Facade Design Pattern

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In this video we will discuss

• What is Facade Design Pattern?
• Implementation Guidelines of Facade design pattern
• And will take a look at simple example to implement this pattern

Facade Design Pattern : As per the GOF definition, Facade Pattern states that we need to “Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.” This pattern Falls under the category of Structural Design Pattern and is also known as Wrapper.


Implementation Guidelines : We need to use Facade Design Pattern when

• We want to provide a simple interface to a complex subsystem. Subsystems often get more complex as they evolve. 
• There are many dependencies between clients and the implementation classes of an abstraction. 
• We want to layer the subsystems. Use a facade to define an entry point to each subsystem level. 

Representation Diagram



Facade  

• Knows which subsystem classes are responsible for a request.
• And it delegates client requests to appropriate subsystem objects.

Subsystem classes 

• Implement their subsystem functionality to handle work assigned by the Facade object.
• These subsystems have no knowledge of the facade; that is, they keep no references to it.

Facade design pattern implementation

Step 1 : Add Subsystem interfaces IAddress, ICart, ITax, IWallet and IAddress

using ShoppingCart.Implementation;

using ShoppingCart.Models;

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Interfaces

{

    public interface IAddress

    {

        Address GetAddressDetails(int userID);

    }

}

using ShoppingCart.Models;

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Interfaces

{

    public interface ICart

    {

        Product GetItemDetails(int itemID);

        bool CheckItemAvailability(Product product);

        bool LockItemInStock(int itemID, int quantity);

        int AddItemToCart(int itemID, int quantity);

        double GetCartPrice(int cartID);

    }

}

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Interfaces

{

    public interface IOrder

    {

        int PlaceOrderDetails(int cartID, int shippingAddressID);

    }

}

using ShoppingCart.Models;

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Interfaces

{

    public interface ITax

    {

        double GetTaxByState(string state);

        void ApplyTax(int cartID, double taxPercent);

    }

}

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Interfaces

{

    public interface IWallet

    {

        double GetUserBalance(int userID);

    }

}

Step 2 : Add the below models used in the subsystems

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Models

{

    public class Address

    {

        public int AddressID { get; set; }

        public string AddressDetails { get; set; }

        public string PinCode { get; set; }

        public string Phone { get; set; }

        public string Country { get; set; }

        public string State { get; set; }

        public string City { get; set; }

    }

}

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Models

{

    public class Cart

    {

        public int CartID { get; set; }

        public int UserID { get; set; }

        public IEnumerable<CartItem> ShoppingCart { get; set; }

    }

}

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Models

{

    public class CartItem

    {

        public int ProductID { get; set; }

        public int Quantity { get; set; }

        public double TaxPercentage { get; set; }

        public double Cost { get; set; }

        public double Price { get; set; }

    }

}

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Models

{

    public class Product

    {

        public int ProductID { get; set; }

        public string Name { get; set; }

        public string Description { get; set; }

        public int Quantity { get; set; }

        public double Cost { get; set; }

        public int LockedQty { get; set; }

    }

}

Step 3 : Implement the Subsystem interfaces as shown below Address, Order, Tax, Wallet, ShoppingCartDetails and Address

using ShoppingCart.Interfaces;

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Implementation

{

    public class AddressDetails : IAddress

    {

        public  Models.Address  GetAddressDetails(int userID)

        {

            Console.WriteLine("\t SubSystem Address : GetAddressDetails");

            return new Models.Address();

        }

        

    }

}

using ShoppingCart.Interfaces;

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Implementation

{

    public class Order : IOrder

    {

        public int PlaceOrderDetails(int cartID, int shippingAddressID)

        {

            Console.WriteLine("\t SubSystem Order : PlaceOrderDetails");

            return 10;

        }

    }

}

using ShoppingCart.Interfaces;

using System;

using System.Collections.Generic;

using System.Text;

using ShoppingCart.Models;

namespace ShoppingCart.Implementation

{

    public class ShoppingCartDetails : ICart

    {

        public ShoppingCartDetails()

        {

        }

        public int AddItemToCart(int itemID, int Quantity)

        {

            Console.WriteLine("\t SubSystem Cart : AddItemToCart");

            return 15;

        }

        public bool CheckItemAvailability(Product product)

        {

            Console.WriteLine("\t SubSystem Cart : CheckItemAvailability");

            return true;

        }

        public double GetCartPrice(int cartID)

        {

            Console.WriteLine("\t SubSystem Cart : GetCartPrice");

            return 15;

        }

        public Product GetItemDetails(int itemID)

        {

            Console.WriteLine("\t SubSystem Cart : GetItemDetails");

            return new Product();

        }

        public bool LockItemInStock(int itemID, int quantity)

        {

            Console.WriteLine("\t SubSystem Cart : LockItemInStock");

            return true;

        }

    }  

}

using ShoppingCart.Interfaces;

using ShoppingCart.Models;

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Implementation

{

    public class Tax : ITax

    {

        public void ApplyTax(int cartID, double taxPercent)

        {

            Console.WriteLine("\t SubSystem Tax : ApplyTax");

        }

        public double GetTaxByState(string state)

        {

            Console.WriteLine("\t SubSystem Tax : GetTaxByState");

            return 10;

        }

    }

}

using ShoppingCart.Interfaces;

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingCart.Implementation

{

    public class Wallet : IWallet

    {

        public double GetUserBalance(int userID)

        {

            Console.WriteLine("\t SubSystem Wallet : GetUserBalance");

            return 100;

        }

    }

}

Step 4 : Add Façade layer with interface and implementation as shown below

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingFacade

{

    public interface IUserOrder

    {

        int AddToCart(int itemId, int qty);

        int PlaceOrder(int cartID, int userID);

    }

}

using ShoppingCart.Implementation;

using ShoppingCart.Interfaces;

using ShoppingCart.Models;

using System;

using System.Collections.Generic;

using System.Text;

namespace ShoppingFacade

{

    public class UserOrder : IUserOrder

    {

        public int AddToCart(int itemId, int qty )

        {

            Console.WriteLine("Start AddToCart");

            ICart userCart = new ShoppingCartDetails();

            int cartID = 0;

            //Step 1 : GetItem

            Product product = userCart.GetItemDetails(itemId);

            //Step 2 : Check Availability

            if (userCart.CheckItemAvailability(product))

            {

                //Step 3 : Lock Item in the Stock

                userCart.LockItemInStock(itemId, qty);

                //Step 4 : Add Item to the cart

                cartID = userCart.AddItemToCart(itemId, qty);

            }

            Console.WriteLine("End AddToCart");

            return cartID;

        }

        public int PlaceOrder(int cartID, int userID)

        {

            Console.WriteLine("Start PlaceOrderDetails");

            int orderID = -1;

            IWallet wallet = new Wallet();

            ITax tax = new Tax();

            ICart userCart = new ShoppingCartDetails();

            IAddress address = new AddressDetails();

            IOrder order = new Order();

            //Step 1 : Get Tax percentage by State

            double stateTax = tax.GetTaxByState("ABC");

            //Step 2 : Apply Tax on the Cart Items

            tax.ApplyTax(cartID, stateTax);

            //Step 3 : Get user Wallet balance

            double userWalletBalance = wallet.GetUserBalance(userID);

            //Step 4 : Get the cart items price

            double cartPrice = userCart.GetCartPrice(cartID);

            //Step 5 : Compare the balance and price

            if (userWalletBalance > cartPrice)

            {

                //Step 6 : Get user Address and set to cart

                Address userAddress = address.GetAddressDetails(userID);

                //Step 7 : Place the order

                orderID = order.PlaceOrderDetails(cartID, userAddress.AddressID);

            }

            Console.WriteLine("End PlaceOrderDetails");

            return orderID;

        }

    }

}

Step 5 : Use the console program and invoke the Façade methods with Console being the client for us

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using ShoppingFacade;

namespace FacadeDemo

{

    class Program

    {

        static void Main(string[] args)

        {

            IUserOrder userOrder = new UserOrder();

            Console.WriteLine("Facade : Start");

            Console.WriteLine("************************************");           

            int cartID = userOrder.AddToCart(10, 1);           

            int userID = 1234;

            Console.WriteLine("************************************");

            int orderID = userOrder.PlaceOrder(cartID, userID);

            Console.WriteLine("************************************");

            Console.WriteLine("Facade : End CartID = {0}, OrderID = {1}",

                cartID, orderID);

            Console.ReadLine();

        }

    }

}

Step 6 : Ensure to maintain the below structure for the code to work



Step 7 : Key notes

• Please note that we are just representing all these methods at high level to show the complexity of the subsystems.
• We have not implemented these methods in detail except that we are printing the details of the methods invoked in each of these subsystems using console.writeline.  
• As you all know, Our idea is to just understand the facade implementation and not to focus on the real implementations of these sub systems
• Notice that  in the output  client invokes the facade methods of add to cart and place order details which  Internally calls many subsystem methods to achieve this functionality

Step 8 : Run the application and notice the below output.