备忘录模式

from __future__ import annotations
from abc import ABC, abstractmethod
from datetime import datetime
from random import sample
from string import ascii_letters, digits


class Originator():
    """
    The Originator holds some important state that may change over time. It also
    defines a method for saving the state inside a memento and another method
    for restoring the state from it.
    """

    _state = None
    """
    For the sake of simplicity, the originator's state is stored inside a single
    variable.
    """

    def __init__(self, state: str) -> None:
        self._state = state
        print(f"Originator: My initial state is: {self._state}")

    def do_something(self) -> None:
        """
        The Originator's business logic may affect its internal state.
        Therefore, the client should backup the state before launching methods
        of the business logic via the save() method.
        """

        print("Originator: I'm doing something important.")
        self._state = self._generate_random_string(30)
        print(f"Originator: and my state has changed to: {self._state}")

    def _generate_random_string(self, length: int = 10) -> None:
        return "".join(sample(ascii_letters, length))

    def save(self) -> Memento:
        """
        Saves the current state inside a memento.
        """

        return ConcreteMemento(self._state)

    def restore(self, memento: Memento) -> None:
        """
        Restores the Originator's state from a memento object.
        """

        self._state = memento.get_state()
        print(f"Originator: My state has changed to: {self._state}")


class Memento(ABC):
    """
    The Memento interface provides a way to retrieve the memento's metadata,
    such as creation date or name. However, it doesn't expose the Originator's
    state.
    """

    @abstractmethod
    def get_name(self) -> str:
        pass

    @abstractmethod
    def get_date(self) -> str:
        pass


class ConcreteMemento(Memento):
    def __init__(self, state: str) -> None:
        self._state = state
        self._date = str(datetime.now())[:19]

    def get_state(self) -> str:
        """
        The Originator uses this method when restoring its state.
        """
        return self._state

    def get_name(self) -> str:
        """
        The rest of the methods are used by the Caretaker to display metadata.
        """

        return f"{self._date} / ({self._state[0:9]}...)"

    def get_date(self) -> str:
        return self._date


class Caretaker():
    """
    The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
    doesn't have access to the originator's state, stored inside the memento. It
    works with all mementos via the base Memento interface.
    """

    def __init__(self, originator: Originator) -> None:
        self._mementos = []
        self._originator = originator

    def backup(self) -> None:
        print("\nCaretaker: Saving Originator's state...")
        self._mementos.append(self._originator.save())

    def undo(self) -> None:
        if not len(self._mementos):
            return

        memento = self._mementos.pop()
        print(f"Caretaker: Restoring state to: {memento.get_name()}")
        try:
            self._originator.restore(memento)
        except Exception:
            self.undo()

    def show_history(self) -> None:
        print("Caretaker: Here's the list of mementos:")
        for memento in self._mementos:
            print(memento.get_name())


if __name__ == "__main__":
    originator = Originator("Super-duper-super-puper-super.")
    caretaker = Caretaker(originator)

    caretaker.backup()
    originator.do_something()

    caretaker.backup()
    originator.do_something()

    caretaker.backup()
    originator.do_something()

    print()
    caretaker.show_history()

    print("\nClient: Now, let's rollback!\n")
    caretaker.undo()

    print("\nClient: Once more!\n")
    caretaker.undo()

Originator: My initial state is: Super-duper-super-puper-super.

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: wQAehHYOqVSlpEXjyIcgobrxsZUnat

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: lHxNORKcsgMWYnJqoXjVCbQLEIeiSp

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: cvIYsRilNOtwynaKdEZpDCQkFAXVMf

Caretaker: Here's the list of mementos:
2019-01-26 21:11:24 / (Super-dup...)
2019-01-26 21:11:24 / (wQAehHYOq...)
2019-01-26 21:11:24 / (lHxNORKcs...)

Client: Now, let's rollback!

Caretaker: Restoring state to: 2019-01-26 21:11:24 / (lHxNORKcs...)
Originator: My state has changed to: lHxNORKcsgMWYnJqoXjVCbQLEIeiSp

Client: Once more!

Caretaker: Restoring state to: 2019-01-26 21:11:24 / (wQAehHYOq...)
Originator: My state has changed to: wQAehHYOqVSlpEXjyIcgobrxsZUnat

// 原发器中包含了一些可能会随时间变化的重要数据。它还定义了在备忘录中保存
// 自身状态的方法，以及从备忘录中恢复状态的方法。
class Editor is
    private field text, curX, curY, selectionWidth

    method setText(text) is
        this.text = text

    method setCursor(x, y) is
        this.curX = curX
        this.curY = curY

    method setSelectionWidth(width) is
        this.selectionWidth = width

    // 在备忘录中保存当前的状态。
    method createSnapshot():Snapshot is
        // 备忘录是不可变的对象；因此原发器会将自身状态作为参数传递给备忘
        // 录的构造函数。
        return new Snapshot(this, text, curX, curY, selectionWidth)

// 备忘录类保存有编辑器的过往状态。
class Snapshot is
    private field editor: Editor
    private field text, curX, curY, selectionWidth

    constructor Snapshot(editor, text, curX, curY, selectionWidth) is
        this.editor = editor
        this.text = text
        this.curX = curX
        this.curY = curY
        this.selectionWidth = selectionWidth

    // 在某一时刻，编辑器之前的状态可以使用备忘录对象来恢复。
    method restore() is
        editor.setText(text)
        editor.setCursor(curX, curY)
        editor.setSelectionWidth(selectionWidth)

// 命令对象可作为负责人。在这种情况下，命令会在修改原发器状态之前获取一个
// 备忘录。当需要撤销时，它会从备忘录中恢复原发器的状态。
class Command is
    private field backup: Snapshot

    method makeBackup() is
        backup = editor.createSnapshot()

    method undo() is
        if (backup != null)
            backup.restore()
    // ...