From 2b07530bb32205b2886b04b8135d57674bae2cb8 Mon Sep 17 00:00:00 2001
From: Jason Zhu <jasonzhuyq@outlook.com>
Date: Mon, 22 Feb 2021 11:25:40 +0000
Subject: [PATCH] Finished Defining an Event & Subscribing to an Event

---
 gradebook/src/GradeBook/Book.cs    | 10 ++++++++++
 gradebook/src/GradeBook/Program.cs | 11 +++++++++++
 2 files changed, 21 insertions(+)

diff --git a/gradebook/src/GradeBook/Book.cs b/gradebook/src/GradeBook/Book.cs
index ceafed4..d45358e 100644
--- a/gradebook/src/GradeBook/Book.cs
+++ b/gradebook/src/GradeBook/Book.cs
@@ -3,6 +3,8 @@ using System.Collections.Generic;
 
 namespace GradeBook
 {
+    public delegate void GradeAddedDelegate(object sender, EventArgs args);
+
     public class Book
     {
         private List<double> grades;
@@ -27,6 +29,12 @@ namespace GradeBook
             if (grade <= 100 && grade >= 0)
             {
                 this.grades.Add(grade);
+
+                if (GradeAdded != null)
+                {
+                    // Somebody is listening/subscribing
+                    GradeAdded(this, new EventArgs()); // sender = this, pass; Raising event
+                }
             } else
             {
                 throw new ArgumentException($"Invalid {nameof(grade)}");
@@ -52,6 +60,8 @@ namespace GradeBook
             }
         }
 
+        public event GradeAddedDelegate GradeAdded;
+
         public Statistics GetStatistics()
         {
             var result = new Statistics();
diff --git a/gradebook/src/GradeBook/Program.cs b/gradebook/src/GradeBook/Program.cs
index 45ab748..54de553 100644
--- a/gradebook/src/GradeBook/Program.cs
+++ b/gradebook/src/GradeBook/Program.cs
@@ -14,6 +14,11 @@ namespace GradeBook
             // book.AddGrade(87.2);
 
             var book = new Book("Jason's Grade Book");
+            book.GradeAdded += OnGradeAdded;
+            book.GradeAdded += OnGradeAdded;
+            book.GradeAdded -= OnGradeAdded;
+            book.GradeAdded -= OnGradeAdded;
+
             string input;
 
             while (true)
@@ -54,5 +59,11 @@ namespace GradeBook
             Console.WriteLine($"The highesst grade is {stats.highGrade}");
             Console.WriteLine($"The average grade is {stats.Average:N1}");
         }
+
+        static void OnGradeAdded(object sender, EventArgs e)
+        {
+            // Main is static, so it can only call static methods, hence return type is static
+            Console.WriteLine("A line has added");
+        }
     }
 }