This article was done using my notes from:
Alexander Shvets (2019), Dive into Design Patterns, Refactoring.Guru
State
State pattern allows an object to change the behavior when its internal state changes. The pattern extracts state-related behaviors into separate state classes and forces the original object to delegate the work to an instance of these classes, instead of acting on its own.
Structure
Code
package main
import (
"fmt"
"log"
)
type State interface {
addItem(int) error
requestItem() error
insertMoney(money int) error
dispenseItem() error
}
type VendingMachine struct {
hasItem State
itemRequested State
hasMoney State
noItem State
currentState State
itemCount int
itemPrice int
}
func newVendingMachine(itemCount, itemPrice int) *VendingMachine {
v := &VendingMachine{
itemCount: itemCount,
itemPrice: itemPrice,
}
hasItemState := &HasItemState{
vendingMachine: v,
}
itemRequestedState := &ItemRequestedState{
vendingMachine: v,
}
hasMoneyState := &HasMoneyState{
vendingMachine: v,
}
noItemState := &NoItemState{
vendingMachine: v,
}
v.setState(hasItemState)
v.hasItem = hasItemState
v.itemRequested = itemRequestedState
v.hasMoney = hasMoneyState
v.noItem = noItemState
return v
}
func (v *VendingMachine) requestItem() error {
return v.currentState.requestItem()
}
func (v *VendingMachine) addItem(count int) error {
return v.currentState.addItem(count)
}
func (v *VendingMachine) insertMoney(money int) error {
return v.currentState.insertMoney(money)
}
func (v *VendingMachine) dispenseItem() error {
return v.currentState.dispenseItem()
}
func (v *VendingMachine) setState(s State) {
v.currentState = s
}
func (v *VendingMachine) incrementItemCount(count int) {
fmt.Printf("Adding %d items\n", count)
v.itemCount = v.itemCount + count
}
type NoItemState struct {
vendingMachine *VendingMachine
}
func (i *NoItemState) requestItem() error {
return fmt.Errorf("Item out of stock")
}
func (i *NoItemState) addItem(count int) error {
i.vendingMachine.incrementItemCount(count)
i.vendingMachine.setState(i.vendingMachine.hasItem)
return nil
}
func (i *NoItemState) insertMoney(money int) error {
return fmt.Errorf("Item out of stock")
}
func (i *NoItemState) dispenseItem() error {
return fmt.Errorf("Item out of stock")
}
type HasItemState struct {
vendingMachine *VendingMachine
}
func (i *HasItemState) requestItem() error {
if i.vendingMachine.itemCount == 0 {
i.vendingMachine.setState(i.vendingMachine.noItem)
return fmt.Errorf("No item present")
}
fmt.Printf("Item requested\n")
i.vendingMachine.setState(i.vendingMachine.itemRequested)
return nil
}
func (i *HasItemState) addItem(count int) error {
fmt.Printf("%d items added\n", count)
i.vendingMachine.incrementItemCount(count)
return nil
}
func (i *HasItemState) insertMoney(money int) error {
return fmt.Errorf("Please select item first")
}
func (i *HasItemState) dispenseItem() error {
return fmt.Errorf("Please select item first")
}
type ItemRequestedState struct {
vendingMachine *VendingMachine
}
func (i *ItemRequestedState) requestItem() error {
return fmt.Errorf("Item already requested")
}
func (i *ItemRequestedState) addItem(count int) error {
return fmt.Errorf("Item Dispense in progress")
}
func (i *ItemRequestedState) insertMoney(money int) error {
if money < i.vendingMachine.itemPrice {
return fmt.Errorf("Inserted money is less. Please insert %d", i.vendingMachine.itemPrice)
}
fmt.Println("Money entered is ok")
i.vendingMachine.setState(i.vendingMachine.hasMoney)
return nil
}
func (i *ItemRequestedState) dispenseItem() error {
return fmt.Errorf("Please insert money first")
}
type HasMoneyState struct {
vendingMachine *VendingMachine
}
func (i *HasMoneyState) requestItem() error {
return fmt.Errorf("Item dispense in progress")
}
func (i *HasMoneyState) addItem(count int) error {
return fmt.Errorf("Item dispense in progress")
}
func (i *HasMoneyState) insertMoney(money int) error {
return fmt.Errorf("Item out of stock")
}
func (i *HasMoneyState) dispenseItem() error {
fmt.Println("Dispensing Item")
i.vendingMachine.itemCount = i.vendingMachine.itemCount - 1
if i.vendingMachine.itemCount == 0 {
i.vendingMachine.setState(i.vendingMachine.noItem)
} else {
i.vendingMachine.setState(i.vendingMachine.hasItem)
}
return nil
}
func main() {
vendingMachine := newVendingMachine(1, 10)
err := vendingMachine.requestItem()
if err != nil {
log.Fatalf(err.Error())
}
err = vendingMachine.insertMoney(10)
if err != nil {
log.Fatalf(err.Error())
}
err = vendingMachine.dispenseItem()
if err != nil {
log.Fatalf(err.Error())
}
fmt.Println()
err = vendingMachine.addItem(2)
if err != nil {
log.Fatalf(err.Error())
}
fmt.Println()
err = vendingMachine.requestItem()
if err != nil {
log.Fatalf(err.Error())
}
err = vendingMachine.insertMoney(10)
if err != nil {
log.Fatalf(err.Error())
}
err = vendingMachine.dispenseItem()
if err != nil {
log.Fatalf(err.Error())
}
}