Why return $this in setter methods?

问题:

Examining Zend Framework, I found that all setter methods (of those I’ve examined) return the instance of the class it lives in. It doesn't only set a value but also returns $this. For example:

  /*   Zend_Controller_Router   */
public function setGlobalParam($name, $value) {
    $this->_globalParams[$name] = $value;
    return $this;
}

  /*    Zend_Controller_Request    */
public function setBaseUrl($baseUrl = null) {
    // ... some code here ...
    $this->_baseUrl = rtrim($baseUrl, '/');
    return $this;
}

  /*    Zend_Controller_Action    */
public function setFrontController(Zend_Controller_Front $front) {
    $this->_frontController = $front;
    return $this;
}

And so on. Every public setter returns $this. And it's not only for setters, there are also other action methods that return $this:

public function addConfig(Zend_Config $config, $section = null) {
    // ... some code here ...
    return $this;
}

Why is this needed? What does returning $this do? Does it have some special meaning?

回答1:

The return $this allows the chaining of methods like:

$foo->bar('something')->baz()->myproperty

回答2:

It's so that method calls on an object can be "chained", like this.

$obj -> setFoo ('foo') -> setBar ('bar') -> setBaz ('baz') -> setFarble ('farble');