Folding range provider API

[rwols]

Problem description

There's this capability in LSP where a server can be a foldingRangeProvider. An editor can ask the server to provide folding ranges.

It is at the moment not possible to bring those fold ranges to the gutter area.

Preferred solution

A new ST callback, much like on_query_completions, where a plugin must return a promise object to provide new fold regions. All fold regions from all plugins are merged in some way or another.

def FooListener(sublime_plugin.ViewEventListener):

    def on_fold_regions(self) -> sublime.FoldRegions:
        promise = sublime.FoldRegions()
        sublime.set_timeout_async(lambda: self.compute_fold_regions(promise))
        return promise

    def compute_fold_regions(self, promise: sublime.FoldRegions) -> None:
        promise.set_result([sublime.Region(0, 1), sublime.Region(10, 12)])

This way, ST can coordinate when to ask for new fold regions (perhaps after saving the view). And ST can also merge results.

Alternatives

None.

Additional Information (optional)

This is one possible solution for #101

[rchl]

This way, ST can coordinate when to ask for new fold regions (perhaps after saving the view).

But the regions would easily get out of sync when editing the files?

[rwols]

If a region is replaced entirely inside of a fold region, then the fold region can just shrink/expand, no?
If a region is replaced entirely outside of a fold region, then the fold region can move.
If the replacement region intersects with the fold region endpoints, then yeah the fold region becomes invalid/stale.

In any case, requesting after a view is saved was just an idea. It's not a hard requirement for me.