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

Placeholder Helper

Plcaeholder view helper 用于在view script和view instances之间保存content，而且提供一些有用的特性例如：聚合content、捕获view script content稍后再用、和添加pre- and post-text 到content（以及内容聚合的自定义分隔符）
Example #11 Placeholders的基本用法
基本用法就是储存view DataSet，然后使用设置好的Placehloder name调用Placeholder helper，返回一个对象用来输出。

<?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
（这个用法还不明确，以后有用再学习）
Example #13 Using Placeholders to Capture Content
偶尔的，你可能在view script中需要placeholder一些内容来做为template（模板）；Placeholder view helper允许你捕获任意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 ?>

类似于laravel中的section的用法，可以引用“代码块”的方式来实现模板化，减少代码的重复。

RenderToPlaceholder Helper

渲染（renders）一个模板（view script）并存储渲染好的输出作为一个placeholder变量稍后使用。
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>
?>