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C# Fundamentals - Events

Events

To understand what Events are you have to know what Delegates are and how they function ( that was actually a pun - learn and come back ), A brief tutorial on Delegates can be found HERE.
Events can be stuff like key presses, player deaths, achievement unlocks, clicks,  mouse movements, etc..., or something like system generated notifications like window resize or minimize.
Events are just basically delegates with some extra protection stuff thrown in to prevent accidents.
We will go over these 'accidents' later on in this tutorial.
Events unlike delegates can not be declared outside the class body  as it is an instance of a delegate and instances can not exist outside a class body. These events are associated with event handlers by using delegates which may exist in the same class or some other class.
The class which contains the event notifies all other class about whether the event occurred.
This class is called the 'Publisher' class. Those classes that accept the event are known as a 'Subscriber' class.
Events greatly improve code quality in a number of ways:
  • Improves modularity of code.
  • Improves the 'One Class, One Job' aspect of the code.
  • Improves code maintainability if used reasonably 😛.
Syntax:
[<modifiers>] event [Delegate Type] <Name>
There are some things that Events prevents us from doing that provides additional protection from ourselves while writing code. The basic functionality of an Event can be done by just using a delegate but that leads to various situations that are undesirable like:
  • When assigning a delegate directly to a method all previous chaining of  methods are now removed.So to prevent this Events only use += & -= and no direct assignment.
  • Delegates can be called directly from the object that it's a part of, Like 'classObj.myDelegate()'. But Events do not allow that and prevents a lot of situations where you might shoot yourself in the foot.
Let's now look at a real-world example of how this might be used:
using System;

namespace Rextester
{
    public class GameManager
    {        
        public void AddPlayer(Player player)
        {
            player.achievementUnlocked += ShowAchievements;
            player.lostGame += PlayerLost;
        }
        private void ShowAchievements(Player player)
        {
            Console.WriteLine("Got Achievement : " + player.name);
        }
        private void PlayerLost(Player player)
        {
            player.achievementUnlocked -= ShowAchievements;
            player.lostGame -= PlayerLost;
            Console.WriteLine(player.name + " lost and is no longer in game");
        }
    }
    public class Player
    {
        public delegate void PlayerDelegate(Player player);
        public event PlayerDelegate achievementUnlocked;
        public event PlayerDelegate lostGame;
        
        private string name = "";
        public Player(string name) { this.name = name; }
        public void GotAchievement()
        {
            if(achievementUnlocked != null)
                achievementUnlocked(this);
        }
        public void LostGame()
        {
            if(lostGame != null)
                lostGame(this);
        }
    }
    
    public class Program
    {
        public static void Main(string[] args)
        {
           GameManager gameManager = new GameManager();
            
           Player player1 = new Player("Rick");
           Player player2 = new Player("Morty");
           Player player3 = new Player("Hideo Kojima");            
           
           gameManager.AddPlayer(player1);
           gameManager.AddPlayer(player2);
           gameManager.AddPlayer(player3);
            
           player1.GotAchievement();
           player2.GotAchievement();
           player3.GotAchievement();
            
           player2.LostGame();
            
           player1.GotAchievement();
           player2.GotAchievement();
           player3.GotAchievement();
            
           player1.LostGame();
            
           player1.GotAchievement();
           player2.GotAchievement();
           player3.GotAchievement();
        }
    }
}
Output :
Got Achievement : Rick
Got Achievement : Morty
Got Achievement : Hideo Kojima
Morty lost and is no longer in game
Got Achievement : Rick
Got Achievement : Hideo Kojima
Rick lost and is no longer in game
Got Achievement : Hideo Kojima
We will now break down all the important bits.
First the Player class:
public class Player
{
    public delegate void PlayerDelegate(Player player);
    public event PlayerDelegate achievementUnlocked;
    public event PlayerDelegate lostGame;
        
    private string name = "";
    public Player(string name) { this.name = name; }
    public void GotAchievement()
    {
        if(achievementUnlocked != null)
             achievementUnlocked(this);
    }
    public void LostGame()
    {
        if(lostGame != null)
             lostGame(this);
    }
}
We declared a delegate called PlayerDelegate which returns void and takes parameter of  Player.
Then we declared two events, both of same delegate type, one for when an achievement is unlocked and another when the player losses the game.
public void GotAchievement()
{
    if(achievementUnlocked != null)
          achievementUnlocked(this);
}
public void LostGame()
{
    if(lostGame != null)
          lostGame(this);
}
We are invoking each of the events but before we do, we are checking if anything is actually subscribed to it; That's why the null check is there.
Now time to look at the GameManager class:
public class GameManager
{        
    public void AddPlayer(Player player)
    {
        player.achievementUnlocked += ShowAchievements;
        player.lostGame += PlayerLost;
    }
    private void ShowAchievements(Player player)
    {
        Console.WriteLine("Got Achievement : " + player.name);
    }
    private void PlayerLost(Player player)
    {
        player.achievementUnlocked -= ShowAchievements;
        player.lostGame -= PlayerLost;
        Console.WriteLine(player.name + " lost and is no longer in game");
    }
}
We have a public AddPlayer method which takes in a Player object and then makes the 'ShowAchievemets' & 'PlayerLost' method subscribe to it.
Now let's take a closer look at the 'PlayerLost' method.
private void PlayerLost(Player player)
{
    player.achievementUnlocked -= ShowAchievements;
    player.lostGame -= PlayerLost;
    Console.WriteLine(player.name + " lost and is no longer in game");
}
This method takes in a player and unsubscribes it from the two methods, so after this, any calls from 'achievementUnlocked' & 'lostGame' will not invoke the ShowAchievements & PlayerLost method unless the player object is added back to the GameManager again as well.
So now you know a little bit more ( hopefully ) about how  Events in C# work.
For More C# Tutorials, go HERE.
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