visitable


A real world example, taken from the gtk module: a Visitable class, providing a framework for a generalized version of the visitor pattern. For information on the visitor pattern, please refer to:

Design Patterns - Elements of Reusable Object-Oriented Software
E.Gamma, R.Helm, R. Johnson, J. Vlissides
Addison Wesley

This example shows how to programmatically call a method given its name: we convert the name to a symbol, then issue the message call with basic.sendwith(), where we pass the arguments as an array. 

package Visitable;

import basic;
import string;

/*
 * Visitable class
 */

public class Visitable extends basic.Object
{
        method visit: visitor with: args ...
        {
                local methodsym;
                local newargs;
                local arg;

                methodsym = string.makesymbol( "visit" + [[self isA] name] );
                if ([visitor doesUnderstand: methodsym])
                {
                        newargs = #[ self ];
                        for (arg in args)
                                newargs[basic.length(newargs)] = arg;
                        return basic.sendwith( visitor, methodsym, newargs );
                }
                return @nil;
        }

        method visit: visitor
        {
                return [self visit: visitor with: @nil];
        }
}