: Marcus Kida <marcus.kida@bearologics.com>
Date: Tue, 7 Feb 2023 00:46:35 +0100
Subject: [PATCH] chore(Feed): Improve Feed performance by fetching statuses in
 batches (#914)

---
 .../HomeTimelineViewController.swift             |  6 ++++++
 .../HomeTimelineViewModel+LoadLatestState.swift  |  2 +-
 .../HomeTimelineViewModel+LoadOldestState.swift  |  2 +-
 .../HomeTimeline/HomeTimelineViewModel.swift     |  6 ++++++
 .../FeedFetchedResultsController.swift           | 16 +++++++++++++++-
 5 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/Mastodon/Scene/HomeTimeline/HomeTimelineViewController.swift b/Mastodon/Scene/HomeTimeline/HomeTimelineViewController.swift
index 3caafef3d..6c390af3b 100644
--- a/Mastodon/Scene/HomeTimeline/HomeTimelineViewController.swift
+++ b/Mastodon/Scene/HomeTimeline/HomeTimelineViewController.swift
@@ -518,6 +518,12 @@ extension HomeTimelineViewController: UITableViewDelegate, AutoGenerateTableView
     }
 
     // sourcery:end
+    
+    func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
+        if indexPath.row == tableView.numberOfRows(inSection: indexPath.section) - 1 {
+            viewModel.timelineDidReachEnd()
+        }
+    }
 }
 
 // MARK: - TimelineMiddleLoaderTableViewCellDelegate
diff --git a/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+LoadLatestState.swift b/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+LoadLatestState.swift
index d243c9a96..fdb9da01e 100644
--- a/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+LoadLatestState.swift
+++ b/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+LoadLatestState.swift
@@ -65,7 +65,7 @@ extension HomeTimelineViewModel.LoadLatestState {
             guard let viewModel else { return }
             
             let latestFeedRecords = viewModel.fetchedResultsController.records.prefix(APIService.onceRequestStatusMaxCount)
-            let parentManagedObjectContext = viewModel.fetchedResultsController.fetchedResultsController.managedObjectContext
+            let parentManagedObjectContext = viewModel.fetchedResultsController.managedObjectContext
             let managedObjectContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
             managedObjectContext.parent = parentManagedObjectContext
 
diff --git a/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+LoadOldestState.swift b/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+LoadOldestState.swift
index e88b5ed5f..2b02f0360 100644
--- a/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+LoadOldestState.swift
+++ b/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+LoadOldestState.swift
@@ -67,7 +67,7 @@ extension HomeTimelineViewModel.LoadOldestState {
             }
             
             Task {
-                let managedObjectContext = viewModel.fetchedResultsController.fetchedResultsController.managedObjectContext
+                let managedObjectContext = viewModel.fetchedResultsController.managedObjectContext
                 let _maxID: Mastodon.Entity.Status.ID? = try await managedObjectContext.perform {
                     guard let feed = lastFeedRecord.object(in: managedObjectContext),
                           let status = feed.status
diff --git a/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel.swift b/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel.swift
index c7c967305..a59ffe7c5 100644
--- a/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel.swift
+++ b/Mastodon/Scene/HomeTimeline/HomeTimelineViewModel.swift
@@ -119,6 +119,12 @@ extension HomeTimelineViewModel {
     }
 }
 
+extension HomeTimelineViewModel {
+    func timelineDidReachEnd() {
+        fetchedResultsController.fetchNextBatch()
+    }
+}
+
 extension HomeTimelineViewModel {
 
     // load timeline gap
diff --git a/MastodonSDK/Sources/MastodonCore/FetchedResultsController/FeedFetchedResultsController.swift b/MastodonSDK/Sources/MastodonCore/FetchedResultsController/FeedFetchedResultsController.swift
index ab555c1c3..27cc5a08d 100644
--- a/MastodonSDK/Sources/MastodonCore/FetchedResultsController/FeedFetchedResultsController.swift
+++ b/MastodonSDK/Sources/MastodonCore/FetchedResultsController/FeedFetchedResultsController.swift
@@ -16,11 +16,19 @@ import MastodonSDK
 
 final public class FeedFetchedResultsController: NSObject {
     
+    private enum Constants {
+        static let defaultFetchLimit = 100
+    }
+    
     public let logger = Logger(subsystem: "FeedFetchedResultsController", category: "DB")
     
     var disposeBag = Set<AnyCancellable>()
     
-    public let fetchedResultsController: NSFetchedResultsController<Feed>
+    private let fetchedResultsController: NSFetchedResultsController<Feed>
+    
+    public var managedObjectContext: NSManagedObjectContext {
+        fetchedResultsController.managedObjectContext
+    }
     
     // input
     @Published public var predicate = Feed.predicate(kind: .none, acct: .none)
@@ -28,6 +36,11 @@ final public class FeedFetchedResultsController: NSObject {
     // output
     private let _objectIDs = PassthroughSubject<[NSManagedObjectID], Never>()
     @Published public var records: [ManagedObjectRecord<Feed>] = []
+
+    public func fetchNextBatch() {
+        fetchedResultsController.fetchRequest.fetchLimit += Constants.defaultFetchLimit
+        try? fetchedResultsController.performFetch()
+    }
     
     public init(managedObjectContext: NSManagedObjectContext) {
         self.fetchedResultsController = {
@@ -36,6 +49,7 @@ final public class FeedFetchedResultsController: NSObject {
             fetchRequest.returnsObjectsAsFaults = false
             fetchRequest.shouldRefreshRefetchedObjects = true
             fetchRequest.fetchBatchSize = 15
+            fetchRequest.fetchLimit = Constants.defaultFetchLimit
             let controller = NSFetchedResultsController(
                 fetchRequest: fetchRequest,
                 managedObjectContext: managedObjectContext,