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#
# == Rosetta Example: Singleton
# For details see https://rosettacode.org/wiki/Singleton
#
# === A Singleton Class
package req nx
package req nx::test
nx::Class create Singleton {
#
# We overload the system method "create". In the modified method we
# save the created instance in the instance variable named
# "instance"
#
:variable instance:object
:public object method create {args} {
return [expr {[info exists :instance] ? ${:instance} : [set :instance [next]]}]
}
}
# === Demonstrating the behavior in a shell:
#
# Calling +Singleton new+ multiple times returns always the same object:
? {expr {[Singleton new] eq [Singleton new]}} 1
#
# === A Singleton Meta-class
#
# Alternatively, we can follow a more generic approach and define a
# metaclass which allows one to define several application classes as
# singletons. The metaclass has the most general metaclass +nx::Class+
# as superclass. In contrary to the example obove, the +create+ method
# is not defined as a class method, but it will be inherited to its
# instances (to the application classes).
#
nx::Class create Singleton -superclass nx::Class {
#
# We overload the system method "create". In the modified method we
# save the created instance in the instance variable named
# "instance"
#
:variable instance:object
:public method create {args} {
return [expr {[info exists :instance] ? ${:instance} : [set :instance [next]]}]
}
}
# Create an application class named +Counter+ as a singleton:
? {Singleton create Counter} ::Counter
# Calling +Counter new+ multiple times returns always the same object:
? {expr {[Counter new] eq [Counter new]}} 1
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