<?php

/**
 * Class for implementing lazily evaluated instance variables in PHP.
 * @author Peter Goodman
 */
abstract class Lazy {
    private $lazy_variables = array();
    
    protected function __get($key) {
        echo "Calling Lazy::__get()\n";
        
        if(isset($this->lazy_variables[$key])) {
            return $this->$key = call_user_func($this->lazy_variables[$key]);
        }
        
        return NULL;
    }
    
    protected function registerLazyVariable($var, $callback) {
        $this->lazy_variables[$var] = $callback;
    }
}

/**
 * The following is an example of how lazy variables can be nifty. Suppose that
 * we want to work with a database result set, but we don't want to execute the
 * query only until we abolsutely have to (in this case, when we want to
 * iterate over it). We can make an IteratorIterator-like class, LazyIterator,
 * that allows iterator over a database result set iterator (example uses
 * ArrayIterator). The LazyIterator::$iterator is the lazy variable. It is
 * only defined when it is first used.
 */


class Query {
    public function execute() {
        
        echo "Calling Query::execute()\n";
        
        return new ArrayIterator(array(
            'hello',
            'world',
        ));
    }
}

class LazyIterator extends Lazy implements Iterator {
    
    protected $query;
    
    public function __construct(Query $query) {
        $this->query = $query;
        $this->registerLazyVariable('iterator', array($this, 'getInstance'));
    }
    
    protected function getInstance() {
        echo "Calling LazyIterator::getInstance()\n";
        return $this->query->execute();
    }
    
    public function next() {
        $this->iterator->next();
    }
    
    public function key() {
        return $this->iterator->key();
    }
    
    public function current() {
        return $this->iterator->current();
    }
    
    public function valid() {
        return $this->iterator->valid();
    }
    
    public function rewind() {
        $this->iterator->rewind();
    }
}

$lazy_it = new LazyIterator(new Query);

foreach($lazy_it as $key => $val)
    echo "{$val}\n";

echo "\n";

foreach($lazy_it as $key => $val)
    echo "{$val}\n";