statec

introduction

Pronounced "static", statec is an Objective-C/Foundation tool to compile Obj-C state machine classes from a little language that describes them.

The goal was to have a simple, clear, language to express a state machine that compiles into an easy to use class to operate them. There are other, similar, tools available for example SMC or Ragel which are more comprehensive, support a wide range of languages, and so forth. They are good tools but they did not immediately fit my needs and thus began a monumental exercise in Yak shaving.

Two days later and I have a "little language" for expressing state machines that is parsed - using a parser built with Tom Davie's CoreParse framework (itself quite young but promising) - into an intermediate representation of a machine and then converted directly into Objective-C source using an Objective-C code generator framework built as part of the framework.

Similar to the approach taken by Rentzsch's mogenerator statec generates two classes for each machine, a completely generated implementation class and a user-managed class that subclasses it. The implementation provides methods to respond to state changes that the user-managed class can override to provide context specific processing.

language

The statec language is a little language for describing state machines. It has a few keywords and a simple structure, here's an example:

@machine "TrafficLight" {
@initial "off"
@state "off" {
@event "on" => "green"
}
@state "green" {
@enter
@exit
@event "amber" => "amber"
@event "off" => "off"
}
@state "amber" {
@enter
@exit
@event "green" => "green"
@event "red" => "red"
}
@state "red" {
@enter
@exit
@event "amber" => "amber"
}
}

@machine

names the machine and contains its states. The name is used as the basis of the classes generated, in our example they would be _TrafficLightMachine (the implementation class) and TrafficLightMachine (the user class).

@initial

defines the initial state of the machine (an enter event is not generated for the machine being started in this state).

@state

defines a named state

@enter

specifies that a callback (e.g. enterGreenState) should be generated and invoked when the machine enters this state.

@exit

specifies than a callback (e.g. exitAmberState) should be generated and invoked before the machine leaves this state.

@event

defines a named event and the state the event transitions the machine into. The event name is used to name the event method in the machine (e.g. amberEvent) to cause the transition.

The current behaviour is to raise an exception when an event is invoked that is not legal in the current state. In the future this might - optionally - return a condition & NSError instead.

running

The statec command line tool requires the CoreParse.framework be installed in /Library/Frameworks.

The statec command line tool has three arguments:

• -i <machine file>
  • e.g. trafficlight.smd
• -d <target folder>
  • e.g. ~/Projects/TrafficLightSim/
• -g
  • Generate a GraphViz digraph of the machine and, if the dot command is available convert it into a PNG image.

The generated files should be added to your Xcode project. The _<Name>Machine.m file should not be edited as this will be regenerated every time the statec command is generated. The <Name>Machine.m is intended for the user to edit and will not be regenerated if it exists.

machine

The generated machine does not employ a switch statement or lookup table but, rather, stateless classes that represent the states in the machine and that have methods that represent the available transitions.

In our example a class TrafficLightState is generated along with a subclass for each state, e.g. TrafficLightOnState. The TrafficLightMachine has a state variable that points at an instance representing the current state. When an event method e.g. greenEvent is called on the machine it is passed to the current state class that either transitions the calling machine into a new state (optionally invoking exit & enter callbacks), or raises an exception if the transition is not legal from that state.