Software Design/Extract Wrapper Object

Extract Wrapper Object is a refactoring primarily aiming at removing logic duplication in the code as per DRY principle. Extract Wrapper Object is related to Extract Class and Introduce Parameter Object refactorings.

Examples

If there is some code working with coordinates x and y and repeatedly performing operations such as negation:

x = -x
y = -y

The coordinates could be grouped into a Point object on which negate() function can be defined:

data class Point(var x: Double, var y: Double) {
    fun negate() {
        x = -x
        y = -y
    }
}

For another example, there may be a class with several operations and in some cases some aspect should be attached to calling those operations, such as logging or notifying some listener:

class Container {
  fun add(elem: Element) { ... }
  fun remove(elem: Element) { ... }
}

// Code repeated in multiple places within some other class:
container.add(elem)
listener.notifyAdded(elem)

// ...
container.remove(elem)
listener.notifyRemoved(elem)

Then a decorator class such as ListeneableContainer can be extracted to gather the repetition of the aspect's logic in one place:

class ListeneableContainer(val container: Container, val listener: Listener) {
  fun add(elem: Element) {
    container.add(elem)
    listener.notifyAdded(elem)
  }
  
  fun remove(elem: Element) {
    container.remove(elem)
    listener.notifyRemoved(elem)
  }
}

Why not

Main article: Extract Class § Why not

Related refactorings and practices

• Extract Class
• Don't repeat logic in several places
• Extract parts of a class into smaller classes

Sources

• Refactoring: Improving the Design of Existing Code (2 ed.). 2018. ISBN 978-0134757599. https://martinfowler.com/books/refactoring.html.  Chapter 3, "Data Clumps" section