Design Pattern by HeadFirst Design Pattern Book.
1. Strategy Pattern (Policy Pattern)
Design Principle:
-
Identify the aspects of your application that vary and seperate them from what stays the same.
when you have an codes to change, investigate new requirements carefully then you should know what behavior need to separates from code block and what need stays the same.
Design Principle:
- Program to an interfacce, not an implementation ( 1 of 5 principals in SOLID )
2. Observer Pattern
Design Principle:
- The observer pattern defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically.
3. Decorator Pattern
Design Principle:
- Decorator is a structural design pattern that lets you attach new behaviors to objects by placing these objects inside special wrapper objects that contain the behaviors.
4.1 Factory Method (a.k.a Virtual Constructor)
- This part is an example for Factory Method. Part 2 belongs to Aabstract factory is here.
This belongs to creational design pattern that provides an interface for creating objects in a supperclass, but allows subclasses to alter/decide type of object that will be created.
Design Principle:
- Depend upon abstractions. Do not depend upon concrete classes.
- Factory Pattern encapsulate object creation
Real life example:
- Imagine you're using Mysql database connections in your backend system.
- One day customer want to switch to use MYSQL Server for some purposes.
- Then we need create new class for MYSQL Server with existing interface using by Mysql before.
- And edit you backend code to use new connection.
- In future, customer wants to change to other DB connector then you needs to do the same thing above...
- Imagine you have 300 lines of code using this connector, then you need edit all of them, it's terrible.
Why Factory Pattern helps here:
- Decoupling the creation creation of object/interface from your code.
- When you need to edit, add or remove some connector, just need to edit 1 method without affect other classes.
Cons:
- Loose Coupling
- Reusability
- Easier to maintain
Design Pattern by HeadFirst Design Pattern Book.
4.2 Abstract Factory
Provides an interface for creating families of related or dependent object without specifying their concrete class
An example for PizzaStore:
Result:
The factory pattern construct a single object != The abstract factory construct multiple objects
As you can see in 2 pictures above, the Abstract Factory actually looks like a Factory Method. Each method is declared abstract and subclasses override it to create some objects.
The job of an Abstract Factory is to define an interface for creating a set of products. Each method in that interface is responsible for creating a concrete product, and we implement a subclass of Abstract Factory to supply those implementations. So factory methods are a natural way to implement your product methods in your factories.
Absctract factory contains multiple factory methods inside
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