Zend Framework: view helper -- Placeholder Helper && RenderToPlcarholder Helper

Placeholder Helper

Plcaeholder view helper 用於在view script和view instances之間保存content，而且提供一些有用的特性例如：聚合content、捕獲view script content稍後再用、和添加pre- and post-text 到content（以及內容聚合的自定義分隔符）
Example #11 Placeholders的基本用法
基本用法就是儲存view DataSet，然後使用設置好的Placehloder name調用Placeholder helper，返回一個對象用來輸出。

<?php $this->placeholder('foo')->set("Some text for later") ?>

<?php
    echo $this->placeholder('foo');
    // outputs "Some text for later"
?>

Example #12 Using Placeholders to Aggregate Content
（這個用法還不明確，以後有用再學習）
Example #13 Using Placeholders to Capture Content
偶爾的，你可能在view script中需要placeholder一些內容來做爲template（模板）；Placeholder view helper允許你捕獲任意content，使用下面的API for later rendering。

• captureStart($type, $key) begins capturing content.

  $type should be one of the Placeholder constants APPEND or SET. If APPEND, captured content is appended to the list of current content in the placeholder; if SET, captured content is used as the sole value of the placeholder (potentially replacing any previous content). By default, $type is APPEND.

  $key can be used to specify a specific key in the placeholder container to which you want content captured.

  captureStart() locks capturing until captureEnd() is called; you cannot nest capturing with the same placeholder container. Doing so will raise an exception.

• captureEnd() stops capturing content, and places it in the container
  object according to how captureStart() was called.

<!-- Default capture: append -->
<?php $this->placeholder('foo')->captureStart();
foreach ($this->data as $datum): ?>
<div class="foo">
    <h2><?php echo $datum->title ?></h2>
    <p><?php echo $datum->content ?></p>
</div>
<?php endforeach; ?>
<?php $this->placeholder('foo')->captureEnd() ?>

<?php echo $this->placeholder('foo') ?>
<!-- Capture to key -->
<?php $this->placeholder('foo')->captureStart('SET', 'data');
foreach ($this->data as $datum): ?>
<div class="foo">
    <h2><?php echo $datum->title ?></h2>
    <p><?php echo $datum->content ?></p>
</div>
 <?php endforeach; ?>
<?php $this->placeholder('foo')->captureEnd() ?>

<?php echo $this->placeholder('foo')->data ?>

類似於laravel中的section的用法，可以引用“代碼塊”的方式來實現模板化，減少代碼的重複。

RenderToPlaceholder Helper

渲染（renders）一個模板（view script）並存儲渲染好的輸出作爲一個placeholder變量稍後使用。
Example #32 Basic Usage of RenderToPlaceholder

<?php
// View script (backlink.phtml):
echo sprintf(
    '<p class="older"><a href="%s">Older posts</a></p>',
    $this->url(array('controller' => 'index', 'action' => 'index'))
)
?>

<?php
// Usage:
$this->renderToPlaceholder('backlink.phtml', 'link');
?>

<?php
echo $this->placeholder('link');
// Output: <p class="older"><a href="index/index">Older posts</a></p>
?>