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Singleton
cdesc-Singleton.yaml
--- !ruby/object:RI::ClassDescription attributes: [] class_methods: [] comment: - !ruby/struct:SM::Flow::P body: The Singleton module implements the Singleton pattern. - !ruby/struct:SM::Flow::P body: "Usage:" - !ruby/struct:SM::Flow::VERB body: " class Klass\n include Singleton\n # ...\n end\n" - !ruby/object:SM::Flow::LIST contents: - !ruby/struct:SM::Flow::LI label: "*" body: this ensures that only one instance of Klass lets call it ``the instance'' can be created. - !ruby/struct:SM::Flow::P body: "a,b = Klass.instance, Klass.instance a == b # => true a.new # NoMethodError - new is private ..." - !ruby/struct:SM::Flow::LI label: "*" body: ``The instance'' is created at instantiation time, in other words the first call of Klass.instance(), thus - !ruby/struct:SM::Flow::VERB body: " class OtherKlass\n include Singleton\n # ...\n end\n ObjectSpace.each_object(OtherKlass){} # => 0.\n" - !ruby/struct:SM::Flow::LI label: "*" body: This behavior is preserved under inheritance and cloning. type: :BULLET - !ruby/struct:SM::Flow::P body: This is achieved by marking - !ruby/object:SM::Flow::LIST contents: - !ruby/struct:SM::Flow::LI label: "*" body: Klass.new and Klass.allocate - as private type: :BULLET - !ruby/struct:SM::Flow::P body: Providing (or modifying) the class methods - !ruby/object:SM::Flow::LIST contents: - !ruby/struct:SM::Flow::LI label: "*" body: Klass.inherited(sub_klass) and Klass.clone() - to ensure that the Singleton pattern is properly inherited and cloned. - !ruby/struct:SM::Flow::LI label: "*" body: "Klass.instance() - returning ``the instance''. After a successful self modifying (normally the first) call the method body is a simple:" - !ruby/struct:SM::Flow::VERB body: " def Klass.instance()\n return @<em>instance</em>\n end\n" - !ruby/struct:SM::Flow::LI label: "*" body: Klass._load(str) - calling Klass.instance() - !ruby/struct:SM::Flow::LI label: "*" body: Klass._instantiate?() - returning ``the instance'' or nil. This hook method puts a second (or nth) thread calling Klass.instance() on a waiting loop. The return value signifies the successful completion or premature termination of the first, or more generally, current "instantiation thread". type: :BULLET - !ruby/struct:SM::Flow::P body: The instance method of Singleton are - !ruby/object:SM::Flow::LIST contents: - !ruby/struct:SM::Flow::LI label: "*" body: clone and dup - raising TypeErrors to prevent cloning or duping - !ruby/struct:SM::Flow::LI label: "*" body: _dump(depth) - returning the empty string. Marshalling strips by default all state information, e.g. instance variables and taint state, from ``the instance''. Providing custom _load(str) and _dump(depth) hooks allows the (partially) resurrections of a previous state of ``the instance''. type: :BULLET constants: [] full_name: Singleton includes: [] instance_methods: - !ruby/object:RI::MethodSummary name: _dump - !ruby/object:RI::MethodSummary name: clone - !ruby/object:RI::MethodSummary name: dup name: Singleton superclass:
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