A small design study to show how to implement state specific behavior in nx via per-object mixins


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# 
# Implementing state specific behavior via per-object mixins
#
package require nx
package require nx::test

#
# Class: StateMachine 
#
# Define a minimalistic state machine that will always be initially in
# a state named "initial". The states are defined as classes for easy
# switching behavior and they are aggregated into the application
# class for locality.
#
nx::Class create StateMachine {
  :public method current_state {} {:info object mixin classes}
  :method transition {newState} {:object mixin [:info class]::$newState}
  :method init {} {:object mixin [:info class]::initial}
}

#
# MetaClass: State
# 
# The meta class provides means for generic state handling (e.g. via
# "info instances") and for better describing the intended semantics
# of these classes.
#
nx::Class create State -superclass nx::Class 

#
# Application Class: conference 
#
nx::Class create conference -superclass StateMachine {
  State create [self]::initial {
    :public method apply_program_committee {} {return ok}
    :public method submit_paper {} {return not-yet}
    :public method publish-cfp {} {:transition cfp-open}
  }
  State create [self]::cfp-open {
    :public method apply_program_committee {} {return too-late}
    :public method submit_paper {} {return ok}
  }
}

conference create tcl2014
? {tcl2014 current_state} "::conference::initial"
? {tcl2014 apply_program_committee} "ok"
? {tcl2014 submit_paper} "not-yet"
tcl2014 publish-cfp

? {tcl2014 current_state} "::conference::cfp-open"
? {tcl2014 apply_program_committee} "too-late"
? {tcl2014 submit_paper} "ok"
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