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add memento pattern
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+342
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‎BehavioralPatterns/BehavioralPatterns.csproj

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<Compile Include="Mediator\ParticipantColleague.cs" />
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<Compile Include="Mediator\ChatRoomMediator.cs" />
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<Compile Include="Mediator\TestMediator.cs" />
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<Compile Include="Memento\Caretaker.cs" />
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<Compile Include="Memento\Memento.cs" />
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<Compile Include="Memento\Originator.cs" />
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<Compile Include="NullObject\ConsoleLog.cs" />
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<Compile Include="NullObject\ILog.cs" />
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<Compile Include="NullObject\NullLog.cs" />
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<None Include="doc\Strategy.cd" />
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<None Include="doc\TemplateMethod.cd" />
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<None Include="Mediator\README.md" />
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<None Include="Memento\README.md" />
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<None Include="NullObject\README.md" />
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<None Include="Observer\README.md" />
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<None Include="Strategy\README.md" />

‎BehavioralPatterns/Memento/Caretaker.cs

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using System;
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using System.Collections.Generic;
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using System.Linq;
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using System.Text;
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using System.Threading.Tasks;
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namespace BehavioralPatterns.Memento
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{
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public class Caretaker
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{
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public static void Run()
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{
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IList<Memento> mementoes = new List<Memento>();
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Originator originator = new Originator();
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originator.SetState("State 1");
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originator.SetState("State 2");
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mementoes.Add(originator.Save()); // checkpoint 1
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originator.SetState("State 3");
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mementoes.Add(originator.Save()); // checkpoint 2
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originator.SetState("State 4");
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originator.Restore(mementoes[0]); // restore to checkpoint 1
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}
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}
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}

‎BehavioralPatterns/Memento/Memento.cs

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using System;
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using System.Collections.Generic;
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using System.Linq;
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using System.Text;
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using System.Threading.Tasks;
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namespace BehavioralPatterns.Memento
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{
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[Serializable]
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class Memento
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{
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public string State { get; }
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public Memento(String state)
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{
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this.State = state;
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}
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}
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}

‎BehavioralPatterns/Memento/Originator.cs

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using System;
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using System.Collections.Generic;
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using System.Linq;
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using System.Text;
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using System.Threading.Tasks;
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namespace BehavioralPatterns.Memento
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{
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class Originator
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{
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private String _state;
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public void SetState(String state)
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{
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this._state = state;
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Console.WriteLine($"State is set to {state}");
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}
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public Memento Save()
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{
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Console.WriteLine($"Saving state to Memento");
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return new Memento(this._state);
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}
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public void Restore(Memento m)
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{
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this._state = m.State;
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Console.WriteLine($"State is restored from Memento: {_state}");
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}
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}
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}

‎BehavioralPatterns/Memento/README.md

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# Momento
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Use the Mediator Pattern to centralize complex communications and control between related objects.
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## Problem
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* Tight coupling between a set of interacting objects should be avoided.
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* It should be possible to change the interaction between a set of objects independently.
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*Tightly coupled objects* are hard to implement, change, test and reuse.
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## Solution
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* Define a `Mediator` object that encapsulates the interaction between a set of objects.
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* Objects delegate their interaction to a mediator object instead of interacting with each other directly.
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This makes the objects *loosely coupled*.
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The Mediator is commonly used to coordinate related GUI components.
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## Benefits
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* Colleagues classes may become more reusable and are decoupled from the system.
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* Simplifies maintenance of the system by centralizing control logic.
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* Simplifies and reduces the variety of messages sent between objects in the system.
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## Drawbacks
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* Without proper design, the mediator object can become overly complex.
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* Single point of failure
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* Hard to test --> Objects should be mocked
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## Common Structure
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![Common structure of mediator pattern](https://upload.wikimedia.org/wikipedia/commons/9/92/W3sDesign_Mediator_Design_Pattern_UML.jpg)
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* Mediator (IChatRoom)
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* defines an interface for communicating with Colleague objects
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* ConcreteMediator (ChatRoom)
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* implements cooperative behavior by coordinating Colleague objects.
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* knows and maintains its colleagues.
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* Colleague (IParticipant)
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* defines an interface for using Colleague objects.
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* ConcreateColleague (Participant)
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* each colleague knows its Mediator object
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* each colleague communicates with its mediator whenever it would have otherwise communicated with another colleague.
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_[Source: http://www.dofactory.com/net/mediator-design-pattern]_
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## Example
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![Mediator Pattern](/Diagrams/Mediator.png)
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Mediator
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```cs
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public interface IChatRoom
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{
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void RegisterParticipant(IParticipant participant);
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void Send(String from, String to, string message);
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}
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```
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ConcreteMediator
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```cs
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public class ChatRoom : IChatRoom
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{
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private readonly IDictionary<string, IParticipant> _participants = new Dictionary<string, IParticipant>();
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public void RegisterParticipant(IParticipant participant)
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{
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this._participants.Add(participant.GetName(), participant);
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}
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public void Send(string from, string to, string message)
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{
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if (this._participants.ContainsKey(to))
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{
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this._participants[to].Receive(from, message);
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}
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else
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{
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throw new ArgumentException("{0} not found", to);
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}
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}
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}
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```
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Colleague
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```cs
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public interface IParticipant
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{
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string GetName();
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void Send(string to, string message);
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void Receive(string from, string message);
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}
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```
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ConcreteColleague
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```cs
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public class Participant : IParticipant
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{
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private readonly string _name;
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private readonly IChatRoom _chatRoom;
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public Participant(string name, IChatRoom chatRoom)
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{
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this._name = name;
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this._chatRoom = chatRoom;
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this._chatRoom.RegisterParticipant(this);
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}
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public string GetName()
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{
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return this._name;
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}
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public void Send(string to, string message)
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{
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this._chatRoom.Send(this._name, to, message);
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}
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public void Receive(string from, string message)
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{
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Console.WriteLine("{0} to {1}: {2}", from, this._name, message);
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}
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}
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```
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Usage
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```cs
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IChatRoom chatRoom = new ChatRoom();
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IParticipant einstein = new Participant("Einstein", chatRoom);
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IParticipant newton = new Participant("Newton", chatRoom);
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IParticipant galileo = new Participant("Galileo", chatRoom);
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newton.Send(galileo.GetName(), "I discoverd laws of motion");
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einstein.Send(newton.GetName(), "I discovered how gravity works");
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```

‎BehavioralPatterns/Momento/README.md

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# Memento
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The Memento pattern provides the abilitiy to restore an object to its previous state later.
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## Problem
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* The internal state of an object should be saved externally so that the object can be restored to this state later.
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* The object's encapsulation must not be violated.
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## Solution
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Make an object (originator) itself responsible for
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* saving its internal state to a `memento` object and
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* restoring to a previous state from a `memento` object.
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Only the originator that created a memento is allowed to access it.
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A client (caretaker) can request a memento from the originator to save the internal state. It can also pass a memento back to the originator to restore to a previous state.
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## Benefits
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* Does not violate the originator's encapsulation.
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* Keeping the saved state external from the originator helps to maintain cohesion.
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* Provides easy-to-implement recovery capability.
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## Drawbacks
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* Saving and restoring state can be time consuming.
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* It may require lots of memory if clients create mementors too often.
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* Clients should track the originator's lifecycle in order to destroy obsolete mementos.
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## Common Structure
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![Common structure of memento pattern](https://upload.wikimedia.org/wikipedia/commons/3/38/W3sDesign_Memento_Design_Pattern_UML.jpg)
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* Memento
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* is a value object that stores internal state of the Originator object.
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* It is common practice to make Memento immutable and pass it data only once, via constructor.
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* Originator
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* creates a memento containing a snapshot of its current internal state.
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* uses the memento to restore its internal state
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* Caretaker
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* keeps a history of originator's state by storing a stack of mementos
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* never operates on or examines the contents of a memento
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* When originator has to go back in history, the caretaker passes the last memento to the originator's restoration method.
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_[Source: http://www.dofactory.com/net/memento-design-pattern]_
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## Example
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Memento
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```cs
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[Serializable]
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class Memento
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{
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public string State { get; }
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public Memento(String state)
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{
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this.State = state;
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}
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}
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```
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Originator
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```cs
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class Originator
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{
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private String _state;
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public void SetState(String state)
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{
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this._state = state;
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Console.WriteLine($"State is set to {state}");
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}
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public Memento Save()
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{
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Console.WriteLine($"Saving state to Memento");
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return new Memento(this._state);
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}
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public void Restore(Memento m)
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{
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this._state = m.State;
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Console.WriteLine($"State is restored from Memento: {_state}");
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}
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}
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```
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CareTaker
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```cs
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IList<Memento> mementoes = new List<Memento>();
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Originator originator = new Originator();
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originator.SetState("State 1");
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originator.SetState("State 2");
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mementoes.Add(originator.Save()); // checkpoint 1
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originator.SetState("State 3");
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mementoes.Add(originator.Save()); // checkpoint 2
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originator.SetState("State 4");
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originator.Restore(mementoes[0]); // restore to checkpoint 1
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```
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Output
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```
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State is set to State 1
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State is set to State 2
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Saving state to Memento
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State is set to State 3
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Saving state to Memento
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State is set to State 4
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State is restored from Memento: State 2
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```

‎BehavioralPatterns/Program.cs

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using System.Text;
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using System.Threading.Tasks;
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using BehavioralPatterns.Mediator;
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using BehavioralPatterns.Memento;
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namespace BehavioralPatterns
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{
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class Program
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{
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static void Main(string[] args)
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{
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TestMediator.Run();
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Caretaker.Run();
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Console.ReadKey();
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}
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}
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}

‎README.md

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[![Build Status](https://travis-ci.org/tk-codes/DesignPatterns.NET.svg?branch=master)](https://travis-ci.org/tk-codes/DesignPatterns.NET)
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![Language C#](https://img.shields.io/badge/language-c%23-blue.svg)
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![status in progress](https://img.shields.io/badge/status-in%20progress-brightgreen.svg)
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![number of patterns](https://img.shields.io/badge/patterns-13-red.svg)
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![number of patterns](https://img.shields.io/badge/patterns-14-red.svg)
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## Creational Patterns
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| | Interpreter
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| | Iterator
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|:heavy_check_mark: | [Mediator](/BehavioralPatterns/Mediator)|
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| | Memento
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|:heavy_check_mark:| [Memento](/BehavioralPatterns/Memento)|
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| :heavy_check_mark: | [Null Object](/BehavioralPatterns/NullObject) |
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| :heavy_check_mark:| [Observer](/BehavioralPatterns/Observer/) |
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| | State | :warning:
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* MSDN - [Patterns & Practices](https://msdn.microsoft.com/en-us/library/ff921345.aspx)
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* dotFactory - http://www.dofactory.com/net/design-patterns
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* Wikipedia - https://en.wikipedia.org/wiki/Software_design_pattern
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* Refactoring.Guru - https://refactoring.guru/design-patterns

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