Refactoring

Extract Class

Overview ¶

You might have heard that classes should have a single responsibility. Even if you try to follow this dogma, in practice classes grow. Some operations are, some more data is added, and sooner or later your class has more than one reponsibility.

The Extract Class refactoring is best applied if there are logical units of data which are grouped together, or operations which are just performed on a subset of the data.

Checklist ¶

Step 1

Determine what to extract
Find logical units of data which are grouped together or operations which are performed on a subset of the data.

Step 2

Create a New Class
Create a new class with the extracted operations and establish a link from the old to new class. This link might be bidirectional.

Step 3

Rename the Old Class
If the name of the old class is not fitting anymore, consider renaming it.

Step 4

Review the interfaces and links of each class
The goal should be minimally coupled classes. Examine both classes to see if you can replace a bidirectional relationship with a unidirectional relationship.

Step 5

Value object vs. reference object
Decide whether you would like to expose the new object as a value object (immutable), or a reference object (may be modified).

Example: Moving Related Fields to Value Object ¶

class BlogPost
{
    // ...
    private $previewTextRst;
    private $previewTextHtml;
    private $textRst;
    private $textHtml;

    // ...

    public function setPreviewText($rst, $html)
    {
        $this->previewTextRst = $rst;
        $this->previewTextHtml = $html;
    }

    public function setText($rst, $html)
    {
        $this->textRst = $rst;
        $this->textHtml = $html;
    }

    // ...
}

class Text
{
    private $rst;
    private $html;

    public function __construct($rst, $html)
    {
        $this->rst = $rst;
        $this->html = $html;
    }

    public function getRst()
    {
        return $this->rst;
    }

    public function getHtml()
    {
        return $this->html;
    }
}

class BlogPost
{
    private $previewText;
    private $text;

    public function setPreviewText($rst, $html)
    {
        $this->previewText = new Text($rst, $html);
    }

    public function setText($rst, $html)
    {
        $this->text = new Text($rst, $html);
    }
}