Preserve Whole Object VS Don't Look For Things

Question

I was reading Fowler\'s Refactoring Book and saw Preserve Whole Object. A different, newer opinion says that this refactoring is the exact opposite of what you should do: The Cl

Answer 1

There are many concepts in the Object Oriented Design such as Patterns, Principles and Practices that may seem to be either similar or contradictory at first. In fact, most of them are neither similar nor contradictory. And the thing that makes them different and consistent is their intent.

The seeming contradiction between the Preserve Whole Object refactoring and the Service Locator pattern mentioned in The Clean Code Talks video occurs when they are treated as a same concept, although they are different in their intent and their essence.

The Preserve Whole Object refactoring is simply a technique used to make code easier to read, understand and maintain by reducing the number of arguments to a function. The Service Locator, on the other hand, is a design pattern that is used to manage dependencies between different components in a system using the Inversion of Control concept. Unlike the Preserve Whole Object refactoring technique which has local effect on a system, that is applied to a small part of the system (a function), the Service Locator pattern has a global effect on the system and addresses a bigger architectural problem (Dependency Management).

When to use the Preserve Whole Object refactoring?

Use the Preserve Whole Object refactoring when You have two or more arguments to a function which are basically the properties of one object, so pass the object instead.

There is a similar concept called Parameter Object (aka Argument Object) (Introduce Parameter Object refactoring) which states that if You have a group of parameters that are not the properties of one object but are conceptually related to each other or naturally go together, wrap them with a class of their own and pass the instance of that class instead. It is mainly used when sending a message to an object.

A quote from Clean Code, Chapter 3: Functions, Function Arguments, page 43 (Robert C. Martin):

Argument Objects

When a function seems to need more than two or three arguments, it is likely that some of those arguments ought to be wrapped into a class of their own. Consider, for example, the difference between the two following declarations:

Circle makeCircle(double x, double y, double radius);
Circle makeCircle(Point center, double radius);

Reducing the number of arguments by creating objects out of them may seem like cheating, but it’s not. When groups of variables are passed together, the way x and y are in the example above, they are likely part of a concept that deserves a name of its own.

When to use the Service Locator pattern?

Use the Service Locator pattern when Your class has dependencies that are not conceptually related and You do not want Your class to depend on concrete implementations. Actually, this is when You would want to use any of the Dependency Management approaches. Another alternative is the Dependency Injection approach which explicitly specifies all the dependencies as a separate arguments to the constructor. Whereas the Service Locator pass all the dependencies in a single container object. In fact, it is this very similarity between the Service Locator pattern and the Preserve Whole Object refactoring of combining the arguments in a single object that serves as a source of confusion. The Dependency Management techniques are mainly used in object construction.

There are pros and cons to both approaches of the Dependency Management which are discussed in the Inversion of Control Containers and the Dependency Injection pattern article by Martin Fowler.

When to use both?

Sometimes there will be situations where Your class will have two or more dependencies that are conceptually related and You might want to combine them in a single object and pass it as a dependency using the Service Locator. So, as You can see these two concepts are not mutually exclusive.