Feature #16471
open
Two feature requests for WeakRef: get original object, callback feature
Description
I'd like to request two features for WeakRef. I'll explain what I want, then provide a real world use case.
First, add the ability to pull the original object out of the WeakRef object, something like this:
myhash = {}
myhash = WeakRef.new(myhash)
myhash = myhash.original_object()
Second, add a callback feature for when a WeakRef's object is being purged by GC. It would work something like this:
wr = WeakRef.new(myobject)
wr.on_garbage() do |wr|
puts 'trashing'
end
# time goes by...
# myobject goes out of scope
# outputs "trashing"
Here's the specific use case I would want it for. I'm developing a database system which includes a class called Node. A Node object holds a reference to a database handle and the primary key of a record in that database. It also has methods for getting and setting values in that record. So a simplified version looks something like this:
class Node
attr_reader :dbh
attr_reader :pk
def initialize(dbh, pk)
@dbh = dbh
@pk = pk
end
def set(fieldname, value)
# a bunch of SQL to set the value
end
def get(fieldname)
# a bunch of SQL to get the value
end
end
node = Node.new dbh, 'abc'
node.set 'name', 'Fred'
The database object will have a node method, so you would usually get nodes like this:
node = dbh.node('abc')
Make sense so far? It's a pretty simple concept. It works, but I'd like to make a small improvement. (Whether or not it's actually an improvement is a judgement call... I expect some disagreement on this point. But work with me here.)
I'd like the database object to keep a cache of Node objects. However, the database doesn't keep those node objects alive: they can fall out of scope and get purged by the GC. However, if that cache is never purged of dead objects, it just grows bigger and bigger. I could occasionally just run a routine to work through the cache and purge dead references, but that seems very inefficient. It would be better to just have them purged as they die.
So I could implement it something like this:
class DataBase
attr_reader :cache
def initialize
@cache = {}
end
def node(pk)
if @cache[pk]
# Here's where we need to get at the original object
return @cache[pk].original_object
else
new_node = Node.new(self, pk)
@cache[pk] = WeakRef.new(new_node)
# Here's where we set the callback
@cache[pk].on_garbage do |wr|
wr.db.cache.delete wr.pk
end
return new_node
end
end
end
So when a Node object is garbage collected, it's deleted from the cache. The cache stays clean of dead objects.
I'll be interested to hear what you think of this idea and how difficult it would be to implement it.
Updated by 
Updated by 
Updated by