Auto Launch Application with Hammerspoon

There is a consensus that macOS has a truly sucky brightness control for extended displays, so an additional app for controlling brightness is always needed.

But due to the notch at the top of the Mac, I am trying my best to reduce the number of apps on the top bar. Therefore, I am looking for a way to auto-launch the controlling app when I connect to the extended display and close it when I use my MacBook independently.

First, I tried to create an automation flow in Shortcuts, but there is no relative trigger that can check the status of the display connection. So I run a shell script, system_profiler SPDisplaysDataType | grep -c "Resolution", in a loop to continuously monitor the connecting status. I set up an if-else condition that opens the application when the output of the command is greater than one and close it when the output equals one.

However, this method has some limitations, such as delays that depend on the timing frequency and the system resource consumption due to the frequent execution. It is not elegant.

So I find another way: to use the tool Hammerspoon, which is a powerful automation tool for macOS. It allows you to use Lua code that interacts with macOS APIs for applications, windows, mouse pointers, filesystem objects, audio devices, batteries, screens, low-level keyboard/mouse events, clipboards, location services, wifi, and more.

The way to use it is editing the file ~/.hammerspoon/init.lua. And below is my sample code for my problem.

function displayWatcher()
    local screens = hs.screen.allScreens()
    
    if #screens > 1 then
        -- An external monitor is connected
        hs.application.launchOrFocus("AppName")
    else
        -- Only the built-in display
        local app = hs.application.find("AppName")
        if app then
            app:kill()
        end
    end
end

-- Monitor display changes
screenWatcher = hs.screen.watcher.new(displayWatcher)
screenWatcher:start()

• Tips: Remember to reload the configuration at the application’s drop-down menu after you change the config file.

Now you can set up many automated actions like this in an ELEGANT way.