Composite + Chain of Responsibility example

Question

Hello!

Can anyone give a practical example of using the design patterns Composite and Chain of Responsibility together?

Thanks

Answer 1

A practical answer may be impossible, but I can see where you would have a composition of chains of responsibility. Here's a pythonish example:

>>> class DevelopmentPerformanceMonitor():
...   def getPerformanceMonitorHandlers():
...     return []
... 
>>> class ProductionPerformanceMonitor():
...   def getPerformanceMonitorHandlers():
...     return [check_cpu_under_load, check_available_hd]
... 
>>> class DevelopmentExceptionMonitor():
...   def getExceptionHandlers():
...     return [email_local_root, log_exception]
... 
>>> class ProductionExceptionMonitor():
...   def getExceptionHandlers():
...     return [emails_system_admin, log_exception, create_ticket]
... 
>>> class SomeSystem:
...    pm = None # Performance Monitor
...    em = None # Exception Monitor
...    def __init__(self, performance_monitor, exception_monitor):
...      pm = performance_monitor
...      em = exception_monitor
...    def on_exception(e):
...      for handler in em.getExceptionHandlers():
...        handler(e)
...    def perform_performance_monitoring(s):
...      for handler in pm.getPerformanceMonitorHandlers():
...        handler(s)

So the SomeSystem object is a composite of a performance_monitor and an exception_monitor. Each of the composites will return a series of handlers for desired chain of responsibility. Although this example is really only complicating a simpler Chains of Responsibility, where the SomeSystem could be initiated with the chains themselves. Although keeping them packaged may be helpful.