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

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