为什么在模型中使用变量（MVC）？

问题描述：

I'm reading Master Joomla! 1.5 book, and I notice in the Revue model has a function like that

// model
function getRevues() {
  $db =& $this->_db;
  if( empty($this->_revues) ) // ?????
  {
    $query = $this->_buildQuery();
    $limitstart = $this->getState('limitstart');
    $limit = $this->getState('limit');

    $this->_revues = $this->_getLimit($query, $limitstart, $limit);
  }
  return $this->_revues;
}

// view
....
revues =& model->getRevues();

why use _revues variable in class model? If I remove _revues variable and rewrite getRevues function as follows:

function getRevues() {
  $db =& $this->_db;

    $query = $this->_buildQuery();
    $limitstart = $this->getState('limitstart');
    $limit = $this->getState('limit');

    $revues = $this->_getLimit($query, $limitstart, $limit);

  return &$revues;
}

what difference between 2 functions?

我正在读Joomla大师！ 1.5本书，我注意到Revue模型中有这样的函数 p>

  // model 
function getRevues（）{
 $ db =＆amp;  $ this-＆gt; _db; 
 if（empty（$ this-＆gt; _revues））// ????? 
 {
 $ query = $ this-＆gt; _buildQuery（）; 
 $ limitstart =  $ this-＆gt; getState（'limitstart'）; 
 $ limit = $ this-＆gt; getState（'limit'）; 
 
 $ this-＆gt; _revues = $ this-＆gt; _getLimit（$ query，  $ limitstart，$ limit）; 
} 
返回$ this-＆gt; _revues; 
} 
 
 // view 
 .... 
revues =＆amp;  model-＆gt; getRevues（）; 
  code>  pre> 
 
 为什么在类模型中使用_revues变量？ 如果我删除_revues变量并重写getRevues函数，如下所示： p> 
 
 
  function getRevues（）{
 $ db =＆amp;  $ this-＆gt; _db; 
 
 $ query = $ this-＆gt; _buildQuery（）; 
 $ limitstart = $ this-＆gt; getState（'limitstart'）; 
 $ limit = $ this-＆gt;  getState（'limit'）; 
 
 $ revues = $ this-＆gt; _getLimit（$ query，$ limitstart，$ limit）; 
 
 return＆amp; $ revues; 
} 
  code>   pre> 
 
  2个函数有什么区别？ p> 
  div>

答

In the second one, you always execute the database query. In the first one you cache the results (in $this->_revues)which might increase performance of the application. So the database is only hit if you call this method the first time.

为什么在模型中使用变量（MVC）？

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