5. Designing the Vending Machine

• Designing a Vending Machine
• User request:
• Build me a new vending machine
• A machine with columns of cans in it. Put in coins. Make a selection. Get a Jolt.
• Simulate to test, reuse the code in the real thing.
• Keys: Flexibility, Reuse
• DON'T GO ANY FURTHER!
• If you're printing these, don't look at the next page yet.
• Coad and Nicola's OOA for the Vending Machine
• 
• (Feel free to critize mightily)
• Doing OOA/OOD/OOP in two pieces
• First, Dispensing Holder and Item
• Second, CashDevice
• A Scenario Script for DispensingHolder and Item
• I'm an item.
• Someone tells me to vend myself.
• I need some help getting enough cash.
• I tell my dispensing holder to dispense.
• I'm a dispensing holder.
• Someone tells me to dispense.
• I look to see if the item that I'm told to dispense is dispensable. If it is, then I dispense it.
• I return a result to the sender - "dispensed" or "not dispensed."
• I'm an item.
• If my dispensing holder returns "dispensed," then I delete myself.
• I need some help making change.
• I return a "dispensed" or "not dispensed" result to the sender.
• Defining Item
• Model subclass: #Item
• instanceVariableNames: 'name price dispensingHolder '
• classVariableNames: ''
• poolDictionaries: ''
• category: 'Vending'
• !Item class methodsFor: 'instance creation'!
• new
• ^super new initialize
• !Item methodsFor: 'initialize-release'!
• initialize
• name := String new.
• price := 0.0
• Item's Accessors
• !Item methodsFor: 'accessing'!
• name
• "Return my name"
• ^name
• name: aName
• "Set the value of name. Notify my dependents
• that I've changed"
• name := aName.
• self changed: #name
• price
• "Return my price "
• ^price
• price: aPrice
• "Set the value of price. Notify my dependents that
• I've changed"
• price := aPrice.
• self changed: #price
• Connecting and Vending in Item
• !Item methodsFor: 'connections'!
• connectToDispensingHolder: aDispensingHolder
• dispensingHolder := aDispensingHolder
• dispensingHolder
• ^dispensingHolder
• !Item methodsFor: 'vending'!
• vend
• ^dispensingHolder dispense: self
• Another way to patch printString
• !Item methodsFor: 'printing'!
• printOn: aStream
• aStream nextPutAll: 'item: '; space; space.
• aStream nextPutAll: name printString; space; space; space.
• aStream nextPutAll: ' price: '; space; space.
• aStream nextPutAll: price printString
• Programming the class DispensingHolder
• Model subclass: #DispensingHolder
• instanceVariableNames: 'items '
• classVariableNames: ''
• poolDictionaries: ''
• category: 'Vending'
• new
• ^super new initialize
• !DispensingHolder methodsFor: 'initialize-release'!
• initialize
• items := OrderedCollection new
• DispensingHolder Connections
• !DispensingHolder methodsFor: 'connections'!
• connectToItem: aItem
• items add: aItem.
• disconnectWithItem: aItem
• items remove: aItem ifAbsent: []
• items
• "Return the current value of items."
• ^items
• Dispensing
• !DispensingHolder methodsFor: 'dispensing'!
• activateDispenser
• ^self
• dispense: itemToDispense
• "Dispense the item.
• Return true if successful, otherwise false."
• self isEmpty ifTrue: [^false].
• (self itemIsDispensable: itemToDispense) ifFalse:
• [^false].
• self printItem: itemToDispense .
• self activateDispenser.
• self disconnectWithItem: itemToDispense.
• ^true
• Testing and Simulating
• !DispensingHolder methodsFor: 'testing'!
• isEmpty
• ^items isEmpty
• itemIsDispensable: itemToDispense
• "Check if the first item and the item to dispense
• are the same object. Be sure to use == rather
• than =."
• ^items first == itemToDispense
• !DispensingHolder methodsFor: 'simulating'!
• printItem: itemToDispense
• Transcript cr.
• Transcript show: 'Releasing ' , itemToDispense printString.
• Example #1
• "EXAMPLE 1"
• "Run the code -- dispense an item (p. 140)"
• | anItem aDispensingHolder |
• aDispensingHolder := DispensingHolder new.
• anItem := Item new.
• anItem name: 'Jolt'; price: 0.75.
• anItem connectToDispensingHolder: aDispensingHolder.
• aDispensingHolder connectToItem: anItem.
• aDispensingHolder dispense: anItem
• Example #2
• "EXAMPLE 2"
• "Run the code -- vend some items (p. 145)"
• | item1 item2 item3 items aDispensingHolder |
• item1 := Item new name: 'Coca-Cola'; price: 0.75.
• item2 := Item new name: 'Pepsi'; price: 0.75.
• item3 := Item new name: 'Jolt'; price: 0.75.
• items := OrderedCollection with: item1 with: item2 with: item3.
• aDispensingHolder := DispensingHolder new.
• items do: [:each |
• aDispensingHolder connectToItem: each.
• each connectToDispensingHolder: aDispensingHolder].
• items do: [:each | each vend].
• Second Part: Adding in the CashDevice
• First Pass Scenario
• I'm an item.
• I know my price.
• I know my corresponding cash device.
• I send it a message to get the amount collected.
• I'm a cash device.
• I get cash.
• I know the amount collected.
• Someone asks me what amount I've collected; I respond with that value.
• I'm an item.
• If the amount collected is greater than or equal to my price
• I tell my dispensing holder to dispense, and
• I tell my cash device to make change.
• Otherwise,
• I return a "not dispensed" result to the sender.
• I'm a cash device.
• I make change.
• I'm an item.
• I return a "dispensed" result to the sender.
• Critique
• o Item does all the work - responsibility not well distributed in design.
• o CashDevice is just a data holder - it has no real functionality.
• An Improved Scenario Script
• I'm an item.
• I know my price.
• I know my corresponding cash device.
• I send a message "got enough (price)" to my cash device.
• I'm a cash device.
• Someone asks me if I've got enough.
• If my amount collected is greater than or equal to the price he gives me,
• then I answer "yes,"
• else I answer "no."
• I'm an item.
• If "yes,"
• I tell my dispensing holder to dispense, and
• I tell my cash device to make change.
• Otherwise,
• I return a "not dispensed" result to the sender.
• I'm a cash device.
• I make change.
• I'm an item.
• I return a "dispensed" result to the sender.
• Building the CashDevice
• Model subclass: #CashDevice
• instanceVariableNames: 'amountCollected '
• classVariableNames: ''
• poolDictionaries: ''
• category: 'Vending'
• CashDevice class methodsFor: 'instance creation'
• new
• ^super new initialize
• CashDevice methodsFor: 'initialize-release'
• initialize
• amountCollected := 0
• CashDevice Accessing
• CashDevice methodsFor: 'accessing'
• amountCollected
• "Return the current value of amount collected."
• ^amountCollected
• amountCollected: newValue
• "Set the value of amount collected. Notify my dependents
• that I've changed."
• amountCollected := newValue.
• self changed: #amountCollected
• Connection and Cash Collecting
• Connection accessors
• (None needed.)
• CashDevice methodsfor: 'cash collecting'
• gotEnough: amountToCollect
• "Return true if the amount collected is greater than
• or equal to the amount to collect."
• ^ amountCollected >= amountToCollect
• return
• "Reset the cash collected. Return false to indicate
• cash not collected."
• self amountCollected: 0
• makeChangeFor: aPrice
• "N.B. This isn't what you'd expect."
• self amountCollected: amountCollected - aPrice
• Simulating
• CashDevice methodsFor: 'simulating'
• addCash
• "Increase my amount collected by some default increment."
• self amountCollected: (amountCollected + self cashIncrement)
• cashIncrement
• ^0.25
• Example with CashDevice
• | cashDevice price |
• "N.B. difference between Cash & cash"
• cashDevice := CashDevice new.
• price := 1.0.
• [cashDevice gotEnough: price]
• whileFalse: [Transcript cr; show: 'need more!'.
• cashDevice addCash].
• Transcript cr; show: 'got enough!'
• Modifications Needed to Item for CashDevice
• Model subclass: #Item
• instanceVariableNames: 'name price cashDevice dispensingHolder'
• "Adding a cashDevice ivar"
• classVariableNames: ''
• poolDictionaries: ''
• category: 'Vending'
• Item methodsFor: 'connections'
• cashDevice
• "Return the current value of cash device."
• ^cashDevice
• connectToCashDevice: aCashDevice
• cashDevice := aCashDevice
• Final Forms of Testing and Vending in Item
• Item methodsFor: 'testing'
• isVendable
• ^cashDevice gotEnough: price
• Item methodsFor: 'vending'
• vend
• | success |
• self isVendable ifFalse:
• [Screen default ringBell. ^false].
• success := dispensingHolder dispense: self.
• success ifFalse: [^false].
• cashDevice makeChangeFor: price.
• ^true.
• The BIG EXAMPLE!
• |item1 item2 item3 items aDispensingHolder aCashDevice |
• item1 := Item new name: 'Coca-Cola'; price: 0.75.
• item2 := Item new name: 'Pepsi'; price: 0.75.
• item3 := Item new name: 'Jolt'; price: 0.75.
• items := Array with: item1 with: item2 with: item3.
• aDispensingHolder := DispensingHolder new.
• aCashDevice := CashDevice new.
• aCashDevice amountCollected: 1.50.
• items do:
• [:anItem |
• aDispensingHolder connectToItem: anItem.
• anItem connectToDispensingHolder: aDispensingHolder.
• anItem connectToCashDevice: aCashDevice].
• "Not both ways on cashDevice."
• items do: [:anItem | anItem vend]