/*************************************************************************
* Copyright 2009-2012 Eucalyptus Systems, Inc.
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package com.eucalyptus.util.fsm;
import com.eucalyptus.util.Callback;
import com.eucalyptus.util.HasName;
import com.eucalyptus.util.Callback.Completion;
/**
* <p>
* A callback interface which is invoked according to the progress of a transitions between two
* states: {@fromState} is the state before the transition and {@toState} is the state
* after the transition. The primary purpose of implementing classes is to enforce pre-conditions
* and apply side effects corresponding with the change of state. The expectation is that
* transitions are applied atomically in the sense that only one can be in progress at a time.
* </p>
* <p>
* The methods are invoked in the following order; their purpose is summarized here with details in
* the documentation for each method:
* <ol>
* <li>{@link #before()}: prior to any change of state -- check preconditions.</li>
* <li>{@link #leave()}: on leaving {@code fromState}.</li>
* <li>{@link #enter()}: on entering {@code toState}.</li>
* <li>
* <li>{@link #after(toState)}: after the transition has been completed.</li>
* </ol>
* </p>
*
* @author decker
*
* @param <S> enum type of the states in the state machine.
*/
public interface TransitionAction<P extends HasName<P>> {
/**
* Invoked before leaving the {@code fromState}. At this time the transition
* has not yet begun and can be aborted. A return of false or a caught
* exception will stop application of the transition.
*
* Implementors should ensure to avoid side-effects.
*
* @return false iff the transition should not be executed.
*/
public abstract boolean before( P parent );
/**
* Applies changes corresponding with leaving {@code fromState} and <strong>signals completion
* using the supplied {@link Callback.Completion}</strong>. This method is invoked
* when the transition begins and after the state has been changed to {@code toState} but before
* any side effects have been applied.
*
* Implementors can assume that no other transitions will execute until this
* transition has completed.
*/
public abstract void leave( P parent, Completion transitionCallback );
/**
* Applies changes corresponding with having entered {@code toState} and is
* invoked when the transition completes leaving the state as {@code toState}.
*
* Implementors can assume that no other transitions will execute until this
* transition has completed.
*/
public abstract void enter( P parent );
/**
* Invoked after the transition completes but before any other transition is
* evaluated against the underlying state machine.
*
* Implementors should ensure to avoid side-effects.
*/
public abstract void after( P parent );
}