The memento pattern is a software design pattern that provides the ability to save and restore an object's state. It is often used in conjunction with undo/redo operations.
The memento pattern is a three-tier structure consisting of the originator, the caretaker, and the memento.
The originator is the object whose state needs to be saved. The caretaker is the object responsible for saving and restoring the originator's state. The memento is an immutable object that stores the originator's state.
The memento pattern is used to:
There are two ways to implement the memento pattern in Kotlin:
A data class is a class that is designed to hold data. Data classes have a number of features that make them ideal for implementing the memento pattern:
copy
methoddata
keywordTo implement the memento pattern using a data class, we need to create a Memento
class that stores the state of the Originator
class. The Originator
class has a createMemento
method that creates a Memento
object and a restoreMemento
method that restores the state of the Originator
from a Memento
object.
Here is an example of how to implement the memento pattern using a data class:
data class Originator(var state: String) {
fun createMemento(): Memento {
return Memento(state)
}
fun restoreMemento(memento: Memento) {
state = memento.state
}
data class Memento(val state: String)
}
A sealed class is a class that can have one or more subclasses. Sealed classes are ideal for implementing the memento pattern because they:
sealed
keywordabstract
To implement the memento pattern using a sealed class, we need to create a Memento
sealed class that has a subclass for each state of the Originator
. The Originator
class has a createMemento
method that creates a Memento
object and a restoreMemento
method that restores the state of the Originator
from a Memento
object.
Here is an example of how to implement the memento pattern using a sealed class:
sealed class Memento
class Originator(var state: String) {
fun createMemento(): Memento {
return when (state) {
"state1" -> State1()
"state2" -> State2()
else -> throw IllegalArgumentException("Invalid state")
}
}
fun restoreMemento(memento: Memento) {
state = when (memento) {
is State1 -> "state1"
is State2 -> "state2"
else -> throw IllegalArgumentException("Invalid memento")
}
}
class State1 : Memento
class State2 : Memento
}
In this article, we have learned about the memento pattern and how to implement it in Kotlin using data classes and sealed classes.