[Sasky]

KillingBlow.CheckForApGain() doesn't trigger the event -- none of the parameters are filled in.

You need to actually register it with an event handler to call.

Does the AP gain event always fire before or after the combat event? If so, you can do something like this to check when the last AP gain was:

local delta = 1 --If events happen within this time, call simultaneous
local lastApGain = nil
 
function KillingBlow.CheckForApGain( alliancePoints, playSound, difference)
    lastApGain = GetTimeStamp() --or a higher precision, maybe GetFrameTimeMilliseconds would work
end
 
 
function KillingBlow.Combat(eventCode, result, isError, abilityName, abilityGraphic, abilityActionSlotType, sourceName, sourceType, targetName, targetType, hitValue, powerType, damageType, log)
     if ... then --Other filtering you were doing here for source damage, etc
 
          if lastApGain and GitDiffBetweenTimestamps(GetTimeStamp(), lastApGain) > delta then
              --Create your popup frame
          end
 
     end
end
 
em:RegisterForEvent(self.addonName, EVENT_COMBAT_EVENT, KillingBlow.Combat )
em:RegisterForEvent(self.addonName, EVENT_ALLIANCE_POINT_UPDATE, KillingBlow.CheckForApGain)

If it's the other way around (combat event always fires first), you can cache the data on a COMBAT event and create the popup on the AP_GAIN event.

If you need values from both, or the events can fire in either order, it's a little bit trickier, but the basic pattern would be:

--Pseudocode:

EVENT1:
    save parameters
    save timestamp
    call FIRE function

EVENT2:
    save parameters
    save timestamp
    call FIRE function

FIRE:
    if timestamps close enough then
         do whatever with saved parameters
    end

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Also, playerName == sourceName will fail for female characters and possibly in the FR or DE clients. Use one of the ZO format functions or a :gsub("%^..$","") to remove from the source name to check.