Previously I assumed it was useless, however I was wrong.
The method is called by the reloader to give the illusion that
the GC is precise. Meaning a class that will be unloaded is
immediately made invisible without waiting for it to be garbage collected.
This is easy to do up to Ruby 3.0 because `DescendantTracker` keeps
a map of all tracked classes.
However on 3.1 we need to use the inverse strategy, we keep a WeakMap
of all the classes we cleared, and we filter the return value of `descendants`
and `subclasses`.
Since `clear` is private API and is only used when reloading is enabled,
to reduce the performance impact in production mode, we entirely remove
this behavior when `config.cache_classes` is enabled.
The descendants tracker is a generic tracker that you can use anywhere.
In particular, outside Rails applications.
Should provide API to clear only a subset of classes, but in my view
should know nothing about autoloading. That is a concern of client code.
The former should be symmetric with ancestors and include all children. However, it should not include self since ancestors + descendants should not have duplicated. The latter is symmetric to superclass in the sense it only includes direct children.
By adopting a convention, we expect to have less conflict with other frameworks, as Datamapper. For this moment, to ensure ActiveModel::Validations can be used with Datamapper, we should always call ActiveSupport::DescendantsTracker.descendants(self) internally instead of self.descendants avoiding conflicts.
Jean Boussier