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Implement Memento Pattern

design pattern

Table of contents

Why Memento pattern

Why Memento pattern

We want to implement undo feature with memento design pattern.

We want three classes that communication between them displayed in the below UML . memento

In this problem Memento class is EditorState and Originator class is Editor and CareTaker class is History.

Editor.java:

Java

public class Editor {
    private String content;
    public EditorState createState(){
        return new EditorState(content);
    }
    public void restore(EditorState state){
        content = state.getContent();
    }
    public String getContent() {
        return content;
    }
    public void setContent(String content) {
        this.content = content;
    }
}

EditorState.java:

Java

public class EditorState {
    private final String content;
    public EditorState(String content) {
        this.content = content;
    }
    public String getContent() {
        return content;
    }
}

History.java:

Java

import java.util.ArrayList;
import java.util.List;
public class History {
    private List<EditorState> states = new ArrayList<>();
    public void push(EditorState state){
        states.add(state);
    }
    public EditorState pop(){
        var lastIndex = states.size()-1;
        var lastState = states.get(lastIndex);
        states.remove(lastState);
        return lastState;
    }
}

memento design pattern in python

Editor.py

Python

from .EditorState import Editor_state
class Editor():
    def __init__(self,content=""):
        self.__content = content
    @property
    def content(self):
        return self.__content
    @content.setter
    def content(self,content):
        self.__content = content
    def create_state(self):
        return Editor_state(self.__content)
    def restore(self,state):
        self.__content = state.content

EditorState.py

Python

class Editor_state():
    def __init__(self,content):
        self.__CONTENT = content
    @property
    def content(self):
        return self.__CONTENT

History.py

Python

class History():
    def __init__(self):
        self.__states = []
    def push(self,state):
        self.__states.append(state)
    def pop(self):
        last_index = len(self.__states) - 1
        last_state = self.__states.pop(last_index)
        return last_state

check package in main file

main.py

Python

from memento.History import History
from memento.Editor import Editor
def main():
     editor = Editor()
     history = History()
     editor.content="a"
     history.push(editor.create_state())
     editor.content="b"
     history.push(editor.create_state())
     editor.content="c"
     editor.restore(history.pop())
     print(editor.content)
# Press the green button in the gutter to run the script.
if __name__ == '__main__':
    main()

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