castbar: Add override for unit checks

Fixes #676

elements/castbar.lua

@@ -200,10 +200,22 @@ local function UpdatePips(element, numStages)
 	end
 end
 
-local function CastStart(self, event, unit)
-	if(self.unit ~= unit) then return end
+--[[ Override: Castbar:ShouldShow(unit)
+Handles check for which unit the castbar should show for.  
+Defaults to the object unit.
 
+* self - the Castbar widget
+* unit - the unit for which the update has been triggered (string)
+--]]
+local function ShouldShow(element, unit)
+	return element.__owner.unit == unit
+end
+
+local function CastStart(self, event, unit)
 	local element = self.Castbar
+	if(not (element.ShouldShow or ShouldShow) (element, unit)) then
+		return
+	end
 
 	local numStages, _
 	local name, text, texture, startTime, endTime, isTradeSkill, castID, notInterruptible, spellID = UnitCastingInfo(unit)
@@ -300,7 +312,10 @@ local function CastStart(self, event, unit)
 end
 
 local function CastUpdate(self, event, unit, castID, spellID)
-	if(self.unit ~= unit) then return end
+	local element = self.Castbar
+	if(not (element.ShouldShow or ShouldShow) (element, unit)) then
+		return
+	end
 
 	local element = self.Castbar
 	if(not element:IsShown() or element.castID ~= castID or element.spellID ~= spellID) then
@@ -357,7 +372,10 @@ local function CastUpdate(self, event, unit, castID, spellID)
 end
 
 local function CastStop(self, event, unit, castID, spellID)
-	if(self.unit ~= unit) then return end
+	local element = self.Castbar
+	if(not (element.ShouldShow or ShouldShow) (element, unit)) then
+		return
+	end
 
 	local element = self.Castbar
 	if(not element:IsShown() or element.castID ~= castID or element.spellID ~= spellID) then
@@ -379,7 +397,10 @@ local function CastStop(self, event, unit, castID, spellID)
 end
 
 local function CastFail(self, event, unit, castID, spellID)
-	if(self.unit ~= unit) then return end
+	local element = self.Castbar
+	if(not (element.ShouldShow or ShouldShow) (element, unit)) then
+		return
+	end
 
 	local element = self.Castbar
 	if(not element:IsShown() or element.castID ~= castID or element.spellID ~= spellID) then
@@ -410,7 +431,10 @@ local function CastFail(self, event, unit, castID, spellID)
 end
 
 local function CastInterruptible(self, event, unit)
-	if(self.unit ~= unit) then return end
+	local element = self.Castbar
+	if(not (element.ShouldShow or ShouldShow) (element, unit)) then
+		return
+	end
 
 	local element = self.Castbar
 	if(not element:IsShown()) then return end