Hammerspoon script for ensuring Sidecar is active when iPad is plugged in (macOS 10.15; see comments for versions up to macOS 13)

init.lua

hs.loadSpoon('SpoonInstall')
spoon.SpoonInstall.use_syncinstall = true
Install = spoon.SpoonInstall

log = hs.logger.new('init', 5)

-- function debugUI(msg, table)
--   log:d(msg)
--   log:d(hs.inspect(table))
-- end

function connectToSidecar(msg, results, count)
  local item

  sidecarItemFound = (count > 0)
  
  if not sidecarItemFound then
    return log:d("Can't get Sidecar connection menu item:", msg)
  end
  item = results[1]
  if item.AXMenuItemMarkChar == nil then
    -- log:d("Connecting to Sidecar...")
    item:doAXPress()
  else
    -- log:d("Closing menu - already connected to Sidecar...")
    item.AXParent:doAXCancel()
  end
end

function stopConnectToSidecarItemSearchTimer()
  if connectToSidecarItemSearchTimer then
    connectToSidecarItemSearchTimer:stop()
    connectToSidecarItemSearchTimer = nil
  end
end

function displaysMenu(msg, results, count)
  local menu, connectToSneezerItemSearch
  
  if count == 0 then
    return log:d("Can't get displays menu:", msg)
  end
  menu = results[1]
  menu:doAXPress()

  connectToSneezerItemSearch = hs.axuielement.searchCriteriaFunction({
      {attribute = 'AXRole',       value = 'AXMenuItem'},
      {attribute = 'AXIdentifier', value = '_deviceSelected:'},
      {attribute = 'AXTitle',      value = 'Sneezer'}})

  -- menu:elementSearch(debugUI, connectToSneezer,
  --                    {objectOnly = false, asTree = true, depth = 2})

  -- iPad may not appear immediately in the Displays menu
  -- wait up to 3 seconds for it to appear
  stopConnectToSidecarItemSearchTimer()
  sidecarItemFound = false
  connectToSidecarItemSearchTimer = hs.timer.doUntil(
    function()
      return sidecarItemFound
    end,
    function()
      menu:elementSearch(connectToSidecar, connectToSneezerItemSearch,
                         {count = 1, depth = 2, noCallback = true})
    end,
    0.5)
  hs.timer.doAfter(3, stopConnectToSidecarItemSearchTimer)
end

function connectSidecar()
  local suisApp, suisAX, displayMenuSearch
  
  suisApp = hs.application.find('com.apple.systemuiserver')
  suisAX = hs.axuielement.applicationElement(suisApp)

  displayMenuSearch = hs.axuielement.searchCriteriaFunction({
    {attribute = 'AXRole',        value = 'AXMenuBarItem'},
    {attribute = 'AXSubrole',     value = 'AXMenuExtra'},
    {attribute = 'AXDescription', value = '^Displays', pattern = true}
  })
  
  -- suisAX:elementSearch(debugUI, displayMenuSearch,
  --                      {objectOnly = false, asTree = true, depth = 2})

  suisAX:elementSearch(displaysMenu, displayMenuSearch,
                       {count = 1, depth = 2})
end

-- what we actually get when the iPad connects:
-- connect, disconnect, connect
-- so, don't trigger twice within half a second
connectSidecarTimer = hs.timer.delayed.new(0.5, function()
  connectSidecar()
end)

function iPadConnected(connected)
  -- log:d("iPad connected?", connected)
  if connected then
    connectSidecarTimer:start()
  else
    connectSidecarTimer:stop()
  end
end

Install:andUse(
  "USBDeviceActions",
  {
    config = {
      devices = {
        iPad = { fn = iPadConnected }
      }
    },
    start = true
  }
)