Note: Interested in understanding how all other design patterns can be implemented in GO. Please see this full reference – All Design Patterns in Go (Golang)

Introduction:

Memento design pattern is a behavioral design pattern. It allows us to save checkpoints for an object and thus allow an object to revert to its previous state. Basically it helps in undo-redo operations on an object. Below are the design components of the Memento Design Pattern.

• Originator: It is the actual object whose state is saved as a memento.
• Memento: This is the object which saves the state of the originator
• Caretaker: This is the object that saves multiple mementos. Given an index, it returns the corresponding memento.

The originator defines two methods. savememento() and restorememento()

• savememento()- in this method the originator saves its internal state into a memento object.
• restorememento()- this method takes input as a memento object. The originator restores itself to the pass memento. Hence a previous state is restored.

Practical Example:

originator.go

package main
type originator struct {
state string
}
func (e *originator) createMemento() *memento {
return &memento{state: e.state}
}
func (e *originator) restorememento(m *memento) {
e.state = m.getSavedState()
}
func (e *originator) setState(state string) {
e.state = state
}
func (e *originator) getState() string {
return e.state

memento.go

package main
type memento struct {
state string
}
func (m *memento) getSavedState() string {
return m.state
}

caretaker.go

Notice that the caretaker contains the mementoArray which holds all the memento.

package main
type caretaker struct {
mementoArray []*memento
}
func (c *caretaker) addMemento(m *memento) {
c.mementoArray = append(c.mementoArray, m)
}
func (c *caretaker) getMenento(index int) *memento {
return c.mementoArray[index]
}

main.go

package main
import "fmt"
func main() {
caretaker := &caretaker{
mementoArray: make([]*memento, 0),
}
originator := &originator{
state: "A",
}
fmt.Printf("Originator Current State: %s\n", originator.getState())
caretaker.addMemento(originator.createMemento())
originator.setState("B")
fmt.Printf("Originator Current State: %s\n", originator.getState())
caretaker.addMemento(originator.createMemento())
originator.setState("C")
fmt.Printf("Originator Current State: %s\n", originator.getState())
caretaker.addMemento(originator.createMemento())
originator.restorememento(caretaker.getMenento(1))
fmt.Printf("Restored to State: %s\n", originator.getState())
originator.restorememento(caretaker.getMenento(0))
fmt.Printf("Restored to State: %s\n", originator.getState())
}

Output:

originator Current State: A
originator Current State: B
originator Current State: C
Restored to State: B
Restored to State: A

Full Working Code:

package main
import "fmt"
type originator struct {
state string
}
func (e *originator) createMemento() *memento {
return &memento{state: e.state}
}
func (e *originator) restoreState(m *memento) {
e.state = m.getSavedState()
}
func (e *originator) setState(state string) {
e.state = state
}
func (e *originator) getState() string {
return e.state
}
type memento struct {
state string
}
func (m *memento) getSavedState() string {
return m.state
}
type caretaker struct {
mementoArray []*memento
}
func (c *caretaker) addMemento(m *memento) {
c.mementoArray = append(c.mementoArray, m)
}
func (c *caretaker) getMenento(index int) *memento {
return c.mementoArray[index]
}
func main() {
caretaker := &caretaker{
mementoArray: make([]*memento, 0),
}
originator := &originator{
state: "A",
}
fmt.Printf("Originator Current State: %s\n", originator.getState())
caretaker.addMemento(originator.createMemento())
originator.setState("B")
fmt.Printf("Originator Current State: %s\n", originator.getState())
caretaker.addMemento(originator.createMemento())
originator.setState("C")
fmt.Printf("Originator Current State: %s\n", originator.getState())
caretaker.addMemento(originator.createMemento())
originator.restoreState(caretaker.getMenento(1))
fmt.Printf("Restored to State: %s\n", originator.getState())
originator.restoreState(caretaker.getMenento(0))
fmt.Printf("Restored to State: %s\n", originator.getState())
}

Output:

originator Current State: A
originator Current State: B
originator Current State: C
Restored to State: B
Restored to State: A