Evocative Development
[print]
Mehdi Bizhani
Design Patterns
## Design Patterns #### Design _reusable_ and _extensible_ object oriented software === ### Introduction - OO Principles - Encapsulation - Abstraction - Inheritance - Polymorphism - Interface - a contract specifying the **capabilities** that a class should provide --- ### Problems with Inheritance - It is all about behavior! - The changes to superclass may affect all subclasses. - Adding an `abstract` method to the superclass forces all subclasses to implement it (code duplication!). - Taking those methods to separate interface(s) is the same problem! - Altering the hierarchy and inserting an abstract class in the middle may solve the problem for **some** behaviors. - Difficult to alter behavior at runtime === ### Design Principles (1/4) - Identify the aspects of your application that vary and separate them from what stays the same. - Take the parts that vary and "encapsulate" them, so that later you can alter or extend the parts that vary without affecting those that don’t. - All patterns provide a way to let _some part of a system vary independently of all other parts_. --- ### Design Principles (2/4) - Program to an interface (_supertype_), not an implementation - Use an interface to represent each behavior - Define a behavior represented by an interface - Delegate the behavior to an implementation - Instead of calling a _set of behaviors_, we’ll start thinking as a _family of algorithms_. --- ### Design Principles (3/4) - **Favor composition over inheritance.** - `HAS-A` can be better than `IS-A` - The class **has** behaviors A and B - You can change behavior at runtime --- ### Design Principles (4/4)  === ### Categories | Creational | Structural | Behavioral | |------------|------------|-----------------------| | | | [Strategy](#strategy) | | | | [Observer](#observer) | === ### Strategy - It defines a family of algorithms - Encapsulates each one into a separate class - Makes them interchangeable === ### Observer === ### References - Head First Design Patterns, O'Reilly, 2021