Allows an object to alter its behavior when its internal state changes.
The object will appear to change its class.
Category:
Behavioral
UML Diagram:
Description of the Demonstration:
In this demonstration an instance of the GumballMachine class implements a
state machine, but all of the states are defined by instances of sub-classes
of the State class. When a function is called in the GumballMachine, like
GumballMachine::insertQuarter(), the GumballMachine just calls insertQuarter
on its current State object. The State object knows how to handle each of
the transition actions. Some of those actions will be invalid for certain
states, e.g. you can't insert a quarter when one has already been inserted,
a valid transition action will be handled by a State telling the
GumballMachine what State to change to.
In essence what is happening is that the behavior of the GumballMachine is
being dynamically changed while the application is running. Some software
engineers consider the State Design Pattern a variation of the Strategy
pattern. The main difference being that the external class that implements
the behavior of a class changes dynamically at run time with the State
Design Pattern.
Code:
Click here to download a
zip file containing all the source files and an executable of the
Demonstration. See the _ReadMe.txt file in the zip.
