Understanding Statemachines, Part 3: Conditional Logic

If you’re doing any significant amount of work with statmachines, you will most certainly encounter some conditional logic in your Statemachines. Take our vending machine.

When ever a coin is inserted, the invoked event will depend on whether the total amount of money inserted is sufficient to buy something.

If enough money has been tendered, the display should suggest that the customer make a selection. If insufficient money has been inserted, the customer should be prompted to insert more.

Conditional logic can be accomplished by using entry actions. See the diagram below.

Starting in the Accept Money state, when a coin is inserted, the coin event is fired and the Statemachine transitions into the Coin Inserted state. This is where it gets fun.

Upon entering of the Coin Inserted state its entry event is invoked: count_amount_tendered. This method will count the money and invoke the not_paid_yet or paid event accordingly. This will cause the Statemachine to transition into the appropriate state.

The Coin Inserted state is unique. You wouldn’t expect to find the Statemachine in the Coin Inserted state for any reason except to make this decision. Once the decision is made, the state changes. States like this are called Decision States.

Code

require 'rubygems'
require 'statemachine'

class VendingMachineContext
		attr_accessor :statemachine

		def initialize
				@amount_tendered = 0
		end

		def add_coin
				@amount_tendered = @amount_tendered + 25
		end

		def count_amount_tendered
				if @amount_tendered >= 100
						@statemachine.paid
				else
						@statemachine.not_paid_yet
				end
		end

		def prompt_money
				puts "$.#{@amount_tendered}: more money please"
		end

		def prompt_selection
				puts "please make a selection"
		end
end

vending_machine = Statemachine.build do
		trans :accept_money, :coin, :coin_inserted, :add_coin
		state :coin_inserted do
				event :not_paid_yet, :accept_money, :prompt_money
				event :paid, :await_selection, :prompt_selection
				on_entry :count_amount_tendered
		end
		context VendingMachineContext.new
end
vending_machine.context.statemachine = vending_machine

vending_machine.coin
vending_machine.coin
vending_machine.coin
vending_machine.coin

Output:

$1.25: more money please
$2.50: more money please
$3.75: more money please
please make a selection

Next lesson: Superstates