This blog post will give you a brief introduction into the Class Interface Size metric, as it is calculated by the static code analysis tool PHP_Depend.
The Class Interface Size or CIS metric is measure of the
public services that a class provides. This metric was orginally defined in
the QMOOD model [1] by Bansiya &
Davis.
The orginal version of the CIS metric was defined as the number of
public methods that a class provides. Each of these methods can be seen as a
service where surrounding application can send messages to or receive messages
from a class.
CIS = public(NOM)
A newer variant of the CIS metric also includes the public attributes
of a class, because theses properties can also be used to transport messages
or information between a class and the surrounding application.
CIS = public(NOM) + public(VARS)
PHP_Depend uses the second variant and counts all public methods and
attributes declared in a class to calculate its Class Interface Size
metric.
This metric is a good indicator for the choosen software design. Several
classes with a high CIS value are a sure sign that the design of the
analyzed software prefers composition over inheritance to share common
functionality between different components. So in most cases a high value is
a good a sign, because composition increases the reusability and flexibility.
But there are also situations where wrongly used composition of functionality
leads to a design that is harder to understand and maintain.
Thresholds
It is not easy to define good thresholds for this metric, because those values
heavy depend on the choosen design, e.g. inheritance or composition. But in
generall we can say that is best practice to limit the public interface that
can be used to alter the internal state of an object. Therefore we suggest
20
as a reference point for the upper limit.
Bibliography
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[1]
-
http://www.ptidej.net/teaching/inf6306/fall09/notes/course4/Bansiya02-QualityModel.pdf
IEEE Transactions on Software Enginerring; Hierarchical Model for
Object-Oriented Design Quality Assessment; Bansiya & Davis; 2002
