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This pattern promotes the idea of [[loose coupling]].
The chain-of-responsibility pattern is structurally nearly identical to the [[decorator pattern]], the difference being that for the decorator, all classes handle the request, while for the chain of responsibility, exactly one of the classes in the chain handles the request. This is a strict definition of the Responsibility concept in the [[Design Patterns|GoF]] book. However, many implementations (such as loggers below, or UI event handling, or servlet filters in Java, etc.) allow several elements in the chain to take responsibility.
==Overview==
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== Structure ==
=== UML class and sequence diagram ===
[[File:w3sDesign Chain of Responsibility Design Pattern UML.jpg|frame|none|A sample UML class and sequence diagram for the Chain of Responsibility design pattern.
In the above [[Unified Modeling Language|UML]] [[class diagram]], the <code>Sender</code> class doesn't refer to a particular receiver class directly.
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{{wikibooks|Computer Science Design Patterns|Chain of responsibility|Chain-of-responsibility implementations in various languages}}
=== Java example ===
Below is an example of this pattern in Java.
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from abc import ABCMeta, abstractmethod
from enum import Enum, auto
class LogLevel(Enum):
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FUNCTIONAL_ERROR = auto()
ALL = auto()
class Logger:
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"""
raise NotImplementedError("You should implement this method.")
class ConsoleLogger(Logger):
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"""
print("Writing to console:", msg)
class EmailLogger(Logger):
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def write_message(self, msg: str) -> None:
print(f"Sending via email: {msg}")
class FileLogger(Logger):
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def write_message(self, msg: str) -> None:
print(f"Writing to log file: {msg}")
def main():
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)
logger.message("OrderDispatched.", LogLevel.FUNCTIONAL_MESSAGE)
if __name__ == "__main__":
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