State Design Pattern
 definition UML diagram participants | sample code in C# |
definition
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medium
UML class diagram
participants
The classes and/or objects participating in this pattern are:
- Context (Account)
- defines the interface of interest to clients
- maintains an instance of a ConcreteState subclass that defines the current state.
- State (State)
- defines an interface for encapsulating the behavior associated with a particular state of the Context.
- Concrete State (RedState, SilverState, GoldState)
- each subclass implements a behavior associated with a state of Context
sample code in C#
This structural code demonstrates the State pattern which allows an object to behave differently depending on its internal state. The difference in behavior is delegated to objects that represent this state.
Hide code
// State pattern -- Structural example
|
using System;
namespace DoFactory.GangOfFour.State.Structural
{
/// <summary>
/// MainApp startup class for Structural
/// State Design Pattern.
/// </summary>
class MainApp
{
/// <summary>
/// Entry point into console application.
/// </summary>
static void Main()
{
// Setup context in a state
Context c = new Context(new ConcreteStateA());
// Issue requests, which toggles state
c.Request();
c.Request();
c.Request();
c.Request();
// Wait for user
Console.ReadKey();
}
}
/// <summary>
/// The 'State' abstract class
/// </summary>
abstract class State
{
public abstract void Handle(Context context);
}
/// <summary>
/// A 'ConcreteState' class
/// </summary>
class ConcreteStateA : State
{
public override void Handle(Context context)
{
context.State = new ConcreteStateB();
}
}
/// <summary>
/// A 'ConcreteState' class
/// </summary>
class ConcreteStateB : State
{
public override void Handle(Context context)
{
context.State = new ConcreteStateA();
}
}
/// <summary>
/// The 'Context' class
/// </summary>
class Context
{
private State _state;
// Constructor
public Context(State state)
{
this.State = state;
}
// Gets or sets the state
public State State
{
get { return _state; }
set
{
_state = value;
Console.WriteLine("State: " +
_state.GetType().Name);
}
}
public void Request()
{
_state.Handle(this);
}
}
}
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Output
State: ConcreteStateA
State: ConcreteStateB State: ConcreteStateA State: ConcreteStateB State: ConcreteStateA |
This real-world code demonstrates the State pattern which allows an Account to behave differently depending on its balance. The difference in behavior is delegated to State objects called RedState, SilverState and GoldState. These states represent overdrawn accounts, starter accounts, and accounts in good standing.
Hide code
// State pattern -- Real World example
|
using System;
namespace DoFactory.GangOfFour.State.RealWorld
{
/// <summary>
/// MainApp startup class for Real-World
/// State Design Pattern.
/// </summary>
class MainApp
{
/// <summary>
/// Entry point into console application.
/// </summary>
static void Main()
{
// Open a new account
Account account = new Account("Jim Johnson");
// Apply financial transactions
account.Deposit(500.0);
account.Deposit(300.0);
account.Deposit(550.0);
account.PayInterest();
account.Withdraw(2000.00);
account.Withdraw(1100.00);
// Wait for user
Console.ReadKey();
}
}
/// <summary>
/// The 'State' abstract class
/// </summary>
abstract class State
{
protected Account account;
protected double balance;
protected double interest;
protected double lowerLimit;
protected double upperLimit;
// Properties
public Account Account
{
get { return account; }
set { account = value; }
}
public double Balance
{
get { return balance; }
set { balance = value; }
}
public abstract void Deposit(double amount);
public abstract void Withdraw(double amount);
public abstract void PayInterest();
}
/// <summary>
/// A 'ConcreteState' class
/// <remarks>
/// Red indicates that account is overdrawn
/// </remarks>
/// </summary>
class RedState : State
{
private double _serviceFee;
// Constructor
public RedState(State state)
{
this.balance = state.Balance;
this.account = state.Account;
Initialize();
}
private void Initialize()
{
// Should come from a datasource
interest = 0.0;
lowerLimit = -100.0;
upperLimit = 0.0;
_serviceFee = 15.00;
}
public override void Deposit(double amount)
{
balance += amount;
StateChangeCheck();
}
public override void Withdraw(double amount)
{
amount = amount - _serviceFee;
Console.WriteLine("No funds available for withdrawal!");
}
public override void PayInterest()
{
// No interest is paid
}
private void StateChangeCheck()
{
if (balance > upperLimit)
{
account.State = new SilverState(this);
}
}
}
/// <summary>
/// A 'ConcreteState' class
/// <remarks>
/// Silver indicates a non-interest bearing state
/// </remarks>
/// </summary>
class SilverState : State
{
// Overloaded constructors
public SilverState(State state) :
this(state.Balance, state.Account)
{
}
public SilverState(double balance, Account account)
{
this.balance = balance;
this.account = account;
Initialize();
}
private void Initialize()
{
// Should come from a datasource
interest = 0.0;
lowerLimit = 0.0;
upperLimit = 1000.0;
}
public override void Deposit(double amount)
{
balance += amount;
StateChangeCheck();
}
public override void Withdraw(double amount)
{
balance -= amount;
StateChangeCheck();
}
public override void PayInterest()
{
balance += interest * balance;
StateChangeCheck();
}
private void StateChangeCheck()
{
if (balance < lowerLimit)
{
account.State = new RedState(this);
}
else if (balance > upperLimit)
{
account.State = new GoldState(this);
}
}
}
/// <summary>
/// A 'ConcreteState' class
/// <remarks>
/// Gold indicates an interest bearing state
/// </remarks>
/// </summary>
class GoldState : State
{
// Overloaded constructors
public GoldState(State state)
: this(state.Balance, state.Account)
{
}
public GoldState(double balance, Account account)
{
this.balance = balance;
this.account = account;
Initialize();
}
private void Initialize()
{
// Should come from a database
interest = 0.05;
lowerLimit = 1000.0;
upperLimit = 10000000.0;
}
public override void Deposit(double amount)
{
balance += amount;
StateChangeCheck();
}
public override void Withdraw(double amount)
{
balance -= amount;
StateChangeCheck();
}
public override void PayInterest()
{
balance += interest * balance;
StateChangeCheck();
}
private void StateChangeCheck()
{
if (balance < 0.0)
{
account.State = new RedState(this);
}
else if (balance < lowerLimit)
{
account.State = new SilverState(this);
}
}
}
/// <summary>
/// The 'Context' class
/// </summary>
class Account
{
private State _state;
private string _owner;
// Constructor
public Account(string owner)
{
// New accounts are 'Silver' by default
this._owner = owner;
this._state = new SilverState(0.0, this);
}
// Properties
public double Balance
{
get { return _state.Balance; }
}
public State State
{
get { return _state; }
set { _state = value; }
}
public void Deposit(double amount)
{
_state.Deposit(amount);
Console.WriteLine("Deposited {0:C} --- ", amount);
Console.WriteLine(" Balance = {0:C}", this.Balance);
Console.WriteLine(" Status = {0}",
this.State.GetType().Name);
Console.WriteLine("");
}
public void Withdraw(double amount)
{
_state.Withdraw(amount);
Console.WriteLine("Withdrew {0:C} --- ", amount);
Console.WriteLine(" Balance = {0:C}", this.Balance);
Console.WriteLine(" Status = {0}\n",
this.State.GetType().Name);
}
public void PayInterest()
{
_state.PayInterest();
Console.WriteLine("Interest Paid --- ");
Console.WriteLine(" Balance = {0:C}", this.Balance);
Console.WriteLine(" Status = {0}\n",
this.State.GetType().Name);
}
}
}
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Output
Deposited $500.00 ---
Balance = $500.00 Status = SilverState Deposited $300.00 --- Balance = $800.00 Status = SilverState Deposited $550.00 --- Balance = $1,350.00 Status = GoldState Interest Paid --- Balance = $1,417.50 Status = GoldState Withdrew $2,000.00 --- Balance = ($582.50) Status = RedState No funds available for withdrawal! Withdrew $1,100.00 --- Balance = ($582.50) Status = RedState |
This .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, object initializers, automatic properties, etc.
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