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package eu.future.earth.gwt.client.ui.actionevent;

import com.google.gwt.event.shared.GwtEvent;

public class ActionEvent extends GwtEvent> {

	/**
	 * Handler type.
	 */
	private static Type> TYPE;

	/**
	 * Fires a before selection event on all registered handlers in the handler manager. If no such handlers exist, this
	 * method will do nothing.
	 * 
	 * @param source
	 *            the source of the handlers
	 * @return the event so that the caller can check if it was canceled, or null if no handlers of this event type have
	 *         been registered
	 */
	public static  ActionEvent fire(HasActionHandlers source, T newValue) {
		// If no handlers exist, then type can be null.
		if (TYPE != null) {
			final ActionEvent event = new ActionEvent(newValue);
			source.fireEvent(event);
			return event;
		}
		return null;
	}

	/**
	 * Convenience method to allow subtypes to know when they should fire a value change event in a null-safe manner.
	 * 
	 * @param source
	 *            the source
	 * @param oldValue
	 *            the old value
	 * @param newValue
	 *            the new value
	 * @return whether the event should be fired
	 */
	protected static  boolean shouldFire(HasActionHandlers source, T oldValue, T newValue) {
		return TYPE != null && oldValue != newValue && (oldValue == null || !oldValue.equals(newValue));
	}

	/**
	 * Fires value change event if the old value is not equal to the new value. Use this call rather than making the
	 * decision to short circuit yourself for safe handling of null.
	 * 
	 * @param source
	 *            the source of the handlers
	 * @param oldValue
	 *            the oldValue, may be null
	 * @param newValue
	 *            the newValue, may be null
	 */
	public static  void fireIfNotEqual(HasActionHandlers source, T oldValue, T newValue) {
		if (shouldFire(source, oldValue, newValue)) {
			ActionEvent event = new ActionEvent(newValue);
			source.fireEvent(event);
		}
	}

	/**
	 * Gets the type associated with this event.
	 * 
	 * @return returns the handler type
	 */
	public static  Type> getType() {
		if (TYPE == null) {
			TYPE = new Type>();
		}
		return TYPE;
	}

	protected ActionEvent(T newValue) {
		super();
		value = newValue;
	}

	private final T value;

	public T getValue() {
		return value;
	}

	@SuppressWarnings({
			"unchecked", "rawtypes"
	})
	@Override
	public final Type> getAssociatedType() {
		return (Type) TYPE;
	}

	@Override
	protected void dispatch(ActionHandler handler) {
		handler.onKeyChange(this);
	}
}