EventAndDelegate.txt

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20200622

Delegates Overview
Delegates have the following properties:
Delegates are similar to C++ function pointers, but delegates are fully object-oriented, and unlike C++ pointers to member functions, delegates encapsulate both an object instance and a method.
Delegates allow methods to be passed as parameters.
Delegates can be used to define callback methods.
Delegates can be chained together; for example, multiple methods can be called on a single event.
Methods do not have to match the delegate type exactly. For more information, see Using Variance in Delegates.
C# version 2.0 introduced the concept of anonymous methods, which allow code blocks to be passed as parameters in place of a separately defined method. C# 3.0 introduced lambda expressions as a more concise way of writing inline code blocks. Both anonymous methods and lambda expressions (in certain contexts) are compiled to delegate types. Together, these features are now known as anonymous functions. For more information about lambda expressions, see Lambda expressions.

public delegate void Del(string message);  // 宣告 delegate 型別為 Del (習慣是將字尾命名為 Callback, Handler, Delegate...)

// Create a method for a delegate.
public static void DelegateMethod(string message)  // 建立(同簽名式的對應函數)
{
    Console.WriteLine(message);
}

// Instantiate the delegate.
Del handler = DelegateMethod; // 建立變數並指定實體.

// Call the delegate.
handler("Hello World");  // 呼叫執行代理實體函數1.

public static void MethodWithCallback(int param1, int param2, Del callback)  // 參數為 delegate 的範例. 
{
    callback("The number is: " + (param1 + param2).ToString());
}

MethodWithCallback(1, 2, handler); // // 呼叫執行代理實體函數2.


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20200622

Events Overview
Events have the following properties:
The publisher determines when an event is raised; the subscribers determine what action is taken in response to the event.
An event can have multiple subscribers. A subscriber can handle multiple events from multiple publishers.
Events that have no subscribers are never raised.
Events are typically used to signal user actions such as button clicks or menu selections in graphical user interfaces.
When an event has multiple subscribers, the event handlers are invoked synchronously when an event is raised. To invoke events asynchronously, see Calling Synchronous Methods Asynchronously.
In the .NET Framework class library, events are based on the EventHandler delegate and the EventArgs base class.

public class Class1
{
    public delegate void Del(string message);  // 宣告 delegate 型別為 Del. (習慣是將字尾命名為 Callback, Handler, Delegate...)
    public event ConnectEventHandler OnHello;  // 宣告 Event. (習慣是將字首命名為 On...)

	public void Method1()
	{
		if (condition matched)
			if (OnHello != null)          
				OnHello("Hello Method1 here."); // 在符合條件時, 若 Event 已被訂閱, 則發出 Event.
	}
}

public class Class2
{
	Class1 Class1_M = new Class1();
	public void Load()
	{
		Class1_M.OnHello += Class1_M_OnHello;  // 訂閱 Event
		Class1_M.Method1();  // 若符合條件時, 則系統會回呼 Class1_M_OnHello
	}
	
	private void Class1_M_OnHello(string message)
	{
        Console.WriteLine(message);
	}
	
}