[ruby-core:107247] [Ruby master Feature#18273] Class#subclasses

Issue #18273 has been updated by byroot (Jean Boussier).


> In my view, this method does not play well with object lifetime, semantically.

This is perfectly fine, Ruby isn't concerned about wether a `Class` is still present in the constant table or not (or if it ever was). That's also how the old Active Support implementation based on `ObjectSpace.each_object` behaved.

That's why [`DescendantTracker` keeps a `WeakMap` of the unregistered classes to give the illusion in reload mode](https://github.com/rails/rails/blob/9062d705ad9055c0f3ec73505150e0e94fff5925/activesupport/lib/active_support/descendants_tracker.rb#L26).

> If GC kicks in between lines 3 and 4, you may get different results. In my opinion, this non-determinism is not good for an API like this one.

That too was considered and is fine.

----------------------------------------
Feature #18273: Class#subclasses
https://bugs.ruby-lang.org/issues/18273#change-96108

* Author: byroot (Jean Boussier)
* Status: Closed
* Priority: Normal
----------------------------------------
Ref: https://github.com/rails/rails/pull/43481

Something we forgot to mention in [Feature #14394], is either a parameter or another method to only get direct descendants.

Active Support has been offering `Class.subclasses` as:

```ruby
  def subclasses
    descendants.select { |descendant| descendant.superclass == self }
  end
```

It seems a bit silly to grab all descendants and then restrict the list when `Class#descendants` had to do some recursion to get them all in the first place.

### Proposal

We could either implement `Class#subclasses` directly, or accept a parameter in `Class#descendants`, e.g. `descendants(immediate = false)`.

cc @eregon



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