In the current Alterac Valley meta, capturing graveyards goes against the interests of the group. Therefore it is of some value to be able to tell who's doing it. In this post I describe the process I followed to create an addon that sends an alert when someone is capping graveyards.
Spell detection
When a flag is being captured in Alterac Valley (or Arathi Basin for that matter) spell#24390 is cast by the player. That spell is marked as hidden but by modifying Spell.dbc we can make the client show it and expose it to the Lua API.
Although the event COMBAT_LOG_EVENT_UNFILTERED shows the spell being cast, it doesn't show the target of the cast, since it's not a unit but a gameobject. This makes it impossible to tell if it's a graveyard or a base that's being captured, so this option is not optimal, since you would be sending alerts when someone started capturing a tower.
I thought of using GetMinimapZoneText to detect if I was in the proximity of a graveyard before sending the alert, but the names of the subzones are inconsistent, and even then most (all?) towers are in close proximity of graveyards.
Therefore I decided this approach wasn't going to work well enough.
Scoreboard scanning
Another option is to wait for the CHAT_MSG_MONSTER_YELL events that are sent once a graveyard has been captured. Those would have to be parsed so the addon would only work for those who play the game in English.
- The Stormpike Aid Station is under attack! If left unchecked, the Horde will capture it!
- The Stormpike Aid Station was taken by the Horde!
The first message is sent whenever a graveyard is about to be captured (i.e. the timer starts to run), while the second message is sent when the graveyard is finally captured and when a graveyard is defended.
We continuously save the data that is shown on the scoreboard by iterating over every player using GetBattlefieldScore and then saving the number of graveyards captured and defended that is returned by GetBattlefieldStatData. Then, every time we receive a CHAT_MSG_MONSTER_YELL event, we compare the current data to the stored data, so we can see whose graveyard capture stats have increased: that's the player who's captured or defended a graveyard.
This worked too well: the addon would alert you and everybody around you of someone capping a graveyard which is far away and no one cares about. Also, by the time a graveyard is capped, it already is too late.
Back to spell detection
Looking at the list of API functions, I stumbled upon GetPlayerMapPosition. Once we receive the COMBAT_LOG_EVENT_UNFILTERED event, we can retrieve the UnitId of the player, pass it to GetPlayerMapPosition, calculate the Pythagorean distance of the player to the list of graveyards that are retrieved by GetMapLandmarkInfo, and only trigger the alert if the player is close to a graveyard.
We only receive COMBAT_LOG_EVENT_UNFILTERED events of units we can see, but since the alerts would be sent in /say, we won't care if someone starts capping a graveyard on the other side of the map.
Since it is possible to continuously start and cancel the cast of the spell that captures the graveyard, some message throttling system would have to be implemented.
I found this to be an acceptable solution.
