From b00ec501bcb08fe2157a52c6c9e72a9e36d0c590 Mon Sep 17 00:00:00 2001
From: Mufan Qiu <mufan@cs.unc.edu>
Date: Tue, 30 Jun 2026 14:21:04 -0700
Subject: [PATCH] feat: optimistic vote updates

Update the vote state and score instantly on tap instead of waiting for the
network round-trip. A pure `VotesModel.predictVote` mirrors NGA's toggle
semantics (re-tapping clears the vote; flipping up/down moves the score by 2),
so the optimistic value matches what the server returns. The request is sent in
the background and reconciled against the pre-vote baseline on success; on
failure the previous state is restored and an error toast is shown.
---
 .../zh-Hans.lproj/Localizable.strings         |  1 +
 app/Shared/Models/VotesModel.swift            | 23 ++++++++++++++
 app/Shared/Views/PostRowView.swift            | 31 ++++++++++++++-----
 3 files changed, 47 insertions(+), 8 deletions(-)

diff --git a/app/Shared/Localization/zh-Hans.lproj/Localizable.strings b/app/Shared/Localization/zh-Hans.lproj/Localizable.strings
index 51973861..0733792a 100644
--- a/app/Shared/Localization/zh-Hans.lproj/Localizable.strings
+++ b/app/Shared/Localization/zh-Hans.lproj/Localizable.strings
@@ -61,6 +61,7 @@
 "Send" = "发送";
 "Quote" = "引用";
 "Vote Up" = "点赞";
+"Vote Failed" = "操作失败";
 "Edit" = "编辑";
 "Edit Signature" = "修改签名";
 "Share" = "分享";
diff --git a/app/Shared/Models/VotesModel.swift b/app/Shared/Models/VotesModel.swift
index 08a82621..1982fadc 100644
--- a/app/Shared/Models/VotesModel.swift
+++ b/app/Shared/Models/VotesModel.swift
@@ -12,6 +12,29 @@ import SwiftUI
 class VotesModel: ObservableObject {
   typealias Vote = (state: VoteState, delta: Int32)
 
+  /// Predict the resulting vote after applying `operation`, mirroring NGA's
+  /// toggle semantics so optimistic updates match what the server returns:
+  /// tapping the current state again clears it; switching across up/down moves
+  /// by 2. Used to update the UI instantly before the network round-trip.
+  static func predictVote(from current: Vote, operation: PostVoteRequest.Operation) -> Vote {
+    switch (operation, current.state) {
+    case (.upvote, .up): // toggle off
+      return (state: .none, delta: current.delta - 1)
+    case (.upvote, .down): // flip down -> up
+      return (state: .up, delta: current.delta + 2)
+    case (.upvote, _): // none -> up
+      return (state: .up, delta: current.delta + 1)
+    case (.downvote, .down): // toggle off
+      return (state: .none, delta: current.delta + 1)
+    case (.downvote, .up): // flip up -> down
+      return (state: .down, delta: current.delta - 2)
+    case (.downvote, _): // none -> down
+      return (state: .down, delta: current.delta - 1)
+    default: // unknown operation (e.g. UNRECOGNIZED): no change
+      return current
+    }
+  }
+
   // TODO: check performance cost here
   @Published private var votes = [PostId: Vote]()
 
diff --git a/app/Shared/Views/PostRowView.swift b/app/Shared/Views/PostRowView.swift
index c77b78c0..9653ab8b 100644
--- a/app/Shared/Views/PostRowView.swift
+++ b/app/Shared/Views/PostRowView.swift
@@ -367,23 +367,38 @@ struct PostRowView: View {
       return
     }
 
+    // Optimistically update the UI immediately, then send the request in the
+    // background and reconcile with the server's authoritative result. On
+    // failure, roll back to the previous state.
+    let previous = vote
+    let predicted = VotesModel.predictVote(from: previous, operation: operation)
+
+    withAnimation {
+      vote = predicted
+      #if os(iOS)
+        if vote.state != .none {
+          HapticUtils.play(style: .light)
+        }
+      #endif
+    }
+
     logicCallAsync(.postVote(.with {
       $0.postID = post.id
       $0.operation = operation
-    })) { (response: PostVoteResponse) in
+    }), errorToastModel: nil) { (response: PostVoteResponse) in
       if !response.hasError {
+        // Reconcile delta against the pre-vote baseline so the server value wins
+        // regardless of our prediction.
         withAnimation {
           vote.state = response.state
-          vote.delta += response.delta
-          #if os(iOS)
-            if vote.state != .none {
-              HapticUtils.play(style: .light)
-            }
-          #endif
+          vote.delta = previous.delta + response.delta
         }
       } else {
-        // not used
+        withAnimation { vote = previous }
       }
+    } onError: { _ in
+      withAnimation { vote = previous }
+      ToastModel.showAuto(.error("Vote Failed"))
     }
   }