castbar: Add override for unit checks (#677)

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)
@@ -300,9 +312,11 @@ 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
+
 	if(not element:IsShown() or element.castID ~= castID or element.spellID ~= spellID) then
 		return
 	end
@@ -357,9 +371,11 @@ 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
+
 	if(not element:IsShown() or element.castID ~= castID or element.spellID ~= spellID) then
 		return
 	end
@@ -379,9 +395,11 @@ 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
+
 	if(not element:IsShown() or element.castID ~= castID or element.spellID ~= spellID) then
 		return
 	end
@@ -410,9 +428,11 @@ 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
+
 	if(not element:IsShown()) then return end
 
 	element.notInterruptible = event == 'UNIT_SPELLCAST_NOT_INTERRUPTIBLE'