An Extensible State Machine Pattern for Interactive Applications
European Conference on Object-Oriented Programming (ECOOP 2008), Paphos, Cyprus, July 7-11, 2008.
Brian Chin, Todd Millstein
The state design pattern is the standard
object-oriented programming idiom for implementing the state machine
logic of interactive applications. While this pattern provides a
number of advantages, it does not easily support the creation of
extended state machines in subclasses. We describe the {\em
extensible state design pattern, which augments the traditional
state pattern with a few additional constraints that allow subclasses
to easily add both new states and new events. Further, we observe
that delimited continuations, a well-known construct from
functional programming languages, supports state refinement in
subclasses as well as the modular expression of control flow in the
presence of interaction. We illustrate our pattern in the context of
Java, leveraging its generics to obviate the need for dynamic
typecasts and employing a small library that implements delimited
continuations. We have used our pattern to simplify and modularize a
widely used application written by others.
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