State pattern

Intent

• Allow an object to adjust its behavior based on its state

Structure

• Client does not need to know about the Object states as well as the associations between the states and the object behaviors.
• The behaviors are now handled by the corresponding State objects.
• The state transition method can be centralized in the Object or distributed over the concrete states.
• The Object can also pass itself as an argument to the operations of State objects. This helps the state objects to access the information in the Object.

Applicable contexts

• You want to modify the object’s behavior at run-time depending on its state.
• You want to eliminate the duplications of the conditional clause on the object’s states currently scatterring over different places.
• You want to emphasize the transitions between states.
• You want to emphasize the associations between states and behaviors.
• There are complicated logics revolving around the object states, and each of those states can transform to another state.

Risks

• Increase the number of classes, as each state is now a class itself
• Increase the number of objects, but this risk can be reduced by reusing the same instance of the state (if possible)
• More complicated code

Example

• github/State

References

[1] State, The GOF book