Facade

Facade

The Facade Design Pattern provides a simple interface and controls access to a series of complicated interfaces and or sub systems.

Some of the important features and advantages of Facade design Pattern are

• A facade can make a software library easier to use and understand, since the facade has convenient methods for common tasks
• A facade can make code that uses the library more readable, for the same reason
• A facade can reduce dependencies of outside code on the inner workings of a library, since most code uses the facade, thus allowing more flexibility in developing the system
• A facade can wrap a poorly-designed collection of APIs with a single well-designed API

In the following example, a developer using the OrderFacade does not need to understand all the requirements of creating a proper Order class.  These details are hidden away behind the facade.

A VB example of the Facade Pattern

 C# Example of above VB code

using System;
using System.Collections.Generic;


namespace Yanesh.DesignPatterns.Facade
{
    public class OrderFacade
    {
        //Places an Order and Returns an Order ID
        public int placeOrder(int CustomerID, List<BasketItem> Products)
        {
            Order anOrder = new Order();
            OrderLine anOrderLine = new OrderLine();
            Address DespatchAddress = Address.getCustomerDespatchAddress(CustomerID);
            int OrderId = anOrder.requestOrderID();

            anOrder.createOrder(OrderId, DespatchAddress);
            anOrderLine.addOrderLinesToOrder(OrderId, Products);

            return OrderId;
        }
    }

    //order class
    public class Order
    {
        public int requestOrderID()
        {
            //Creates and Returns a Unique Order ID
        }

        public void createOrder(int OrderId, Address DespatchAddress)
        {
        }
    }

    //OrderLine Class
    public class OrderLine
    {
        public void addOrderLinesToOrder(int OrderId, List<BasketItem> Products)
        {
        }
    }
    
    //Public Customer
    public class Address
    {
        public static Address getCustomerDespatchAddress(int CustomerID)
        {
            return new Address();
        }
        //Address properties
    }

    public class BasketItem
    {
        //BasketItem Properties
    }
}

UML

The Facade however, can itself become too complex for a huge subsystem. Also it's a good idea to actually have an abstract Facade over the Facade. One of the most common and successfull examples is using this pattern through a webservice, making the webservice acting as the Facade or the interface to many different dll's each representing a subsystem.

Articles

• Facade Design Pattern: ASP Alliance
• Facade pattern

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