According to the Single Responsibility Principle, introduced by Robert C. Martin in his book "Principles of Object Oriented Design", a class should
have only one responsibility:
If a class has more than one responsibility, then the responsibilities become coupled.
Changes to one responsibility may impair or inhibit the class' ability to meet the others.
This kind of coupling leads to fragile designs that break in unexpected ways when changed.
Classes which rely on many other classes tend to aggregate too many responsibilities and should be split into several smaller ones.
Nested classes dependencies are not counted as dependencies of the outer class.
Noncompliant code example
class Foo { // Noncompliant - Foo depends on too many classes: T1, T2, T3, T4, T5, T6 and T7
/**
* @var T1
*/
public $a1; // Foo is coupled to T1
/**
* @var T2
*/
protected $a2; // Foo is coupled to T2
/**
* @var T3
*/
private $a3; // Foo is coupled to T3
/**
* @param T5
* @param T6
*
* @return T4
*/
public function compute(T5 $a, $b) { // Foo is coupled to T4, T5 and T6
$result = new T7(); // Foo is coupled to T7
return $result;
}
}
