Chain of Responsibility em Python
O Chain of Responsibility é um padrão de projeto comportamental que permite passar a solicitação ao longo da cadeia de handlers em potencial até que um deles lide com a solicitação.
O padrão permite que vários objetos tratem a solicitação sem acoplar a classe remetente às classes concretas dos destinatários. A cadeia pode ser composta dinamicamente em tempo de execução com qualquer handler que siga uma interface de handler padrão.
Complexidade:
Popularidade:
Exemplos de uso: O padrão Chain of Responsibility não é um padrão frequente em um programa Python, pois é relevante apenas quando o código opera com cadeias de objetos.
Identificação: O padrão é reconhecível pelos métodos comportamentais de um grupo de objetos que indiretamente chamam os mesmos métodos em outros objetos, enquanto todos os objetos seguem a interface comum.
Exemplo conceitual
Este exemplo ilustra a estrutura do padrão de projeto Chain of Responsibility. Ele se concentra em responder a estas perguntas:
- De quais classes ele consiste?
- Quais papéis essas classes desempenham?
- De que maneira os elementos do padrão estão relacionados?
main.py: Exemplo conceitual
from __future__ import annotations
from abc import ABC, abstractmethod
from typing import Any, Optional
class Handler(ABC):
"""
The Handler interface declares a method for building the chain of handlers.
It also declares a method for executing a request.
"""
@abstractmethod
def set_next(self, handler: Handler) -> Handler:
pass
@abstractmethod
def handle(self, request) -> Optional[str]:
pass
class AbstractHandler(Handler):
"""
The default chaining behavior can be implemented inside a base handler
class.
"""
_next_handler: Handler = None
def set_next(self, handler: Handler) -> Handler:
self._next_handler = handler
# Returning a handler from here will let us link handlers in a
# convenient way like this:
# monkey.set_next(squirrel).set_next(dog)
return handler
@abstractmethod
def handle(self, request: Any) -> str:
if self._next_handler:
return self._next_handler.handle(request)
return None
"""
All Concrete Handlers either handle a request or pass it to the next handler in
the chain.
"""
class MonkeyHandler(AbstractHandler):
def handle(self, request: Any) -> str:
if request == "Banana":
return f"Monkey: I'll eat the {request}"
else:
return super().handle(request)
class SquirrelHandler(AbstractHandler):
def handle(self, request: Any) -> str:
if request == "Nut":
return f"Squirrel: I'll eat the {request}"
else:
return super().handle(request)
class DogHandler(AbstractHandler):
def handle(self, request: Any) -> str:
if request == "MeatBall":
return f"Dog: I'll eat the {request}"
else:
return super().handle(request)
def client_code(handler: Handler) -> None:
"""
The client code is usually suited to work with a single handler. In most
cases, it is not even aware that the handler is part of a chain.
"""
for food in ["Nut", "Banana", "Cup of coffee"]:
print(f"\nClient: Who wants a {food}?")
result = handler.handle(food)
if result:
print(f" {result}", end="")
else:
print(f" {food} was left untouched.", end="")
if __name__ == "__main__":
monkey = MonkeyHandler()
squirrel = SquirrelHandler()
dog = DogHandler()
monkey.set_next(squirrel).set_next(dog)
# The client should be able to send a request to any handler, not just the
# first one in the chain.
print("Chain: Monkey > Squirrel > Dog")
client_code(monkey)
print("\n")
print("Subchain: Squirrel > Dog")
client_code(squirrel)
Output.txt: Resultados da execução
Chain: Monkey > Squirrel > Dog
Client: Who wants a Nut?
Squirrel: I'll eat the Nut
Client: Who wants a Banana?
Monkey: I'll eat the Banana
Client: Who wants a Cup of coffee?
Cup of coffee was left untouched.
Subchain: Squirrel > Dog
Client: Who wants a Nut?
Squirrel: I'll eat the Nut
Client: Who wants a Banana?
Banana was left untouched.
Client: Who wants a Cup of coffee?
Cup of coffee was left untouched.
