MouseOverCast.lua

local feature = ns.Register({
  identifier = "MouseOverCast",
  description = "Enables /mcast command that uses mouseover to detect the unit.",
  category = "utility",
  config = {}
})

local frame = CreateFrame("Frame")

frame:RegisterEvent("ADDON_LOADED")

local function MouseoverUnit()
  local unit = "mouseover"

  if not UnitExists(unit) then
    local frame = GetMouseFocus()
    local fparent = frame:GetParent()
    
    if frame.label and frame.id then unit = frame.label .. frame.id
    elseif frame.unit then unit = frame.unit
    elseif fparent and fparent.unit then unit = fparent.unit
    elseif UnitExists("target") then unit = "target"
    elseif GetCVar("autoSelfCast") == "1" then unit = "player"
    else unit = nil end
  end

  return unit
end 

frame:SetScript("OnEvent", function()
  if not ns.IsEnabled(feature.identifier) then return end

  do
    SLASH_STCAST1 = "/mcast"
    SlashCmdList["STCAST"] = function(msg, editbox)
      local spell = msg or nil
      local unit = MouseoverUnit()
      local restoreHostileTarget = false

      -- Target hostile. Will need to revert back to it.
      if UnitCanAttack("player", "target") then
        restoreHostileTarget = true
      end

      TargetUnit(unit)
      SpellTargetUnit(unit)
      CastSpellByName(spell)

      -- Restoring back to the hostile target.
      if restoreHostileTarget then
        TargetLastEnemy()
      end
    end
  end
end)