The State Pattern [HFDP]

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What is the State Pattern?

  • It allows an object to alter its behavior when the internal state changes
  • The object will appear to change its class

How Does It Work?

  • A State interface is created that has all the actions the main class needs.
  • A concrete class is created for each state the system can be in (implementing the State)
  • Each state knows when a state change happens and sets the container's state class to the new state (object)
  • The container always calls state.method()
  • The states have to have a reference to the container since they changed its State

State Vs Strategy

  • User of context in state isn't aware of state objects
  • User of Strategy specifies objects to change the algorithm
  • Strategy is a more flexible alternative to subclassing (changing the composition changes the behavior)
  • State is an alternative to using conditionals

Context Transitions Vs State Transitions

  • You can decouple the states from the Context by having the Context choose the next state
    • If Transitions are fixed, put in Context
    • If Transitions are dynamic, put in state (state knows the Transition to the next state)

Do States Maintain State?

  • No, they choose Transitions
  • This means contexts can share the same strategies