Oh! Perhaps we should have some utility for this; we define lists of these now, but they are strongly typed (which should still be fine, as they're just a string under the hood);

Considering some options for that;

We could create slice for all event.Type and event.Action

var Types = []Type{
	ContainerEventType,
	ImageEventType,
	// ...
}

var Actions = []Action{
	ActionCreate,
	ActionStart,
	// ...
}

And/or create a struct with a slice of actions per type, e.g. something like;

var PerType = map[Type][]Action{
	ContainerEventType: {
		ActionCreate, ActionAttach, ActionDelete,
	},
	ImageEventType: {
		ActionCreate, ActionDelete,
	},
}

Or a utility function for that can provide this;

func ForType(t Type) []Action {
	if string(t) == "all" {
		return AllActions
	}

	return PerType[t]
}

This would be added in the moby codebase?

Yes, correct; we vendor those types from the Moby repo.

Converting to a string (or slice of strings) would then probably be left to the consumer (so that's something we can do in this repository).

But before those changes are made there, we could implement them locally here as non-exported / private utilities (and see what a good "shape" would be), then make the change in moby, and once those are vendored here, we can swap the local ones; similar to how #5480 swapped a local construct to using the new module to provide those

Great. I'll make a suggestion. This might take some time, though.
Thanks for your support.

done, PTAL.

@thaJeztah I added completions for the remaining filters.
At that stage, the completeFilters was way to long and ugly, so I decided to introduce some helper functions.
These helper functions simplify error handling very much, as they just return empty arrays in case of API errors.
PTAL, IMHO the PR should be complete now.

Nice!! I was wondering the other day if something like this would work. I wanted to experiment if completion would be possible for the "advanced" CSV flags, but didn't give it a try yet.

We could probably optimize this to prevent matching prefix multiple times;

// completeEventFilters provides completion for the filters that can be used with `--filter`.
func completeEventFilters(dockerCLI completion.APIClientProvider) completion.ValidArgsFn {
	return func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
		key, _, ok := strings.Cut(toComplete, "=")
		if !ok {
			return postfixWith("=", eventFilters), cobra.ShellCompDirectiveNoSpace
		}
		switch key {
		case "container":
			return prefixWith("container=", containerNames(dockerCLI, cmd, args, toComplete)), cobra.ShellCompDirectiveNoFileComp
		case "daemon":
			return prefixWith("daemon=", daemonNames(dockerCLI, cmd)), cobra.ShellCompDirectiveNoFileComp
		case "event":
			return prefixWith("event=", validEventNames()), cobra.ShellCompDirectiveNoFileComp
		case "image":
			return prefixWith("image=", imageNames(dockerCLI, cmd)), cobra.ShellCompDirectiveNoFileComp
		case "label":
			return nil, cobra.ShellCompDirectiveNoFileComp
		case "network":
			return prefixWith("network=", networkNames(dockerCLI, cmd)), cobra.ShellCompDirectiveNoFileComp
		case "node":
			return prefixWith("node=", nodeNames(dockerCLI, cmd)), cobra.ShellCompDirectiveNoFileComp
		case "scope":
			return prefixWith("scope=", []string{"local", "swarm"}), cobra.ShellCompDirectiveNoFileComp
		case "type":
			return prefixWith("type=", eventTypeNames()), cobra.ShellCompDirectiveNoFileComp
		case "volume":
			return prefixWith("volume=", volumeNames(dockerCLI, cmd)), cobra.ShellCompDirectiveNoFileComp
		default:
			return postfixWith("=", eventFilters), cobra.ShellCompDirectiveNoSpace
		}
	}
}

I was even contemplating if we should re-purpose the constants where suitable, but perhaps confusing, because not all filters refer to a type; i.e., was considering;

case string(events.ContainerEventType):

But not sure if that's a good thing to do

That's much better, thank you. I did not go for the constants because I think this produces asymmetry that makes the code harder to comprehend.

Yes, makes sense; I started to replace the strings with the consts, then realised not all of them were event-types 😂