Strategy decorator

Solution

• We decompose strategies into their operations, so that each strategy operation can be implemented in a separate class.
• We make it possible to chain implementations of the same strategy interface in any combination and in any order.
• We make it possible to add and use strategy operations without an implementation for the respective operation.
• We implement a generic default behavior once and for all invocations of strategy operations where no other behavior applies.

Class Diagram I

Class Diagram I Benefits

This design allows to

• select algorithms in any combination and in any order, separately for any operation of the strategy interface and determined by any number, any type and any combination of parameters.
• apply behavior hierarchically where it is needed, with an unlimited depth of the hierarchy, for instance if the parameters relate to each other hierarchically.
• to choose the same behavior for different sets of parameter values or for different parameter types, without having to duplicate the implementation of that behavior.
• to parallelize the work easily, since the implementations will be much smaller pieces of code and decoupled from each other.
• add operations to the strategy interface without having to add or change any strategy or decorator. Just the forwarding to the decorated strategy has to be added.
• automatically apply a generic default behavior where no other behavior has been added. A strategy operation can be used even without any decorator for that operation.
• generate a proxy for each decorated strategy in order to apply AOP behavior for cross-cutting concerns like logging or authorization.

Class Diagram I Principles

The Strategy Decorator design pattern adheres to the SOLID principles:

• Single-Responsibility Principle (SRP)
• Open-Closed Principle (OCP)
• Liskov Substitution Principle (LSP)
• Interface Segregation Principle (ISP)
• Dependency Inversion Principle (DIP)

As well as to the following principles:

• Composition over Inheritance (CoI)
• Don‘t Repeat Yourself (DRY)