State Pattern

CSC-430

Phillip Wright

The State Pattern

Allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

KA: Objects For States

State Machines

State Machine

(You remember these, right?)

State Machines

public enum State {
  START,TWO,THREE,ERROR;
}

State Machines

public class StateMachine {
  State state = START;
  public void doA(){
    if(state==START){
      state=TWO;
    }else if(state==TWO){
      state=ERROR;
    }else if(state==THREE){
      state=ERROR;
    }
  }

  //public void doB()
  //public void doC()
  //public void doD()
}

State Machines

What happens when we need to make a change to the state machine?

Encapsulate What Varies

The State Pattern

Localize the behavior of each state into its own class
Each state closed for modification, client open for extension

The State Pattern

Define state interface
Define concrete state class for each state
Delegate transitions, behavior, etc. to states

State Interface

public interface State {
  public void doA();
  public void doB();
  public void doC();
  public void doD();
}

States

public class StartState implements State{
  public StartState(StateMachine machine){
    // initialize things...         
  }
  public void doA(){ machine.state=machine.getStateTwo(); }
  public void doB(){ machine.state=machine.getErrorState(); }
  public void doC(){ machine.state=machine.getErrorState(); }
  public void doD(){ machine.state=machine.getErrorState(); }
}

State Machine

public class StateMachine{
  State start = new StartState(this);
  //... initialize other states
  State state = start; // initial state

  public void doA(){state.doA()};
  public void doB(){state.doB()};
  public void doC(){state.doC()};
  public void doD(){state.doD()};
}

State vs Strategy

The client chooses the behavior with a strategy
The state is often chosen by prior states
Client is much less aware of the behavior it is using
Strategies tend to not change

Warning

States may be tightly coupled
Including state in state objects may require a large number of states

Another Way To State

public class StateMachine{
  State state = START;
  F<Unit,State>[Input][State] fsm = // wut?
  public void nextInput(Input i){
    // Use current state and input to look up
    // a function which will perform the transition
    F<Unit,State> f = fsm[i][state];
    // apply the function for the new state
    state = f();
  }
}

Another Way To State

fsm[START][A]= {
  // do some side effects, print messages, get crazy
  return TWO;
}

fsm[TWO][B]= {
  // do some side effects, print messages, get crazy
  return THREE;
}