Example of an optional command block in a NetLogo extension primitive

gistfile1.java

	public static class Reflect 
		extends DefaultCommand
		implements org.nlogo.nvm.CustomAssembled
	{
		public Syntax getSyntax()
		{
			int[] right = { Syntax.TYPE_NUMBER , Syntax.TYPE_BOOLEAN_BLOCK , 
							Syntax.TYPE_COMMAND_BLOCK | Syntax.TYPE_OPTIONAL } ;
			return Syntax.commandSyntax( right ) ;
		}
		public String getAgentClassString()
		{
			return "T:--P-" ;
		}	
		public void assemble( org.nlogo.nvm.AssemblerAssistant a )
		{
			a.block() ;
			a.done() ;
		}		
		public void perform( Argument args[] , final Context context )
			throws ExtensionException, LogoException
		{
			try
			{
				// we're breaking the rules here a little bit but not it a way that it
				// hasn't been before either for cities or p2p. ev 7/13/07
				reflect( (Turtle) context.getAgent() , args[ 0 ].getDoubleValue() , 
						 ((org.nlogo.nvm.ExtensionContext) context).nvmContext() , 
						 ((org.nlogo.nvm.Argument)args[ 1 ]).getReporter() ) ;
			}
			catch( org.nlogo.api.AgentException e )
			{
				throw new ExtensionException( e.getMessage() ) ;
			}
		}
		public void reflect( Turtle turtle , double dist , 
							 org.nlogo.nvm.Context context ,
							 org.nlogo.nvm.Reporter condition )
			throws org.nlogo.api.AgentException , LogoException
		{
			org.nlogo.agent.Patch nextPatch = (org.nlogo.agent.Patch) nextPatch( turtle ) ;
			double distToBoundary = distanceToBoundary( turtle , nextPatch ) ;
			org.nlogo.nvm.Context freshContext = new org.nlogo.nvm.Context( context , nextPatch ) ;
			condition.checkAgentClass( nextPatch , context ) ;
			while ( dist >= distToBoundary )
			{
				Patch lastPatch = turtle.getPatchHere() ;
				turtle.jump( distToBoundary ) ;
				Object value = freshContext.evaluateReporter( nextPatch , condition ) ;
				if( value instanceof Boolean &&  ( ( Boolean ) value ).booleanValue() )
				{
					if( lastPatch.pycor() == nextPatch.pycor() )
					{
						turtle.heading( - turtle.heading() ) ;
					}
					else
					{
						turtle.heading( 180 - turtle.heading() ) ;
					}
					org.nlogo.agent.AgentSet patchSet = new org.nlogo.agent.ArrayAgentSet
						( org.nlogo.agent.Patch.class , 1 , false , nextPatch.world() ) ;
					patchSet.add( nextPatch ) ;
					context.runExclusiveJob( patchSet , context.ip + 1 ) ;
				}
				dist -= distToBoundary ;
				nextPatch = (org.nlogo.agent.Patch) nextPatch( turtle ) ;
				distToBoundary = distanceToBoundary( turtle , nextPatch ) ;
			}
			turtle.jump( dist ) ;
		}