C# Event (事件)

之前我只寫過 UEFI 的事件，UEFI 的事件比 C# 簡單多了。C# Event 的重點在於 Handler, Callback and Listener。 Handler 處理事件的訂閱和 Callback，Callback 是事件觸發時要做的事，Listener 會監聽事件並捕獲事件。

下面的例子中 waiter 會通知客人去用餐，客人會等待被通知要吃午餐或晚餐。用餐的內容是一個等待被呼喚的事件，觸發事件時顧客也會取得用餐的資訊。

using System;

namespace DotNetCoreTest
{
    class Program
    {
　　　　// create a callback.
        public class MealInfo : EventArgs
        {
            public MealInfo (string meal)
            {
                Meal = meal;
            }
            public string Meal { get; set; }       
        }

        // Handle subscription and invoke the callback.
        public class Waiter
        {
            public EventHandler MealIsReady;

            public void InformCustomer(string meal)
            {
                MealIsReady?.Invoke(this, new MealInfo(meal));
            }
        }

        // Listen to the event.
        public class Customer
        {
           private string _name;
 
            public Customer(string name)
            {
                Name = name;
            }

            public string Name
            {
                get { return _name; }
                set { _name = value; }
            }

            // Linsten to an event.
            public void EatMeal(object waiter, MealInfo e)
            {
                Console.WriteLine($"{_name} is informed to have a {e.Meal}");
            }
        }

        static void Main(string[] args)
        {
            var waiter = new Waiter();
            var Bob = new Customer("Bob");
            var Alice = new Customer("Alice");
            // Subscribe the event.
            waiter.MealIsReady += Bob.EatMeal;
            waiter.MealIsReady += Alice.EatMeal;

            // Trigger event.
            waiter.InformCustomer("lunch");
            waiter.InformCustomer("dinner");
        }
    }
}