Super-simple example of C observer observable with delegates

Question

I recently started digging into C  but I can t by my life figure out how delegates work when implementing the observer observable pattern in the language   Could someone give me a super-simple example of how it is done  I have googled this  but all of the examples I found were either too problem-specific or too  bloated

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Applying the Observer Pattern with delegates and events in c  is named  Event Pattern  according to MSDN which is a slight variation   In this Article you will find well structured examples of how to apply the pattern in c  both the classic way and using delegates and events   Exploring the Observer Design Pattern  public class Stock          declare a delegate for the event     public delegate void AskPriceChangedHandler object sender            AskPriceChangedEventArgs e         declare the event using the delegate     public event AskPriceChangedHandler AskPriceChanged         instance variable for ask price     object  askPrice         property for ask price     public object AskPrice                set                         set the instance variable              askPrice   value                 fire the event             OnAskPriceChanged                      AskPrice property        method to fire event delegate with proper name     protected void OnAskPriceChanged                  AskPriceChanged this  new AskPriceChangedEventArgs  askPrice            AskPriceChanged     Stock class    specialized event class for the askpricechanged event public class AskPriceChangedEventArgs   EventArgs          instance variable to store the ask price     private object  askPrice         constructor that sets askprice     public AskPriceChangedEventArgs object askPrice     askPrice   askPrice           public property for the ask price     public object AskPrice   get   return  askPrice          AskPriceChangedEventArgs

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Here s a simple example   public class ObservableClass       private Int32  Value       public Int32 Value               get   return  Value            set                       if   Value    value                                 Value   value                  OnValueChanged                                       public event EventHandler ValueChanged       protected void OnValueChanged                 if  ValueChanged    null              ValueChanged this  EventArgs Empty            public class ObserverClass       public ObserverClass ObservableClass observable                observable ValueChanged    TheValueChanged             private void TheValueChanged Object sender  EventArgs e                Console Out WriteLine  Value changed to                   ObservableClass sender  Value            public class Program       public static void Main                 ObservableClass observable   new ObservableClass            ObserverClass observer   new ObserverClass observable           observable Value   10            Note    This violates a rule in that I don t unhook the observer from the observable  this is perhaps good enough for this simple example  but make sure you don t keep observers hanging off of your events like that  A way to handle this would be to make ObserverClass IDisposable  and let the  Dispose method do the opposite of the code in the constructor No error-checking performed  at least a null-check should be done in the constructor of the ObserverClass

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In this model  you have publishers who will do some logic and publish an  event    Publishers will then send out their event only to subscribers who have subscribed to receive the specific event     In C   any object can publish a set of events to which other applications can subscribe  When the publishing class raises an event  all the subscribed applications are notified   The following figure shows this mechanism     Simplest Example possible on Events and Delegates in C      code is self explanatory  Also I ve added the comments to clear out the code     using System   public class Publisher   main publisher class which will invoke methods of all subscriber classes       public delegate void TickHandler Publisher m  EventArgs e     declaring a delegate     public TickHandler Tick        creating an object of delegate     public EventArgs e   null      set 2nd paramter empty     public void Start         starting point of thread               while  true                        System Threading Thread Sleep 300               if  Tick    null      check if delegate object points to any listener classes method                               Tick this  e      if it points i e  not null then invoke that method                                   public class Subscriber1                  1st subscriber class       public void Subscribe Publisher m     get the object of pubisher class               m Tick    HeardIt                 attach listener class method to publisher class delegate object           private void HeardIt Publisher m  EventArgs e      subscriber class method               System Console WriteLine  Heard It by Listener             public class Subscriber2                     2nd subscriber class       public void Subscribe2 Publisher m       get the object of pubisher class               m Tick    HeardIt                  attach listener class method to publisher class delegate object           private void HeardIt Publisher m  EventArgs e      subscriber class method               System Console WriteLine  Heard It by Listener2              class Test       static void Main                 Publisher m   new Publisher           create an object of publisher class which will later be passed on subscriber classes         Subscriber1 l   new Subscriber1       create object of 1st subscriber class         Subscriber2 l2   new Subscriber2      create object of 2nd subscriber class         l Subscribe m         we pass object of publisher class to access delegate of publisher class         l2 Subscribe2 m       we pass object of publisher class to access delegate of publisher class          m Start               starting point of publisher class           Output     Heard It by Listener    Heard It by Listener2    Heard It by Listener      Heard It by Listener2    Heard It by Listener        infinite times

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I ve tied together a couple of the great examples above  thank you as always to Mr  Skeet and Mr  Karlsen  to include a couple of different Observables and utilized an interface to keep track of them in the Observer and allowed the Observer to to  observe  any number of Observables via an internal list   namespace ObservablePattern       using System      using System Collections Generic       internal static class Program               private static void Main                         var observable   new Observable                var anotherObservable   new AnotherObservable                 using  IObserver observer   new Observer observable                                 observable DoSomething                    observer Add anotherObservable                   anotherObservable DoSomething                               Console ReadLine                         internal interface IObservable               event EventHandler SomethingHappened             internal sealed class Observable   IObservable               public event EventHandler SomethingHappened           public void DoSomething                         var handler   this SomethingHappened               Console WriteLine  About to do something                 if  handler    null                                handler this  EventArgs Empty                                      internal sealed class AnotherObservable   IObservable               public event EventHandler SomethingHappened           public void DoSomething                         var handler   this SomethingHappened               Console WriteLine  About to do something different                 if  handler    null                                handler this  EventArgs Empty                                      internal interface IObserver   IDisposable               void Add IObservable observable            void Remove IObservable observable              internal sealed class Observer   IObserver               private readonly Lazy lt IList lt IObservable gt  gt  observables               new Lazy lt IList lt IObservable gt  gt       gt  new List lt IObservable gt               public Observer                                public Observer IObservable observable    this                         this Add observable                      public void Add IObservable observable                        if  observable    null                                return                             lock  this observables                                this observables Value Add observable                   observable SomethingHappened    HandleEvent                                   public void Remove IObservable observable                        if  observable    null                                return                             lock  this observables                                observable SomethingHappened -  HandleEvent                  this observables Value Remove observable                                    public void Dispose                         for  var i   this observables Value Count - 1  i  gt   0  i--                                this Remove this observables Value i                                     private static void HandleEvent object sender  EventArgs args                        Console WriteLine  Something happened to     sender

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I did t want to change my source code to add additional observer   so I have written following simple example     EVENT DRIVEN OBSERVER PATTERN public class Publisher       public Publisher                 var observable   new Observable            observable PublishData  Hello World                Server will send data to this class s PublishData method public class Observable       public event Receive OnReceive       public void PublishData string data                  Add all the observer below           1st observer         IObserver iObserver   new Observer1            this OnReceive    iObserver ReceiveData            2nd observer         IObserver iObserver2   new Observer2            this OnReceive    iObserver2 ReceiveData             publish data          var handler   OnReceive          if  handler    null                        handler data                      public interface IObserver       void ReceiveData string data        Observer example public class Observer1   IObserver       public void ReceiveData string data                  sample observers does nothing with data             public class Observer2   IObserver       public void ReceiveData string data                  sample observers does nothing with data

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The observer pattern is usually implemented with events   Here s an example   using System   class Observable       public event EventHandler SomethingHappened       public void DoSomething     gt          SomethingHappened  Invoke this  EventArgs Empty      class Observer       public void HandleEvent object sender  EventArgs args                Console WriteLine  Something happened to     sender            class Test       static void Main                 Observable observable   new Observable            Observer observer   new Observer            observable SomethingHappened    observer HandleEvent           observable DoSomething              See the linked article for a lot more detail   Note that the above example uses C  6 null-conditional operator to implement DoSomething safely to handle cases where SomethingHappened has not been subscribed to  and is therefore null  If you re using an older version of C   you d need code like this   public void DoSomething         var handler   SomethingHappened      if  handler    null                handler this  EventArgs Empty

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Something like this      interface implementation publisher public delegate void eiSubjectEventHandler eiSubject subject    public interface eiSubject       event eiSubjectEventHandler OnUpdate       void GenereteEventUpdate           class implementation publisher class ecSubject   eiSubject       private event eiSubjectEventHandler  OnUpdate   null      public event eiSubjectEventHandler OnUpdate               add                       lock  this                                 OnUpdate -  value                   OnUpdate    value                                  remove   lock  this     OnUpdate -  value                 public void GenereteEventUpdate                 eiSubjectEventHandler handler    OnUpdate           if  handler    null                        handler this                          interface implementation subscriber public interface eiObserver       void DoOnUpdate eiSubject subject          class implementation subscriber class ecObserver   eiObserver       public virtual void DoOnUpdate eiSubject subject                    observer pattern C  with event   link to the repository

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Simple Example                               class Program                       static void Main string   args                                Parent p   new Parent                                                                                           public delegate void DelegateName string data            class Child                       public event DelegateName delegateName               public void call                                 delegateName  Narottam                                                                                          class Parent                       public Parent                                 Child c   new Child                    c delegateName    new DelegateName print                     or like this                   c delegateName    print                  c call                               public void print string name                                Console WriteLine  yes we got the name       name

[c#] Super-simple example of C# observer/observable with delegates

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