finish lecture 11, fix theme not being applied in cli html conversion
- update readmemain
borb
2 weeks ago
parent
0f0335833e
commit
88c8c953c6
File diff suppressed because one or more lines are too long
@ -1,289 +1,448 @@
|
||||
# Delegates and Events
|
||||
|
||||
|
||||
|
||||
---
|
||||
marp: true
|
||||
paginate: true
|
||||
math: mathjax
|
||||
theme: buutti
|
||||
title: N. Delegates and events
|
||||
---
|
||||
|
||||
# Overview
|
||||
|
||||
Delegates
|
||||
# Delegates and events
|
||||
|
||||
Multicast Delegates
|
||||
<!-- headingDivider: 5 -->
|
||||
<!-- class: invert -->
|
||||
|
||||
Anonymous Methods
|
||||
## Overview
|
||||
|
||||
Events
|
||||
* Delegates
|
||||
* Multicast Delegates
|
||||
* Anonymous Methods
|
||||
* Events
|
||||
|
||||
# Delegates
|
||||
## Delegates
|
||||
|
||||
* __Delegates __ are reference type variables that hold a __reference to a method__ or multiple methods
|
||||
* *__Delegates__* are reference type variables that hold a *__reference to a method__* or multiple methods
|
||||
* Class objects hold a reference to a class instance, delegate objects hold a reference to a method / methods
|
||||
* Similar to function pointers in C and C++, or how any function in JavaScript works
|
||||
* Allows for methods to be passed as variables, useful for creating, for example, events
|
||||
|
||||
# Creating a Delegate
|
||||
|
||||
Declare a delegate using the following syntax:
|
||||
|
||||
delegate \<return type> \<delegate name>(\<parameters>);
|
||||
|
||||
Example:
|
||||
|
||||
delegate void PrintDelegate(string output);
|
||||
|
||||
This creates a new delegate of type void, named PrintDelegate and one parameter of type string
|
||||
|
||||
The referenced method return and parameter types have to match the delegate!
|
||||
|
||||
# Referencing a Delegate
|
||||
|
||||
After creating the delegate, it can be instantiated and the method assigned to it with the method name:
|
||||
|
||||
delegate void PrintDelegate(string output);
|
||||
|
||||
static void Main(string[] args)
|
||||
|
||||
{
|
||||
|
||||
void PrintInLower(string text)
|
||||
|
||||
{
|
||||
|
||||
Console.WriteLine(text.ToLower());
|
||||
|
||||
}
|
||||
|
||||
PrintDelegate print = PrintInLower;
|
||||
|
||||
print("AaaBbbCcc"); // Outputs "aaabbbccc"
|
||||
|
||||
}
|
||||
|
||||
# Using Multicast Delegates
|
||||
|
||||
Delegates can be __composed of__ multiple methods using the "+" operator
|
||||
|
||||
Using the same PrintDelegate delegate as before, we could do this:
|
||||
|
||||
delegate void PrintDelegate(string output);
|
||||
|
||||
static void Main(string[] args)
|
||||
|
||||
{
|
||||
|
||||
void PrintInLower(string text)
|
||||
|
||||
{
|
||||
|
||||
Console.WriteLine(text.ToLower());
|
||||
|
||||
}
|
||||
|
||||
void PrintInUpper(string text)
|
||||
|
||||
{
|
||||
|
||||
Console.WriteLine(text.ToUpper());
|
||||
|
||||
}
|
||||
|
||||
PrintDelegate print = PrintInLower;
|
||||
|
||||
print += PrintInUpper;
|
||||
|
||||
print("AaaBbbCcc");
|
||||
|
||||
}
|
||||
### Creating a delegate
|
||||
|
||||
* Declare a delegate using the following syntax:
|
||||
```csharp
|
||||
delegate returnType DelegateName(parameters);
|
||||
```
|
||||
* Example:
|
||||
|
||||
```csharp
|
||||
delegate void PrintDelegate(string output);
|
||||
```
|
||||
* This creates a new delegate of type `void`, named `PrintDelegate` and one parameter of type `string`
|
||||
* The referenced method return and parameter types have to match the delegate!
|
||||
|
||||
### Referencing a delegate
|
||||
|
||||
* After creating the delegate, it can be instantiated and the method assigned to it with the method name:
|
||||
```csharp
|
||||
delegate void PrintDelegate(string output);
|
||||
|
||||
static void Main(string[] args)
|
||||
{
|
||||
void PrintInLower(string text)
|
||||
{
|
||||
Console.WriteLine(text.ToLower());
|
||||
}
|
||||
|
||||
PrintDelegate print = PrintInLower;
|
||||
print("AaaBbbCcc"); // Outputs "aaabbbccc"
|
||||
}
|
||||
```
|
||||
|
||||
### Using multicast delegates
|
||||
|
||||
* Delegates can be composed of multiple methods using the `+` operator
|
||||
* Using the same `PrintDelegate` delegate as before, we could do this:
|
||||
<div class='columns21' markdown='1'>
|
||||
<div markdown='1'>
|
||||
|
||||
```csharp
|
||||
delegate void PrintDelegate(string output);
|
||||
static void Main(string[] args)
|
||||
{
|
||||
void PrintInLower(string text)
|
||||
{
|
||||
Console.WriteLine(text.ToLower());
|
||||
}
|
||||
void PrintInUpper(string text)
|
||||
{
|
||||
Console.WriteLine(text.ToUpper());
|
||||
}
|
||||
PrintDelegate print = PrintInLower;
|
||||
print += PrintInUpper;
|
||||
print("AaaBbbCcc");
|
||||
}
|
||||
```
|
||||
|
||||
</div>
|
||||
<div markdown='1'>
|
||||
|
||||
|
||||
|
||||
# Using Multicast Delegates (continued)
|
||||
|
||||
Methods can be removed from the delegate with the "-" operator:
|
||||
|
||||
delegate void PrintDelegate(string output);
|
||||
|
||||
static void Main(string[] args)
|
||||
|
||||
{
|
||||
|
||||
...
|
||||
|
||||
print -= PrintInLower;
|
||||
|
||||
print("AaaBbbCcc"); // Outputs "AAABBBCCC"
|
||||
|
||||
...
|
||||
|
||||
}
|
||||
</div>
|
||||
</div>
|
||||
|
||||
### Removing methods
|
||||
|
||||
* Methods can be removed from the delegate with the `-` operator:
|
||||
```csharp
|
||||
delegate void PrintDelegate(string output);
|
||||
static void Main(string[] args)
|
||||
{
|
||||
...
|
||||
print -= PrintInLower;
|
||||
print("AaaBbbCcc"); // Outputs "AAABBBCCC"
|
||||
...
|
||||
}
|
||||
```
|
||||
|
||||
### Delegates: An example
|
||||
|
||||
* Let's extend our previous example (without the `-=` part) by creating a new class called `DelegateTest`, and giving it a constructor that takes a `PrintDelegate` object as a parameter:
|
||||
```csharp
|
||||
public class DelegateTest
|
||||
{
|
||||
public DelegateTest(PrintDelegate printDelegate)
|
||||
{
|
||||
printDelegate("This Method Was Called From Another Class!");
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
# Delegates - Example
|
||||
|
||||
Let's extend our previous example (without the "-=" part) by creating a new class called DelegateTest, and giving it a constructor that takes a PrintDelegate object as a parameter:
|
||||
|
||||
public class DelegateTest
|
||||
|
||||
{
|
||||
|
||||
public DelegateTest(PrintDelegate printDelegate)
|
||||
|
||||
{
|
||||
|
||||
printDelegate("This Method Was Called From Another Class!");
|
||||
|
||||
}
|
||||
|
||||
}
|
||||
|
||||
# Delegates - Example (continued)
|
||||
---
|
||||
|
||||
Now we can create a new DelegateTest object and pass the delegate to the object constructor:
|
||||
<div class='columns21' markdown='1'>
|
||||
<div markdown='1'>
|
||||
|
||||
```csharp
|
||||
void PrintInLower(string text)
|
||||
|
||||
{
|
||||
|
||||
Console.WriteLine(text.ToLower());
|
||||
|
||||
Console.WriteLine(text.ToLower());
|
||||
}
|
||||
|
||||
void PrintInUpper(string text)
|
||||
|
||||
{
|
||||
|
||||
Console.WriteLine(text.ToUpper());
|
||||
|
||||
Console.WriteLine(text.ToUpper());
|
||||
}
|
||||
|
||||
// Initialize new delegate which is composed of PrintInLower method
|
||||
|
||||
PrintDelegate print = PrintInLower;
|
||||
|
||||
// Add PrintInUpper method to the delegate
|
||||
|
||||
print += PrintInUpper;
|
||||
|
||||
// Send the delegate to the class constructor
|
||||
|
||||
DelegateTest delegateTest = new DelegateTest(print);
|
||||
```
|
||||
|
||||
|
||||
|
||||
# Anonymous Methods
|
||||
|
||||
Delegates can be initialized anonymously (without a specified name)
|
||||
|
||||
Anonymous method in variable declaration:
|
||||
|
||||
delegate void PrintDelegate(string output);
|
||||
|
||||
bool printUpper = true;
|
||||
|
||||
PrintDelegate printCheckUpper =
|
||||
</div>
|
||||
<div markdown='1'>
|
||||
|
||||
delegate (string text)
|
||||
|
||||
{
|
||||
|
||||
if (printUpper)
|
||||
|
||||
Console.WriteLine(text.ToUpper());
|
||||
|
||||
else
|
||||
Now we can create a new DelegateTest object and pass the delegate to the object constructor:
|
||||
|
||||
Console.WriteLine(text);
|
||||
|
||||
|
||||
};
|
||||
</div>
|
||||
</div>
|
||||
|
||||
printCheckUpper("I'm not angry!"); // Outputs I'M NOT ANGRY!
|
||||
|
||||
Notice that the actual method that prints the text is not declared anywhere
|
||||
### Anonymous methods
|
||||
|
||||
You can use an empty anonymous method to initialize a delegate which does nothing:
|
||||
* Delegates can be initialized anonymously (without a specified name)
|
||||
* Anonymous method in variable declaration:
|
||||
```csharp
|
||||
delegate void PrintDelegate(string output);
|
||||
bool printUpper = true;
|
||||
PrintDelegate printCheckUpper =
|
||||
delegate (string text)
|
||||
{
|
||||
if (printUpper)
|
||||
Console.WriteLine(text.ToUpper());
|
||||
else
|
||||
Console.WriteLine(text);
|
||||
};
|
||||
|
||||
delegate void SomeDelegate();
|
||||
printCheckUpper("I'm not angry!"); // Outputs I'M NOT ANGRY!
|
||||
```
|
||||
* Notice that the actual method that prints the text is not declared anywhere!
|
||||
|
||||
class Program
|
||||
---
|
||||
|
||||
* You can use an empty anonymous method to initialize a delegate that does nothing:
|
||||
```csharp
|
||||
delegate void SomeDelegate();
|
||||
|
||||
class Program
|
||||
{
|
||||
static void Main(string[] args)
|
||||
{
|
||||
// Initialize an empty delegate, add method later...
|
||||
SomeDelegate myDelegate = new SomeDelegate(delegate { });
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
## Events
|
||||
|
||||
### The problem
|
||||
|
||||
<div class='columns' markdown='1'>
|
||||
<div markdown='1'>
|
||||
|
||||
```csharp
|
||||
class Game
|
||||
{
|
||||
Sound gameOverSound;
|
||||
Window gameOverScreen;
|
||||
|
||||
void OnGameOver()
|
||||
{
|
||||
gameOverSound.Play(); // plays some sound
|
||||
gameOverScreen.Show(); // shows a screen
|
||||
}
|
||||
}
|
||||
...
|
||||
class Program
|
||||
{
|
||||
static void Main()
|
||||
{
|
||||
var game = new Game();
|
||||
|
||||
// somewhere in the game logic...
|
||||
// game.OnGameOver();
|
||||
}
|
||||
}
|
||||
```
|
||||
</div>
|
||||
<div markdown='1'>
|
||||
|
||||
* Consider a game engine with three classes, `Sound`, `Window` and `Game`.
|
||||
* If implemented like this, the `Game` class has to know about the `Sound` and `Window` classes
|
||||
* $\Rightarrow$ `Game` is ***tightly coupled***, and thus ***dependent*** on the `Sound` and `Window` classes
|
||||
* Changes in either of the classes could break the code!
|
||||
|
||||
</div>
|
||||
</div>
|
||||
|
||||
### The solution: Events
|
||||
|
||||
* A solution to this problem is the [Publisher-subscriber pattern](https://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern)
|
||||
* In C#, this pattern is implemented with the [`event` keyword](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/event?redirectedfrom=MSDN)
|
||||
* Events are ***signals*** that are raised by a *__Publisher__* and received by a *__Subscriber__*
|
||||
* The publisher does not know or care who, if anyone, receives the signal
|
||||
* Changes in the subscriber classes do not affect the publisher
|
||||
* In C#, events are multicast delegates
|
||||
* When an object triggers an event, the event invokes ***event handlers***
|
||||
* Event handlers are delegate instances added to the event
|
||||
|
||||
### Raising events
|
||||
|
||||
* Events consist of two elements:
|
||||
* A delegate that identifies the method that provides the response to the event
|
||||
* In the simplest case, we can use the built-in [`EventHandler`](https://learn.microsoft.com/en-us/dotnet/api/system.eventhandler?view=net-9.0) delegate
|
||||
* An optional class to hold event data if the event provides data.
|
||||
```csharp
|
||||
public class Publisher
|
||||
{
|
||||
public event EventHandler SampleEvent;
|
||||
// Wrap the event in a protected virtual method
|
||||
// to enable derived classes to raise the event.
|
||||
protected virtual void OnSampleEvent()
|
||||
{
|
||||
// Raise the event in a thread-safe manner using the ?. operator.
|
||||
SampleEvent?.Invoke(this);
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
### Raising the event
|
||||
|
||||
Events can only be invoked (i.e., raised) from within the class (or derived classes) or struct where they're declared (the publisher class)
|
||||
|
||||
<div class='columns' markdown='1'>
|
||||
<div markdown='1'>
|
||||
|
||||
```csharp
|
||||
public class Publisher
|
||||
{
|
||||
public event EventHandler SampleEvent;
|
||||
protected virtual void OnSampleEvent()
|
||||
{
|
||||
SampleEvent?.Invoke(this);
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
static void Main(string[] args)
|
||||
</div>
|
||||
<div markdown='1'>
|
||||
|
||||
```csharp
|
||||
class Program
|
||||
{
|
||||
static void Main()
|
||||
{
|
||||
var pub = new Publisher();
|
||||
|
||||
// Initialize an empty delegate, add method later...
|
||||
|
||||
SomeDelegate myDelegate = new SomeDelegate(delegate { });
|
||||
|
||||
pub.OnSampleEvent(); // ✅ Works fine
|
||||
// pub.SampleEvent(); // ❌ Not allowed!
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
}
|
||||
|
||||
# Events
|
||||
|
||||
* Consider the following Game class:
|
||||
* class Game
|
||||
* {
|
||||
* Sound gameOverSound;
|
||||
* Window gameOverScreen;
|
||||
* // Some game logic here...
|
||||
* // ...
|
||||
* void OnGameOver()
|
||||
* {
|
||||
* gameOverSound.Play(); // This plays some sound
|
||||
* gameOverScreen.Show(); // This shows some screen
|
||||
* }
|
||||
* }
|
||||
* This raises a couple of problems:
|
||||
* The Game class has to know about the Sound and Window classes
|
||||
* Changes in either of the classes could break the code
|
||||
* = Game is dependent of Sound and Window classes
|
||||
---
|
||||
|
||||
Tight coupling
|
||||
</div>
|
||||
</div>
|
||||
|
||||
# Events (continued)
|
||||
### Adding subscribers to the event
|
||||
|
||||
Events are signals that are raised by a __Publisher __ and received by a __Subscriber__
|
||||
* Now that we know how to raise the event, we can add subscribers to it
|
||||
* i.e., functions that get called when the event is raised
|
||||
|
||||
The publisher does not know or care what, if any, object receives the signal
|
||||
|
||||
Changes in the subscriber classes do not affect the publisher
|
||||
|
||||
# Events - Example
|
||||
|
||||
Events are created in two steps:
|
||||
|
||||
Declare a delegate
|
||||
|
||||
Declare a variable of the delegate with the event keyword
|
||||
|
||||
public delegate void GameOver();
|
||||
|
||||
class Game
|
||||
<div class='columns' markdown='1'>
|
||||
<div markdown='1'>
|
||||
|
||||
```csharp
|
||||
public class Publisher
|
||||
{
|
||||
|
||||
public event GameOver GameOverEventHandler;
|
||||
|
||||
// Some game logic here...
|
||||
|
||||
// ...
|
||||
|
||||
public event EventHandler SampleEvent;
|
||||
protected virtual void OnSampleEvent()
|
||||
{
|
||||
SampleEvent?.Invoke(this);
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
# Exercise 1: A Rudimentary Event System
|
||||
|
||||
Create a console application for controlling a plant treatment system, that has three methods which print the following outputs:
|
||||
</div>
|
||||
<div markdown='1'>
|
||||
|
||||
| void ReleaseWater() | Releasing water... |
|
||||
| :-: | :-: |
|
||||
| void ReleaseFertilizer() | Releasing fertilizer... |
|
||||
| void IncreaseTemperature() | Increasing temperature... |
|
||||
```csharp {7-8}
|
||||
class Program
|
||||
{
|
||||
static void Main()
|
||||
{
|
||||
var pub = new Publisher();
|
||||
|
||||
pub.SampleEvent +=
|
||||
() => Console.WriteLine("Sample event!");
|
||||
|
||||
pub.OnSampleEvent();
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
</div>
|
||||
</div>
|
||||
|
||||
|
||||
### Custom event handler
|
||||
|
||||
* We can define the delegate ourselves, for example if we want to send some data to the event.
|
||||
```csharp
|
||||
public class Publisher
|
||||
{
|
||||
public delegate void SampleEventHandler(object sender);
|
||||
|
||||
// Declare the event.
|
||||
public event SampleEventHandler SampleEvent;
|
||||
|
||||
protected virtual void OnSampleEvent()
|
||||
{
|
||||
SampleEvent?.Invoke(this);
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
### Event arguments
|
||||
|
||||
* Finally, we can send arguments to the event like this:
|
||||
```csharp
|
||||
public class SampleEventArgs
|
||||
{
|
||||
public SampleEventArgs(string text) { Text = text; }
|
||||
public string Text { get; } // readonly
|
||||
}
|
||||
|
||||
public class Publisher
|
||||
{
|
||||
public delegate void SampleEventHandler(object sender, SampleEventArgs e);
|
||||
|
||||
public event SampleEventHandler SampleEvent;
|
||||
|
||||
protected virtual void RaiseSampleEvent()
|
||||
{
|
||||
SampleEvent?.Invoke(this, new SampleEventArgs("Hello"));
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
|
||||
### "Fixing" the Game Over example
|
||||
|
||||
<div class='columns' markdown='1'>
|
||||
<div markdown='1'>
|
||||
|
||||
```csharp
|
||||
public delegate void GameOverHandler();
|
||||
class Game
|
||||
{
|
||||
public event GameOverHandler GameOver;
|
||||
|
||||
protected virtual void OnGameOver()
|
||||
{
|
||||
GameOver?.Invoke(this);
|
||||
}
|
||||
}
|
||||
...
|
||||
class Program
|
||||
{
|
||||
static void Main()
|
||||
{
|
||||
var game = new Game();
|
||||
Sound gameOverSound;
|
||||
Window gameOverScreen;
|
||||
|
||||
game.GameOver += gameOverSound.Play;
|
||||
game.GameOver += gameOverScreen.Show;
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
</div>
|
||||
<div markdown='1'>
|
||||
|
||||
1) Declare an event handler delegate (`GameOverHandler`)
|
||||
2) Declare an instance of the handler with the `event` keyword (`GameOVer`)
|
||||
3) Declare a virtual method that invokes the event
|
||||
4) Add methods that subscribe to the event
|
||||
|
||||
</div>
|
||||
</div>
|
||||
|
||||
## Exercise 1: A rudimentary event system
|
||||
<!--_class: "exercise invert" -->
|
||||
|
||||
Create a console application for controlling a plant treatment system with three methods that print the following outputs:
|
||||
|
||||
| Method | Output |
|
||||
| :-- | :-- |
|
||||
| `void ReleaseWater()` | `Releasing water...` |
|
||||
| `void ReleaseFertilizer()` | `Releasing fertilizer...` |
|
||||
| `void IncreaseTemperature()` | `Increasing temperature...` |
|
||||
|
||||
* All methods are off by default. Create a main loop where the user can...
|
||||
* ...type the name of the method to switch each method on (add it to the delegate)
|
||||
* ...type `run` to execute all the methods that are on
|
||||
|
||||
Create a main loop where the user can switch each method on (= add to a delegate) by writing its name. All methods are off by default. (Hint: you can just use switch-case for defining which method should be added to the delegate)
|
||||
---
|
||||
|
||||
If the user types run, all the methods that are on (= added to the delegate), will be executed.
|
||||
<!--_class: "exercise invert" -->
|
||||
|
||||
|
||||
* ***Hint:*** you can just use switch-case for defining which method should be added to the delegate
|
||||
|
||||
* Here's an example console input & output:
|
||||
|
||||
|
||||
Loading…
Reference in New Issue