Visitor pattern: Difference between revisions

Visiting car.

Kicking my front left wheel.

Kicking my front right wheel.

Kicking my back left wheel.

Kicking my back right wheel.

Moving my body.

Starting my engine.

Starting my car.

</pre>

=={{header|Python}}==

<syntaxhighlight lang="python">

"""An example of the visitor pattern using structural pattern matching.

Requires Python >= 3.10.

"""

from __future__ import annotations

from abc import ABC

from abc import abstractmethod

class CarElement(ABC):

def accept(self, visitor: CarElementVisitor) -> None:

visitor.visit(self)

class CarElementVisitor(ABC):

@abstractmethod

def visit(self, car_element: CarElement) -> None:

"""Override this in `CarElementVisitor` subclasses."""

class Body(CarElement):

"""Car body."""

class Engine(CarElement):

"""Car engine."""

class Wheel(CarElement):

"""Car wheel"""

def __init__(self, name: str) -> None:

self.name = name

class Car(CarElement):

def __init__(self) -> None:

self.elements: list[CarElement] = [

Wheel("front left"),

Wheel("front right"),

Wheel("back left"),

Wheel("back right"),

Body(),

Engine(),

]

def accept(self, visitor: CarElementVisitor) -> None:

for element in self.elements:

visitor.visit(element)

super().accept(visitor)

class CarElementDoVisitor(CarElementVisitor):

def visit(self, car_element: CarElement) -> None:

match car_element:

case Body():

print("Moving my body.")

case Car():

print("Starting my car.")

case Wheel() as wheel:

print(f"Kicking my {wheel.name} wheel.")

case Engine():

print("Starting my engine.")

class CarElementPrintVisitor(CarElementVisitor):

def visit(self, car_element: CarElement) -> None:

match car_element:

case Body():

print("Visiting body.")

case Car():

print("Visiting car.")

case Wheel() as wheel:

print(f"Visiting my {wheel.name} wheel.")

case Engine():

print("Visiting my engine.")

if __name__ == "__main__":

car = Car()

car.accept(CarElementPrintVisitor())

car.accept(CarElementDoVisitor())

</syntaxhighlight>

{{out}}

<pre>

Visiting my front left wheel.

Visiting my front right wheel.

Visiting my back left wheel.

Visiting my back right wheel.

Visiting body.

Visiting my engine.