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 😛.
[<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.
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();
}
}
}
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 KojimaWe 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);
}
}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);
}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");
}
}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");
}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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