frame: add Frame:IsEventRegistered

Vanilla has RegisterEvent / UnregisterEvent as frame methods but not the
query IsEventRegistered (added in a later expansion). There's no engine
Script_* to delegate to, so we replicate the membership check already
embedded in Frame::RegisterEvent (FUN_00702140): find the event by name
in the live event table (SStrCmpI), then walk that entry's subscriber
chain (head@+0xC, next@+4, frame@+8, low-bit/null sentinel) for this
frame. self is resolved via Game::Lua::ResolveObject, yielding the same
CFrameScriptObject* the chain stores.

Registered on the Frame method registry (VAR_FRAME_METHOD_REGISTRY,
alongside RegisterEvent) so it resolves on any frame. Pure read, no new
hooks. Returns a boolean, matching the 3.3.5-era contract; verified
in-game (true for a registered event, false otherwise). Documented in
docs/API.md and README.md.

Author: brues-code

README.md

@@ -33,7 +33,7 @@ Full per-function reference: **[docs/API.md](docs/API.md)**.
 | [Expansion](docs/API.md#expansion) | `ClassicExpansionAtLeast`, `ClassicExpansionAtMost`, `GetClassicExpansionLevel` |
 | [Faction](docs/API.md#faction) | `C_Reputation.GetFactionDataByIndex`, `C_Reputation.GetFactionStandings`, `C_Reputation.GetLastStandingChange`, `C_Reputation.GetWatchedFactionData`, `C_Reputation.SetWatchedFactionByID`, `GetFactionIDByIndex`, `GetFactionInfoByID`, `GetFactionParentID` |
 | [Focus](docs/API.md#focus) | `ClearFocus`, `FocusUnit` |
-| [Frame](docs/API.md#frame) | `region:SetPoint("point")` (one-arg form), `region:SetSize`, `region:GetSize`, `frame:SetShown`, `frame:SetResizeBounds`, `frame:HookScript` |
+| [Frame](docs/API.md#frame) | `region:SetPoint("point")` (one-arg form), `region:SetSize`, `region:GetSize`, `frame:SetShown`, `frame:SetResizeBounds`, `frame:HookScript`, `frame:IsEventRegistered` |
 | [FriendList](docs/API.md#friendlist) | `C_FriendList.IsWhoQueryPending`, `C_FriendList.SendWhoQueryByName` |
 | [GameObject](docs/API.md#gameobject) | `C_GameObjectInfo.GetGameObjectInfoByID`, `C_GameObjectInfo.RequestLoadGameObjectByID` |
 | [Gossip](docs/API.md#gossip) | `C_GossipInfo.CloseGossip`, `C_GossipInfo.GetActiveQuests`, `C_GossipInfo.GetAvailableQuests`, `C_GossipInfo.GetNumActiveQuests`, `C_GossipInfo.GetNumAvailableQuests`, `C_GossipInfo.GetNumOptions`, `C_GossipInfo.GetOptions`, `C_GossipInfo.GetText`, `C_GossipInfo.SelectActiveQuest`, `C_GossipInfo.SelectAvailableQuest`, `C_GossipInfo.SelectOption`, `C_GossipInfo.SelectOptionByIndex` |

docs/API.md

@@ -137,6 +137,7 @@ build instructions.
   - [`frame:SetShown(shown)`](#framesetshownshown)
   - [`frame:SetResizeBounds(minWidth, minHeight [, maxWidth, maxHeight])`](#framesetresizeboundsminwidth-minheight--maxwidth-maxheight)
   - [`frame:HookScript(scriptType, handler)`](#framehookscriptscripttype-handler)
+  - [`frame:IsEventRegistered(event)`](#frameiseventregisteredevent)
 
 - [FriendList](#friendlist)
   - [`C_FriendList.SendWhoQueryByName(name)`](#c_friendlistsendwhoquerybynamename)
@@ -3143,6 +3144,22 @@ button:HookScript("OnEnter", function(self)
 end)
 ```
 
+### `frame:IsEventRegistered(event)`
+
+Vanilla ships `RegisterEvent` / `UnregisterEvent` but not the query.
+Returns `true` if the frame is currently registered for `event`,
+`false` otherwise (including for an unknown event name). Reuses the
+engine's own subscriber-chain membership check — the same walk
+`RegisterEvent` performs before appending — so the answer is exactly
+what the event dispatcher sees. Works on any frame.
+
+```lua
+local f = CreateFrame("Frame")
+f:RegisterEvent("PLAYER_LOGIN")
+f:IsEventRegistered("PLAYER_LOGIN")   -- true
+f:IsEventRegistered("PLAYER_LOGOUT")  -- false
+```
+
 ## FriendList
 
 ### `C_FriendList.SendWhoQueryByName(name)`

src/Offsets.h

@@ -3463,6 +3463,15 @@ enum Offsets {
     OFF_EVENT_ENTRY_NAME = 0x00,
     OFF_EVENT_ENTRY_HEAD = 0x0C,
 
+    // Subscriber-chain node layout — the linked list hanging off each
+    // entry's +0x0C head, derived from Frame::RegisterEvent
+    // (FUN_00702140): each node is `{ ?, next@+0x04, frame@+0x08 }`, where
+    // `frame` is the CFrameScriptObject* subscriber. A node value with its
+    // low bit set (or null) is the end/empty sentinel. Walked by
+    // Frame:IsEventRegistered to test membership.
+    OFF_EVENT_NODE_NEXT = 0x04,
+    OFF_EVENT_NODE_FRAME = 0x08,
+
     // `Frame::RegisterEvent` — the C++ helper called by the Lua
     // `frame:RegisterEvent(eventName)` method (`Script_RegisterEvent` at
     // `0x00774A40`). `__thiscall` with `(this=frame, eventName)`. Walks

src/frame/Modern.cpp

@@ -186,6 +186,65 @@ int __fastcall Script_HookScript(void *L) {
     return 0;
 }
 
+// ---- IsEventRegistered (Frame) ---------------------------------------------
+
+// `frame:IsEventRegistered("event")` — vanilla has RegisterEvent /
+// UnregisterEvent but not the query. There's no engine Script_* to
+// delegate to, so we replicate the membership check that lives inside
+// Frame::RegisterEvent (FUN_00702140): find the event by name in the live
+// event table, then walk that entry's subscriber chain for this frame.
+// The frame pointer we compare against is the same CFrameScriptObject*
+// the engine stores (Game::Lua::ResolveObject resolves self exactly as
+// Script_RegisterEvent does).
+using SStrCmpI_t = int(__stdcall *)(const char *a, const char *b, int n);
+
+bool FrameHasEvent(void *frame, const char *eventName) {
+    const uint32_t count = *reinterpret_cast<const uint32_t *>(
+        static_cast<uintptr_t>(Offsets::VAR_EVENT_TABLE_COUNT));
+    auto *base = *reinterpret_cast<const uint8_t *const *>(
+        static_cast<uintptr_t>(Offsets::VAR_EVENT_TABLE_BASE_PTR));
+    if (count == 0 || base == nullptr)
+        return false;
+
+    auto sstrcmpi = reinterpret_cast<SStrCmpI_t>(
+        static_cast<uintptr_t>(Offsets::FUN_SSTR_CMP_I));
+
+    for (uint32_t i = 0; i < count; ++i) {
+        const uint8_t *entry = base + i * Offsets::EVENT_ENTRY_STRIDE;
+        const char *name = *reinterpret_cast<const char *const *>(
+            entry + Offsets::OFF_EVENT_ENTRY_NAME);
+        if (name == nullptr)
+            continue;
+        if (sstrcmpi(name, eventName, 0x7FFFFFFF) != 0)
+            continue;
+        // Names are unique, so this is the one entry — walk its chain.
+        uintptr_t node = *reinterpret_cast<const uintptr_t *>(
+            entry + Offsets::OFF_EVENT_ENTRY_HEAD);
+        while ((node & 1) == 0 && node != 0) {
+            if (*reinterpret_cast<void *const *>(
+                    node + Offsets::OFF_EVENT_NODE_FRAME) == frame)
+                return true;
+            node = *reinterpret_cast<const uintptr_t *>(
+                node + Offsets::OFF_EVENT_NODE_NEXT);
+        }
+        return false;
+    }
+    return false;
+}
+
+int __fastcall Script_IsEventRegistered(void *L) {
+    if (!Game::Lua::IsString(L, 2)) {
+        Game::Lua::Error(L, "Usage: frame:IsEventRegistered(\"event\")");
+        return 0;
+    }
+    void *frame = Game::Lua::ResolveObject(L, 1);
+    const char *eventName = Game::Lua::ToString(L, 2);
+    const bool registered =
+        frame != nullptr && eventName != nullptr && FrameHasEvent(frame, eventName);
+    Game::Lua::PushBool(L, registered);
+    return 1;
+}
+
 // ---- SetPoint one-arg form (Script_SetPoint co-hook) -----------------------
 
 // Vanilla's Script_SetPoint accepts every modern form except the bare
@@ -222,6 +281,7 @@ const Game::Lua::FrameMethodEntry g_frameMethods[] = {
                                   Offsets::FUN_SCRIPT_FRAME_HIDE>},
     {"SetResizeBounds", &Script_SetResizeBounds},
     {"HookScript", &Script_HookScript},
+    {"IsEventRegistered", &Script_IsEventRegistered},
 };
 
 const Game::Lua::FrameMethodEntry g_textureMethods[] = {