robust-iterator; detaching observers automaticly ?

[Simon Strandgaard]

I am working on a Robust-Iterator (tm).. where the iterators
gets registerd at the data-structure they traverse.
When doing insert/remove in the data-structure, then the iterators
still points to the same element.
Clone such an iterator will add an extra observer to the list.

What I cannot figure out is how to detach observers when an iterator is no
longer used.. kind of similar to the Ruby-destructor problem.

Question#1: should I introduce a #close method in all my object ?

Question#2: how do you unregister objects (pseudo automaticly) ?

--
Simon Strandgaard



server> cat robust.rb
require 'iterator'
require 'observer'

# purpose:
# a array with robust iterators
#
# functions:
# * iterators keeps pointing to the same element,
# when doing insert/remove (primary feature).
# * normal array behavier.
#
# issues:
# * you must #detach allocated iterators.
class RobustArray
# purpose:
# a robust iterator, which keeps its position
# even though you insert/remove elements.
#
# functions:
# * in case of changes in the array the iterator
# gets notify from the parent about the change.
# * #clone attaches the new instance to the parent.
#
# todo:
# * policies for how the iterator should behave
# detach_on_remove, prev_on_remove, next_on_remove
class Iterator < Iterator::Collection
def initialize(data)
super(data)
@data.add_observer(self)
end
def clone
instance = super
@data.add_observer(instance)
instance
end
def detach
@data.delete_observer(self)
@data = nil
@position = nil
end
def update(where, delta)
if delta > 0
if @position >= where
@position += delta
end
elsif (where+delta..where).member?(@position)
detach # remove ourselves from the iterator list
elsif @position >= where
@position += delta - 1
end
end
end

include Observable
def initialize(ary)
@data = ary
end
def to_a
@data
end
def size
@data.size
end
def [](index)
@data[index]
end
def []=(index, value)
@data[index] = value
end
def RobustArray.build_from_array(ary)
RobustArray.new(ary)
end
def create_iterator
Iterator.new(self)
end
def remove(range)
res = @data.slice!(range)
a = range.first
b = range.last
changed
notify_observers(b, -(b-a))
RobustArray.build_from_array(res)
end
def insert(where, data)
ary = data.to_a
@data[where, 0] = ary
changed
notify_observers(where, ary.size)
end
end
server>

 

[Nathan Weston]

You could use a block, a la File.open {}:
robust_array.create_iterator { |i|
do some stuff with the iterator...
}

Where the iterator is created and passed to the block, and then
detached after the block is executed.

or even

robust_array.each { |x| ... }
where the each is implemented with a robust iterator.

Neither of these solves the clone problem, but it at least makes
things easy in the common case.

Nathan

I am working on a Robust-Iterator (tm).. where the iterators
gets registerd at the data-structure they traverse.
When doing insert/remove in the data-structure, then the iterators
still points to the same element.
Clone such an iterator will add an extra observer to the list.

What I cannot figure out is how to detach observers when an iterator is no
longer used.. kind of similar to the Ruby-destructor problem.

Question#1: should I introduce a #close method in all my object ?

Question#2: how do you unregister objects (pseudo automaticly) ?

--
Simon Strandgaard



server> cat robust.rb
require 'iterator'
require 'observer'

# purpose:
# a array with robust iterators
#
# functions:
# * iterators keeps pointing to the same element,
# when doing insert/remove (primary feature).
# * normal array behavier.
#
# issues:
# * you must #detach allocated iterators.
class RobustArray
# purpose:
# a robust iterator, which keeps its position
# even though you insert/remove elements.
#
# functions:
# * in case of changes in the array the iterator
# gets notify from the parent about the change.
# * #clone attaches the new instance to the parent.
#
# todo:
# * policies for how the iterator should behave
# detach_on_remove, prev_on_remove, next_on_remove
class Iterator < Iterator::Collection
def initialize(data)
super(data)
@data.add_observer(self)
end
def clone
instance = super
@data.add_observer(instance)
instance
end
def detach
@data.delete_observer(self)
@data = nil
@position = nil
end
def update(where, delta)
if delta > 0
if @position >= where
@position += delta
end
elsif (where+delta..where).member?(@position)
detach # remove ourselves from the iterator list
elsif @position >= where
@position += delta - 1
end
end
end

include Observable
def initialize(ary)
@data = ary
end
def to_a
@data
end
def size
@data.size
end
def [](index)
@data[index]
end
def []=(index, value)
@data[index] = value
end
def RobustArray.build_from_array(ary)
RobustArray.new(ary)
end
def create_iterator
Iterator.new(self)
end
def remove(range)
res = @data.slice!(range)
a = range.first
b = range.last
changed
notify_observers(b, -(b-a))
RobustArray.build_from_array(res)
end
def insert(where, data)
ary = data.to_a
@data[where, 0] = ary
changed
notify_observers(where, ary.size)
end
end
server>

 

[Simon Strandgaard]

You could use a block, a la File.open {}:
robust_array.create_iterator { |i|
do some stuff with the iterator...
}

Where the iterator is created and passed to the block, and then
detached after the block is executed.

or even

robust_array.each { |x| ... }
where the each is implemented with a robust iterator.

My problem is that I want to build other kinds of iterators
using robust-iterator. For instance I have a Iterator::Range which
is created with a first and a last iterator, which may be a
robust-iterator. The #each operation is not used there at all,
the Iterator::Range exercises #is_done? and #next.

Neither of these solves the clone problem, but it at least makes
things easy in the common case.

True, I am C++brainwashed, well knowing that destructors cannot help me
here. But I hesitate too much implementing #close and &blocks in the whole
class hierarchy.

I was hoping that others had knowledge about #clone and how to automaticly
unregister instances when they are no longer in usage. If there are
any rubists which has such knowledge, then say something