append metadata to hs.window instance

[mogenson]

Hello, I'd like to append some metadata to an instance of an hs.window so I can retrieve and inspect it later. For example, use the hs.window:newWatcher() method to create a uielement.watcher and store it with the window.

myWindow = hs.window.focusedWindow()
myWindow.myWatcher = myWindow:newWatcher( -- can't index an hs.window value
        function(element, event, watcher)
            print("myWatcher for myWindow called with event " .. event)
        end
    )
myWindow.myWatcher:start({hs.uielement.watcher.windowMoved})

Are there any strategies in Lua to storing Lua variables/tables alongside a userdata type like hs.window? Wold debug. setuservalue() be an option? I can't find any good examples of it's use.

[Rhys-T]

I don't think there's any way to add that info to the userdata itself. You're better off maintaining any information like that in a table. (debug.setuservalue seems to be limited to messing with the userdata's existing fields, and can't add new ones.)

Using the hs.window instance as a table key directly won't work like you expect, though: Each time Hammerspoon gives you a reference to a window, it's a new userdata, even if it's pointing to the same actual window. They compare as equal using the == operator, but they aren't rawequal to each other, and they count as different table keys.¹ What you can do instead is get each window's :id() and use that as an index into your table. Something like this:

-- somewhere global:
myWindowWatchers = {}

-- wherever you're creating each watcher:
myWindow = hs.window.focusedWindow()
myWindowID = myWindow:id()
myWindowWatchers[myWindowID] = myWindow:newWatcher(
        function(element, event, watcher)
            print("myWatcher for myWindow called with event " .. event)
            -- clean up watchers for windows that no longer exist:
            if event == hs.uielement.watcher.elementDestroyed then
                myWindowWatchers[myWindowID] = nil
            end
        end
    )
myWindow.myWatcher:start({hs.uielement.watcher.windowMoved, hs.uielement.watcher.elementDestroyed})
-- make sure that elementDestroyed gets added to this call: ↑

Footnotes

1. See hs.application constructors returning duplicate objects #2720, Hash/identifier for userdata objects for hs.axuielement to allow usage as table keys? #3532, and audio device event not captured #3057 for more instances of that sort of thing in Hammerspoon's other userdata-based types. This is another reason why you don't really want to attach extra fields directly to the userdata - they won't be there on the next userdata for that window! ↩

[mogenson]

Thanks for the response. I ended up using window IDs as keys for some locally managed tables here: https://github.com/mogenson/PaperWM.spoon/blob/e64e0d864d2db4cb6f65a92929bbcdf4a1b07fa4/init.lua#L121