在 Python 中使用命令模式执行/撤消

2022-01-23 00:00:00 python command design-patterns

问题描述

我已经读到使用命令模式是完成执行/撤消功能的最流行的方法之一.事实上，我已经看到可以堆叠一堆动作并反转它们以达到给定状态.但是，我不太确定如何在 Python 中做到这一点，而且我阅读的大多数教程都涉足概念，但没有展示 Python 中的实际实现.

I have read that using command pattern is one of the most popular ways to accomplish do/undo functionality. In fact, I have seen that it's possible to stack a bunch of actions and reverse them in order to reach a given state. However, I'm not quite sure how that can be done in Python and most of the tutorials I have read, dabble into concepts but don't show an actual implementation in Python.

有谁知道 Python 中的执行/撤消功能是如何工作的?

Does anyone know how do/undo functionality work in Python?

作为参考，这是我的(幼稚且可能充满错误)代码:

For reference, this is my (naive and probably ridden with errors) code:

# command
class DrawCommand:
    def __init__(self, draw, point1, point2):
        self.draw = draw
        self.point1 = point1
        self.point2 = point2
    def execute_drawing(self):
        self.draw.execute(self.point1, self.point2)
    def execute_undrawing(self):
        self.draw.unexecute(self.point1, self.point2)
# invoker
class InvokeDrawALine:
    def command(self, command):
        self.command = command
    def click_to_draw(self):
        self.command.execute_drawing()
    def undo(self):
        self.command.execute_undrawing()
# receiver
class DrawALine:
    def execute(self, point1, point2):
        print("Draw a line from {} to {}".format(point1, point2))
    def unexecute(self, point1, point2):
        print("Erase a line from {} to {}".format(point1, point2))

实例化如下:

invoke_draw = InvokeDrawALine()
draw_a_line = DrawALine()
draw_command = DrawCommand(draw_a_line, 1, 2)
invoke_draw.command(draw_command)
invoke_draw.click_to_draw()
invoke_draw.undo()

输出:

Draw a line from 1 to 2
Erase a line from 1 to 2

显然，此测试不允许堆叠多个操作来撤消.也许我完全错了，所以我会很感激一些帮助.

Obviously, this test doesn't allow stack several actions to undo. Maybe I'm completely mistaken so I would appreciate some help.

解决方案

这是一个将命令保存在列表中的实现.

Here is an implementation keeping the commands in a list.

# command
class DrawCommand:
    def __init__(self, draw, point1, point2):
        self.draw = draw
        self.point1 = point1
        self.point2 = point2
    def execute_drawing(self):
        self.draw.execute(self.point1, self.point2)
# invoker
class InvokeDrawLines:
    def __init__(self, data):
        self.commandlist = data
    def addcommand(self, command):
        self.commandlist.append(command)
    def draw(self):
        for cmd in self.commandlist:
            cmd.execute_drawing()
    def undocommand(self, command):
        self.commandlist.remove(command)

# receiver
class DrawALine:
    def execute(self, point1, point2):
        print("Draw a line from" , point1, point2)

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