StateManager.java

package oripa.appstate;

import oripa.paint.EditMode;


/**
 * This class holds current state and (only one) previous state 
 * to help getting back.
 * 
 * @author koji
 *
 */
public class StateManager implements StateManagerInterface<EditMode>{

	//-----------------------------------------------------------------
	// singleton implementation
	private static StateManager instance = null;

	public static StateManager getInstance(){
		if(instance == null){
			instance = new StateManager();
		}

		return instance;
	}

	//-----------------------------------------------------------------
	// Instance implementation

	private ApplicationState<EditMode> current, lastInputCommand, previous;


	@Override
	public ApplicationState<EditMode> getCurrent() {
		return current;
	}

	/**
	 * push {@code s} as a new state to be held. 
	 * the current state will be dropped to previous state.
	 * @param s new state
	 */
	@Override
	public void push(ApplicationState<EditMode> s){

		
		if(s.getGroup() == EditMode.INPUT){
			// keep for popLastInputCommand()
			lastInputCommand = s;
		}
		else if(current != null){

		}
		

		if(current != null){
			// pushing copy or cut causes empty pasting
			if(current.getGroup() != EditMode.COPY && 
					current.getGroup() != EditMode.CUT){
				previous = current;
			}
		}
		current = s;
	}

	/**
	 * pop previous state. It  will be set to current state.
	 * @return previous state. null if empty.
	 */
	@Override
	public ApplicationState<EditMode> pop(){
		if(current == previous){
			return null;
		}

		current = previous;
		return current;
	}

	/**
	 * This method accepts INPUT only.
	 * the current state will be dropped to previous state.
	 * @param group ID.
	 * @return last state of the group. 
	 * {@code null} if {@code group} is not {@code oripa.paint.EditMode.INPUT}.
	 */
	@Override
	public ApplicationState<EditMode> popLastOf(EditMode group) {
		if(group != EditMode.INPUT){
			return null;
		}

		return popLastInputCommand();

	}

	/**
	 * for the action of "input" radio button.
	 * the current state will be dropped to previous state.
	 * @return state of the last input command
	 */
	public ApplicationState<EditMode> popLastInputCommand(){
		if(current == lastInputCommand){
			return null;
		}
		previous = current;
		current = lastInputCommand;

		return current;
	}

}