如何在Drupal7中使用Ajax寻呼机

我想在我的区块中使用ajax_pager。 我已经安装了库API和Ajax寻呼机API（如AP API页面所述）。但它仍然不起作用。 当我使用“寻呼机”而不是“ajax_寻呼机”时，它就工作了。 我做错了什么

<?php
function latest_news_block_info() {    
    $blocks['latest_news_block'] = array(
    // info: The name of the block.
    'info' => t('Latest News'),
);

  return $blocks;
}

function latest_news_block_view($delta = '') {
    // The $delta parameter tells us which block is being requested.
    switch ($delta) {
        case 'latest_news_block':
        // Create your block content here
        $block['subject'] = t('Last news');

    $query = new EntityFieldQuery();
    //change news by the name of your content type
    $entities = $query->entityCondition('entity_type', 'node')
        ->pager(5, 0)
        ->entityCondition('bundle', 'news')
        ->propertyOrderBy("created", "DESC")
        ->execute();

    $content = "";
    foreach($entities['node'] as $obj) {
         $node = node_load($obj->nid);
         $content .= "<img src='" .file_create_url($node->field_image['und'][0]['uri']) . "' width='200px' height='200px'>" .  l($node->title, "node/" . $node->nid) . truncate_utf8($node->body['und'][0]['value'], 200, true, true) . '<br><hr>';
         $block['content'] = $content;
    }

    break;
   }


   $pager = theme('ajax_pager', array('parameters' => array('selector' => 'block-system-main')));
   $block['content'] .= $pager;

   return $block;
}

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怎么不工作？寻呼机不显示