Design Patterns in Ruby: Strategy
A clear example of the Strategy pattern
A clear example
The intent
According to the GoF, the intent of the Strategy pattern is to:
“Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.”
The example
In the example below, an instance of the Car class accepts an engine strategy. There are two engine strategies defined, StraightSixEngine and V8Engine. Each encapsulates a variant of an engine algorithm. Each shares a common interface (meaning that they share the same public methods).
Some tidbits
The strategies share an interface
When using the Strategy pattern you should be sure to couple clients to the interface shared by each of the strategies, and not to any concrete strategy. In Ruby, which lacks formal interfaces, this merely means that each strategy in the family of algorithms share the same public API (the same public methods).
Getting rid of if statements
The Strategy pattern is often used to eliminate conditional statements within a class. Sometimes, when you have different algorithms for a desired behavior and the algorithm is chosen conditionally, it makes sense to encapsulate those algorithms into classes (strategies) and eliminate the conditionals altogether.
Composition over Inheritance
Because the Strategy pattern uses composition instead of inheritance, it is often considered superior to solutions favoring inheritance.
Going deeper
There are many contexts in which the use of the Strategy pattern is prudent. To learn more, check out the seminal work on Design Patterns: The GOF Design Pattern book.
