Example of observer mode of C -- ordering milk

One , Introduction to observer mode

1.1 Observer mode definition

Intention ： Defines a one-to-many dependency between objects , When the state of an object changes , All objects that depend on it can be notified and updated automatically .

Applicability ：

1. When the state of one object changes, other objects need to be changed , Or when the actual object is unknown or dynamically changing in advance , You can use observer mode .
2. When some objects in the application observe other objects , You can use this pattern . Subscription lists are dynamic , Therefore, subscribers can join or leave this list at any time .

1.2 Observer pattern structure

• Publisher ： Will send interesting events to other objects . Events occur after the publisher's own state changes or performs a specific behavior . The publisher contains a subscription framework that allows new subscribers to join and current subscribers to leave the list .

  When something new happens , The sender will traverse the subscription list and call the notification method of each subscriber object . The method is declared in the subscriber interface .

• subscriber ： The interface declares the notification interface . In the vast majority of cases , The interface contains only one update Update method . The method can have multiple parameters , Enable publishers to pass event details when updating .

• Specific subscribers ： You can do something to respond to a publisher's notification . All concrete subscriber classes implement the same interface , Therefore, publishers do not need to be coupled to specific classes .

  Subscribers often need some context information to properly handle updates . therefore , Publishers usually pass some context data as parameters of notification methods . Publishers can also pass themselves as parameters , Enable subscribers to directly obtain the required data .

• client ： The publisher and subscriber objects are created separately , Then register publisher updates for subscribers .

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Two , Example of ordering milk

2.1 Problem description

If you subscribe to a milk , Then there is no need to go to the dairy to buy milk . Milk station （ That is... In application “ Publisher ”） After the milk arrives （ Even ahead of time ） Send the milk directly to your home .

The milk station is responsible for maintaining the subscriber list , Find out what kind of milk subscribers are interested in . When subscribers want to stop subscribing to milk , They can withdraw from this list at any time .

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2.2 Realize the idea

1. Declare the subscriber interface . This interface declares a notification method .
2. Declare the publisher interface and define some interfaces to add and delete subscription objects in the list .
3. Create a specific publisher class . Every time a publisher has an important event, all subscribers must be notified .
4. Implement the method of notification update in the specific subscriber class .

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2.3 The sample code

using System;
using System.Collections.Generic;

namespace MilkDemo
{
    
    //  Observer interface 
    public interface IObserver
    {
    
        //  Observer properties  -->  Names and things like that  
        string Name {
     get; set; }

        //  Observer method  -->  Receive notice （ Called ）
        void ReceiveMilk(IMilkStation subject);
    }
  
    //  subscribers A
    class ObserverUserA : IObserver
    {
    
        public string Name
        {
    
            get; set;
        }

        public ObserverUserA(string name)
        {
    
            this.Name = name;
        }

        public void ReceiveMilk(IMilkStation subject)
        {
    
            Console.WriteLine(this.Name + "： I got it. , Subscribed milk ");
            Console.WriteLine(this.Name + " Take a look at the quantity of remaining milk in the remaining milk station ：" + subject.MilkCount);
        }
    }

    //  subscribers 
    class ObserverUserB : IObserver
    {
    
        public string Name
        {
    
            get; set;
        }

        public ObserverUserB(string name)
        {
    
            this.Name = name;
        }

        public void ReceiveMilk(IMilkStation subject)
        {
    
            Console.WriteLine(this.Name + "： I got it. , Subscribed milk ");
        }
    }
    
	//  Milk station interface 
    public interface IMilkStation
    {
    
        int MilkCount {
     get; set; }

        //  Order milk  -->  Add you to the milk delivery list （ Push notification ）-->  Attach the observer to the subject 
        void Attach(IObserver observer);

        //  Unsubscribe milk  -->  Remove from the remove delivery milk list  -->  Remove the observer from the subject 
        void Detach(IObserver observer);

        //  Deliver milk  -->  Notify observer 
        void Distribution();
    }

    /// <summary>
    ///  Milk delivery clerk 
    ///  Deliver milk according to the milk list ordered by the milk station  -->  Notify observer 
    /// </summary>
    public class MilkCourier : IMilkStation
    {
           
        //  The amount of milk left in the milk station 
        public int MilkCount {
     get; set; }

        //  All customers who order milk  -->  Observers who need to be informed  
        private List<IObserver> observerList = new List<IObserver>();

        public MilkCourier(int milkCount)
        {
    
            this.MilkCount = milkCount;
        }

        //  How to order milk  --  Add observer implementation 
        public void Attach(IObserver observer)
        {
    
            Console.WriteLine(" Milk station ： Received a customer ordering milk ");
            this.observerList.Add(observer);
        }

        //  Unsubscribe method  -->  Remove the observer implementation 
        public void Detach(IObserver observer)
        {
    
            this.observerList.Remove(observer);
            Console.WriteLine(" Milk station ： Received a user unsubscribe ");
        }

        //  Trigger updates in each subscriber .
        public void Distribution()
        {
    
            Console.WriteLine(" Milk station ： Milk delivery ...");
            foreach (var observer in observerList)
            {
    
                observer.ReceiveMilk(this);
            }
        }

        public void SomeLogical()
        {
    
            if (MilkCount > observerList.Count)
            {
    
                Console.WriteLine(" The amount of milk left in the milk station : " + this.MilkCount);
                this.MilkCount -= observerList.Count;
                this.Distribution();
                Console.WriteLine(" The remaining quantity after this delivery : " + this.MilkCount);
            }
            else
            {
    
                Console.WriteLine(" The amount of milk left in the milk station : " + this.MilkCount + ", Not enough to deliver all subscribers ;");
            }
        }
    }

    class Program
    {
    
        static void Main(string[] args)
        {
    
            //  Initialize the milk station 
            MilkCourier milkCourier = new MilkCourier(10);

            //  Initialize user A,B  And order milk 
            IObserver observerA = new ObserverUserA(" Chen will do what he says ");
            milkCourier.Attach(observerA);

            IObserver observerB = new ObserverUserB("Czhenya");
            milkCourier.Attach(observerB);

            //  Milk station delivery 
            milkCourier.SomeLogical();

            Console.WriteLine();
            Console.WriteLine("--------  After a period of simulation  ----------");
            Console.WriteLine();

            //  user B unsubscribe 
            milkCourier.Detach(observerB);

            //  Milk station delivery 
            milkCourier.SomeLogical();

            Console.Read();
        }
    }
}

Running results ：