[lightspark]

AssignedArtifactPower = LibStub("AceAddon-3.0"):NewAddon("Assigned Artifact Power", "AceEvent-3.0")
 
function AssignedArtifactPower:calc_ap()
    local c, ge, gc, xp, ps, n, _ = _G.C_ArtifactUI
    _, _, n, _, xp, ps = c.GetEquippedArtifactInfo()
    for i=1,ps-1 do
        xp = xp + c.GetCostForPointAtRank(i)
    end
    AP = xp
end
 
function AssignedArtifactPower:OnEnable()
    self:RegisterEvent("ARTIFACT_UPDATE")
    self:RegisterEvent("ARTIFACT_XP_UPDATE")
    self:RegisterEvent("ARTIFACT_MAX_RANKS_UPDATE")
    self:Refresh()
end
 
function AssignedArtifactPower:ARTIFACT_UPDATE()
    self:Refresh()
end
 
function AssignedArtifactPower:ARTIFACT_XP_UPDATE()
    self:Refresh()
end
 
function AssignedArtifactPower:ARTIFACT_MAX_RANKS_UPDATE()
    self:Refresh()
end
 
function AssignedArtifactPower:Overlay()
    AFP = ArtifactFrame.PerksTab
    if not AFP.AssignedArtifactPower then
        AF = AFP.TitleContainer
        AFPoints = AF.PointsRemainingLabel
        AFLabel = AF.ArtifactPower
        --New Frame for available Artifactpower
        AFP.ArtifactPower = CreateFrame("Frame", "ArtifactPower", AFP)
        AFP.ArtifactPower:SetPoint("TOP", -75, -120, "CENTER")
        AFP.ArtifactPower:SetWidth(150)
        AFP.ArtifactPower:SetHeight(40)
 
        AFLabel:ClearAllPoints()
        AFLabel:SetPoint("TOP", AFP.ArtifactPower, 0, 0, "CENTER")
        AFPoints:ClearAllPoints()
        AFPoints:SetPoint("BOTTOM", AFLabel, 0, -15, "CENTER")
 
        --New Frame for assigned Artifactpower
        AFP.AssignedArtifactPower = CreateFrame("Frame", "AssignedArtifactPower", AFP)
        AFP.AssignedArtifactPower:SetPoint("TOP", 75, -120, "CENTER")
        AFP.AssignedArtifactPower:SetWidth(150)
        AFP.AssignedArtifactPower:SetHeight(40)
 
        tAPL = AFP.AssignedArtifactPower:CreateFontString("ArtifactPowerLabel", "ARTWORK")
        tAPL:SetFont(AFLabel:GetFont())
        tAPL:SetPoint("TOP", AFP.AssignedArtifactPower, 0, 0, "CENTER")
        tAPL:SetTextColor(AFLabel:GetTextColor())
        tAPL:SetText("Assigned")
 
        tAP = AFP.AssignedArtifactPower:CreateFontString("ArtifactPowerPoints", "ARTWORK")
        Mixin(tAP, AnimatedNumericFontStringMixin)
        tAP:SetFont(AFPoints:GetFont())
        tAP:SetPoint("BOTTOM", tAPL, 0, -15, "CENTER")
        tAP:SetTextColor(AFPoints:GetTextColor())
        self:calc_ap()
        tAP:SetAnimatedValue(0) -- for extra prettiness :p
        tAP:SetAnimatedValue(AP)
 
        -- TBH, it's not the best of setting OnUpdate script, but style is style :p
        -- Read [url]https://www.wowinterface.com/forums/showpost.php?p=257783&postcount=2[/url]
        -- (2) Defining a function inside another function
        AFP.AssignedArtifactPower:SetScript("OnUpdate", function(self, elapsed)
            tAP:UpdateAnimatedValue(elapsed)
        end)
 
        AFP.AssignedArtifactPower:SetScript("OnEnter", function(self, motion)
            GameTooltip:SetOwner(self, "ANCHOR_CURSOR")
            GameTooltip:ClearLines()
            GameTooltip:SetText("Total assigned artifact power")
            GameTooltip:AddLine("Resetting your weapons artifact power will reset this counter!", 1, 1, 1)
            GameTooltip:Show()
        end)
 
        AFP.AssignedArtifactPower:SetScript("OnLeave", function(self, motion)
            GameTooltip:Hide()
        end)
 
    else
        self:calc_ap()
        tAP:SetAnimatedValue(AP)
    end
end
 
function AssignedArtifactPower:Refresh()
    if not ArtifactFrame or not ArtifactFrame.PerksTab then return end
    self:Overlay()
end

As I noted in code comments, setting "OnUpdate" the way I did isn't optimal, like at all... Read this post for more info.

I also hope that tAPL, and many other variables are locals...