Delegates and Events.ppt

8/14/2019 Delegates and Events.ppt

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C# Delegates and Events

C# Delegates and EventsCNS 3260

C# .NET Software Development


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Delegate types

A delegate declaration defines a new type

Delegates are similar to function pointers Delegates are type-safe

Delegates are object-oriented

Delegate types are derived fromSystem.MulticastDelegate


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The Command Pattern

Exposed Command Variable Command object instance

Command executor

Command Class

Execute()

Command issuer

Command Subclass

Execute()

Knows when the

event happens but

doesnt know what to

do about it

Knows what to do

when an event

happens but doesnt

know when


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Simple Delegate Command Pattern

Delegate Host Class

(Publisher)

Exposed Delegate

Knows when the

event happens but

doesnt know what to

do about it

Delegate User Class

(Subscriber)

Knows what to do

when an event

happens but doesnt

know when

Subscribing Method

AKA: The Observer Pattern or .NET Event Model


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Two reasons to use Delegates

When youre not sure what should happenwhen an event occurs GUI events

Threading situations

Callbacks

Command Pattern

To keep your interface clean Looser coupling


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Defining and using Delegates

three steps:

1. Declaration

2. Instantiation3. Invocation


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Delegate Declarationnamespace some_namespace

{

delegate void MyDelegate(int x, int y);

class MyClass

{

public delegate int AnotherDelegate(object o, int y);

}

}

Delegate Type Name


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Delegate Instantiationdelegate void MyDelegate(int x, int y);

class MyClass

{

private MyDelegate myDelegate = new MyDelegate( SomeFun );

public static void SomeFun(int dx, int dy)

{

}

}

Invocation Method

Invocation Methodname (no params

or perens)


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More Realisticallyclass MyClass

{

private MyDelegate myDelegate;

public MyDelegate MyDelegate

{

set { this.myDelegate = value; }}

}

class MainClass{

[STAThread]

static void Main()

{MyClass mc = new MyClass();

mc.MyDelegate = new MyDelegate( MainClassMethod );

}

private static void MainClassMethod(int x, int y) { }

}

Delegate Variable (null)

Passed in and assigned

in the constructor


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Instance Methodsclass MyClass

{


public MyDelegate MyDelegate

{

set { this.MyDelegate = value; }

}}

class MainClass{

[STAThread]

static void Main(){

MainClass mainClass = new MainClass();MyClass mc = new MyClass();

mc.MyDelegate = new MyDelegate( mainClass.MainClassMethod );}

private void MainClassMethod(int x, int y) { }

}

Method attached to

an instance


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Delegate-Method Compatibility

A Method is compatible with a Delegate if

They have the same parameters

They have the same return type


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Delegate Invocationclass MyClass

{


public MyClass(MyDelegate myDelegate)

{

this.MyDelegate = myDelegate;}

private void WorkerMethod()

{int x = 500, y = 1450;

if(myDelegate != null)

myDelegate(x, y);}

}

Attempting to invoke a delegate instance whose value is null results in an exception of type

System.NullReferenceException.


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Delegates Multicast Nature

Delegate is really an array of function pointers

Now when Invoked, mc.MyDelegate will execute allthree Methods

Notice that you dont have to instantiate the delegatebefore using += The compiler does it for you when calling +=

mc.MyDelegate += new MyDelegate( mc.Method1 );


mc.MyDelegate = mc.MyDelegate + new MyDelegate( mc.Method3 );


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The Invocation List Methods are executed in the order they are added

Add methods with + and +=

Remove methods with - and -= Attempting to remove a method that does not exist is not an error

Return value is whatever the last method returns

A delegate may be present in the invocation list more than once The delegate is executed as many times as it appears (in the appropriate

order)

Removing a delegate that is present more than once removes only the lastoccurrence


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Multicast example

mc.MyDelegate = new MyDelegate( mc.Method1 );


mc.MyDelegate = mc.MyDelegate + new MyDelegate( mc.Method3 );

// The call to:

mc.MyDelegate(0, 0);

// executes:

// mc.Method1

// mc.Method2// mc.Method3 (See Delegates Demo)


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System.Delegate

abstract

Special Only the system and compiler can inherit fromSystem.Delegate

Members: Method

Returns A MethodInfo object

Target

The this pointer belonging to the method null if the method was static

operator == and operator !=


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System.MulticastDelegate

Target

Method

operator== and operator !=_prev

Linked list of System.Delegateobjects

GetInvocationList() Returns the Invocation List as a Delegate[]


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Delegate Dangers

Exposing a delegate might be bad if:

You dont want others to invoke your delegate

You have assigned to the delegate internally

Enter the event keyword...


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Events Events are safe delegates

But they are delegates

Restricts use of the delegate (event) to the target of a += or -=operation No assignment

No invocation No access of delegate members (like GetInvocation List)

Allow for their own Exposure Event Accessors


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Event Accessorspublic delegate void FireThisEvent();class MyEventWrapper

{

private event FireThisEvent fireThisEvent;

public void OnSomethingHappens()

{if(fireThisEvent != null)

fireThisEvent();

}

public event FireThisEvent FireThisEvent

{

add { fireThisEvent += value; }remove { fireThisEvent -= value; }

}

}

add and remove

keywords

(See Event Demo)


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On Word on Exceptions

Exceptions thrown from a delegate:

Are thrown to the calling context

All subsequent delegates in the Invocation List are ignored


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Library Delegates ThreadStart

TimerCallback

ASyncCallback

EventHandler

KeyPressEventHandler KeyEventHandler

etc.

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Delegates and Events.ppt