require "delegate"

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# WeakRef is a class to represent a reference to an object that is not seen by

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# the tracing phase of the garbage collector. This allows the referenced

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# object to be garbage collected as if nothing is referring to it. Because

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# WeakRef delegates method calls to the referenced object, it may be used in

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# place of that object, i.e. it is of the same duck type.

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#

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# Usage:

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#

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# foo = Object.new

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# foo = Object.new

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# p foo.to_s # original's class

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# foo = WeakRef.new(foo)

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# p foo.to_s # should be same class

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# ObjectSpace.garbage_collect

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# p foo.to_s # should raise exception (recycled)

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class WeakRef<Delegator

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# RefError is raised if an object cannot be referenced by a WeakRef.

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class RefError<StandardError

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end

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@@id_map = {} # obj -> [ref,...]

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@@id_rev_map = {} # ref -> obj

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@@final = lambda{|id|

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__old_status = Thread.critical

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Thread.critical = true

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begin

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rids = @@id_map[id]

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if rids

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for rid in rids

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@@id_rev_map.delete(rid)

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end

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@@id_map.delete(id)

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end

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rid = @@id_rev_map[id]

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if rid

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@@id_rev_map.delete(id)

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@@id_map[rid].delete(id)

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@@id_map.delete(rid) if @@id_map[rid].empty?

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end

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ensure

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Thread.critical = __old_status

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end

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}

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# Create a new WeakRef from +orig+.

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def initialize(orig)

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super

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__setobj__(orig)

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end

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# Return the object this WeakRef references. Raises RefError if the object

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# has been garbage collected. The object returned is the object to which

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# method calls are delegated (see Delegator).

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def __getobj__

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unless @@id_rev_map[self.__id__] == @__id

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raise RefError, "Illegal Reference - probably recycled", caller(2)

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end

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begin

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ObjectSpace._id2ref(@__id)

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rescue RangeError

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raise RefError, "Illegal Reference - probably recycled", caller(2)

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end

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end

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def __setobj__(obj)

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@__id = obj.__id__

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__old_status = Thread.critical

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begin

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Thread.critical = true

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unless @@id_rev_map.key?(self)

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ObjectSpace.define_finalizer obj, @@final

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ObjectSpace.define_finalizer self, @@final

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end

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@@id_map[@__id] = [] unless @@id_map[@__id]

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ensure

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Thread.critical = __old_status

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end

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@@id_map[@__id].push self.__id__

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@@id_rev_map[self.__id__] = @__id

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end

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# Returns true if the referenced object still exists, and false if it has

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# been garbage collected.

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def weakref_alive?

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@@id_rev_map[self.__id__] == @__id

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end

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end

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if __FILE__ == $0

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require 'thread'

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foo = Object.new

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p foo.to_s # original's class

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foo = WeakRef.new(foo)

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p foo.to_s # should be same class

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ObjectSpace.garbage_collect

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p foo.to_s # should raise exception (recycled)

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end

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