FR: generic methods for setup, suspend, resume, and teardown

[dcolascione]

I'm thinking of using a transient plus a custom window configuration for a modal interaction flow and I'm not seeing the right spot to inject contextual setup and teardown in the transient lifecycle. There's transient-exit-hook, but this hook runs not only on exit, but also on suspend. I'd love to have a few cl-defgeneric methods on prefix objects for subclasses to add behaviors for different lifecycle transitions.

[tarsius]

I am currently focused on other packages and won't get into this now. Please consider explaining what you want to do in more detail now anyway. Currently this is too vague, and I will be more inclined to prioritize this over other transient related ideas, once I turn my focus back here, if it is more actionable.

One thing you might be interested in is the :display-action suffix slot/argument.

[tarsius]

This might or might not be an XY problem. I understand that you want some hook or generic function as a means to be notified that a certain state has been reached. You have now added more information about which states you want to be informed about. What I don't know is what you want to do in response to those events. If you describe that, I might be able to point you to existing solutions that allow you to do those things, despite the "notification mechanism" not working the way you expected.

[dcolascione]

I have a long lived transient for managing merge conflict resolution state. It's effectively an interactive iterator over conflict regions. The transient controls two ediff windows (the ediff native control panel window being hidden). The window configuration for the two buffers has to be cleaned up (previous state restored) when the transient exits. There's no obvious public API for doing so. I can just advise various internal functions now, but I'd rather have a reasonable API for this use case, especially considering that stock Emacs set-transient-keymap already has such a teardown notification callback.

[tarsius]

Such a hook was requested before. I implemented it and keep it on my private idea branch. If I remember correctly, I did not install it on main because the user who originally requested it, did not actually need it. I also think there's usually a better way.

That might look something like this:

(defvar pre-ediff-winconf nil)

(transient-define-prefix my-ediff-menu ()
  :transient t
  [;; ...
   ("q  " "Quit menu and ediff" my-ediff-quit)
   ("C-g" "Quit menu only" transient-quit-one)]
  (interactive)
  (unless (ediff-active-p)
    (setq pre-ediff-winconf (current-window-configuration))
    (setup-ediff-without-control-window))
  (transient-setup 'my-ediff-menu))

(transient-define-suffix my-ediff-quit ()
  :transient #'transient--do-quit-all
  (interactive)
  (quit-ediff-as-usual)
  (set-window-configuration pre-ediff-winconf))

[dcolascione]

Thanks. That approach addresses some of the problem, but not all of it. For example, transients also get torn down on frame switch --- how do we respond to that? What if a user installs a custom terminal suffix in the transient? It's hard to maintain the invariant that the temporary windows get torn down and the old window configuration restored upon transient exit if the only means for doing so is making sure each command in the transient manually invokes the cleanup code. A single hook would be more convenient and future-proof.

[tarsius]

For example, transients also get torn down on frame switch --- how do we respond to that?

The transient behavior for handle-switch-frame needs some work in general, but it is already possible to prevent that from quitting individual menus using :transient-switch-frame t.

What if a user installs a custom terminal suffix in the transient? It's hard to maintain the invariant that the temporary windows get torn down and the old window configuration restored upon transient exit if the only means for doing so is making sure each command in the transient manually invokes the cleanup code.

Yes, but IMO that's actually a good thing in this case. I tried to hint at that in the above code, by making the C-g binding explicit and showing how my-ediff-menu can be taught to support starting the ediff session, as well as only bringing up the menu again to control an already running ediff session.

You prefer different behavior, which I would qualify as destructive, but that's your choice.

Anyway, I'll probably add the hook.

The new (and still experimental) :advice or :advice* slots might also be useful. See 6e45366.