/**
 * AmbientTalk/2 Project
 * SignalListener.java created on Jun 29, 2008 at 11:39:06 AM
 * (c) Programming Technology Lab, 2006 - 2007
 * Authors: Tom Van Cutsem & Stijn Mostinckx
 * 
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use,
 * copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following
 * conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 */
package edu.vub.at.signals;

import java.util.EventListener;

import edu.vub.at.objects.ATMessage;

/**
 * SignalListener is an interface specifying which messages can be sent to any dependent
 * reactive computation. This interface provides methods to get the dependency height of
 * a dependent computation and update it if necessary.
 * 
 * It also provides methods to unhook signal listeners,  such that obsolete branches can
 * be removed from the dataflow graph.
 * 
 * @author smostinc
 *
 */
public interface SignalListener extends EventListener {

	/**
	 * This method is called when one of the parent signal has been updated.  The update
	 * is not performed immediately when this method is invoked. There are two important
	 * reasons for delaying the update:  First of all,  the parent signal may not belong
	 * to the same event loop as its child. This is typically the case when modelling an
	 * external event source such as time, key strokes, etc.  In this case, the external
	 * process should not be able to perform computation inside another event loop.
	 * <p>
	 * Even if parent and child signal belong to the same event loop, performing updates
	 * of the child signal immediately after having updated the parent is not acceptable
	 * since this may introduce update glitches. Consider the following example:
	 * <pre><code>
	 * (seconds < (seconds + 1))
	 * </code></pre>
	 * In the above example, both the comparison <code>&lt;</code> and the addition have
	 * a direct dependency on the seconds signal.  If dependent signals were immediately
	 * updated, the comparison could be updated first, yielding <code>false</code>.
	 * <p>
	 * To avoid update glitches updates are not performed immediately, but are scheduled
	 * to be performed when the dataflow engine reached the proper dependency height. As
	 * such, update glitches can be avoided, since the dependency height of   <code>&lt;
	 * </code> in the example is 2 since it depends on <code>seconds</code>  (height: 0) 
	 * and <code>seconds + 1</code> (height: 1). 
	 * 
	 * @param theParent the parent signal that has been updated
	 * @param theUpdate the update that was propagated by that parent
	 */
	public void scheduleUpdate(ATSignal theParent, ATMessage theUpdate);

	/**
	 * 
	 * @param theParent
	 * @param theUpdate
	 * @return whether the listener wants to receive subsequent updates
	 */
	public boolean performUpdate(ATSignal theParent, ATMessage theUpdate);
	
	/**
	 * Every signal is assigned a dependency height,  which determines the stratus where
	 * the signal is situated in the dataflow graph. Updates are propagated first to the
	 * signals with the lowest dependency height to ensure that signals are only updated
	 * when the signals they depend on are guaranteed not to contain stale information.
	 * 
	 * @return The dependency height of this signal listener.
	 */
	public int dependencyHeight();
	
	/**
	 * When the value of a behavior is set to another behavior, this behavior may have a
	 * higher dependency height. Updates from the newly assigned value should henceforth
	 * be propagated to all dependent computations.  However, before these update can be
	 * propagated the dependency graph should be stratified anew.  There are essentially
	 * two solutions: we can keep the strata static and establish a "delayed" dependency
	 * between both behaviors, which implies that updates are only propagated during the
	 * next turn. The alternative is to reassign the dependency heights of all dependent
	 * signals, such that they are strictly greater than those of the new parent signal.
	 * <p>
	 * The latter approach is adopted in <b>FrTime</b>, as well as in AmbientTalk.  This
	 * method is used to update the dependency height of a signal listener.  It verifies
	 * whether the new height is strictly greater than its current dependency height and
	 * transitively updates dependent signals if necessary.
	 */
	public void updateDependencyHeight(int newDependencyHeight);
	
	/**
	 * When constructing parts of the dataflow network inside a method which was invoked
	 * with a signal as receiver or parameter,  the constructed dataflow will implicitly
	 * depend on the signals that were passed to the method as well.  When these signals
	 * change, the method will be executed anew, and the dataflow network that was built
	 * during the previous invocation is no longer valid.
	 * <p>
	 * This observation is a generalization of the technique in <b>FrTime</b>, where the
	 * problem only surfaces when using signals in conditions or in function position.
	 * <p>
	 * The unhook method is used to deconstruct the dataflow graph, whenever a signal on
	 * which the signal listener depended implicitly is changed. The signal listener has
	 * to ensure that updates will no longer be propagated to rule out already scheduled
	 * update attempts.  
	 */
	public void unhook();
}