How to model delegate objects in .NET edition

Answer

The solution is organized as follows:

• PART A: REFERENCES ON DELEGATES
  This part provides background on how the delegate model works
  
  
  
• PART B: MODELING DELEGATES WITH XDE, AN EXAMPLE
  This part uses an example to explain how to model
  and forward engineer delegates and events in XDE
  
  
  
• PART C: IMPLEMENTING THE CODE
  This part shows how to implement the code generated by XDE
  to build and run the example.
  
  
  

Delegates, Part 2

Implementation of Events with Delegates

(mscorlib.dll) System.Delegate

(mscorlib.dll) System.MulticastDelegate

PART B: MODELING DELEGATES WITH XDE, AN EXAMPLE


B.1. CREATING THE 'Account' CLASS

This excerise uses the familiar example of a bank account. You can model this class in XDE.
To start, open .NET and create a new C# Console Application. Then, right click on the project and Synchronize. This creates a code model in the Model Explorer.
In the Model Explorer, create an 'Account' class that has a private attribute called 'balance', of type 'int'.

This class also has two public operations called 'Withdraw' and 'Deposit', each taking an 'int' [in] parameter (the amount to be withdrawn/deposited) and returning void.

B.2. CREATING THE DELEGATE 'Account.AccountEventHandler'

The class Account should also have a nested class called AccountEventHandler, stereotyped as <<delegate>>.

To complete the delegate,add to AccountEventHandler only one operation called 'Invoke'.
The list of parameters of the Invoke operation expresses the signature of the callback function invoked when the event is fired.

It is custom to give the delegate the following arguments:
1. an object, representing the object that fired the event
2. a class inheriting from System.EventArgs to pass any needed parameters.

In this case, to fire events that communicate the actual balance, create a nested class Account.AccountEventArgs that inherits from System.EventArgs, and has one parameter of type 'int' called 'amount', one non-default constructor that takes this int parameter as an argument, and one getter that returns the 'amount'.

Now give the Account.AccountEventHandler.Invoke operation
1 [IN] parameter of type object,
1 [IN] parameter of type Account.AccountEventArgs,
1 [return] parameter of type void.

After Syncronizing the project, an inheritance relationship will be added in the model
from the delegate to the class System.Delegate even if the Intermediate Language subsequently produced by the compiler shows that the delegate inherits from System.MulticastDelegate instead. This is reported as defect RATLC00591955.

B.3. CREATING THE EVENTS 'Debit' and 'Credit'

Finally class Account will have two attributes (or associations) stereotyped as <<event>>, called 'Debit' and 'Credit', that should be fired whenever a change of the balance takes place, to signal if the updated balance is positive or negative.

If created as attributes, make their TypeExpression = Account.AccountEventHandler

If created as directed associations, draw the associations from
Account to AccountEventHandler.

To make these events static:

- for attributes: in the Property Editor, set their OwnerScope to 'Classifier' (rather than 'Instance')

- for association roles: in the property editor for the associations, locate which end
of the association has the name Debit (Credit). Then set End1OwnerScope or End2OwnerScope to 'classifier' (rather than 'Instance').

B.4. FORWARD ENGINEER THE CODE:

After creating the above class Account, right click on it in the Model Explorer
and choose 'Generate Code'.

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public class Account
{
public static event Account.AccountEventHandler Credit;
public static event Account.AccountEventHandler Debit;
private int balance;



public class AccountEventArgs : System.EventArgs
{
public int Get_Amount()
{
return amount;
}

private int amount;

public AccountEventArgs(int arg0)
{
amount=arg0;
}
}

public delegate void AccountEventHandler(object sender, AccountEventArgs e);


public void Deposit(int amount)
{
System.Console.WriteLine("Called Deposit with amount: " + amount);
int CurrentBalance = balance + amount;

//CurrentBalance holds the updated balance while it is checked

if(CurrentBalance < 0)
{
//fire the event
Debit(this,new AccountEventArgs(CurrentBalance));
}
if (CurrentBalance >= 0)
{
//fire the event
Credit(this, new AccountEventArgs(CurrentBalance));
}
balance = CurrentBalance;

}

public void Withdraw(int amount)
{
System.Console.WriteLine("Called Deposit with amount: " + amount);
int CurrentBalance = balance - amount;

//CurrentBalance holds the updated balance while it is checked

if(CurrentBalance < 0)
{
//fire the event
Debit(this,new AccountEventArgs(CurrentBalance));
}
if (CurrentBalance >= 0)
{
//fire the event
Credit(this, new AccountEventArgs(CurrentBalance));
}
balance = CurrentBalance;

}


}


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A driver application that creates an account , that calls the Deposit and Withdraw methods (triggering the events), and that reacts to the events is needed.

Create a class called DriverApplication, with a main function and two static methods that handle the events Credit and Debit: OnCredit and OnDebit.
These methods must have the same signature as the Invoke operation of the delegate.

Eventually the resulting code could be something like:
------------------------------------------------------------------------------------------------------------------------------------



using System;

class DriverApplication
{
//Reaction to the event 'Debit'
public static void OnDebit(object sender,Account.AccountEventArgs e)
{
Console.WriteLine("Event Debit: "+e.Get_Amount());
}

//Reaction to the event 'Credit'
public static void OnCredit(object sender,Account.AccountEventArgs e)
{
Console.WriteLine("Event Credit: "+e.Get_Amount());
}

static void Main(string[] args)
{

//Create an account
Account theAccount = new Account();
//Add to the Delegate a list of operations
//that have the same signature
//as the Invoke operation
Account.Credit += new
Account.AccountEventHandler(OnCredit);
Account.Debit += new
Account.AccountEventHandler(OnDebit);

// perform actions that will trigger events
theAccount.Deposit(100);
theAccount.Withdraw(250);

}
}

------------------------------------------------------------------------------------------------------------------------------------
After the account is built and run, the typical output will be:



C:\DelegatesEvents.exe
Called Deposit with amount: 100
Event Credit: 100
Called WithDraw with amount: 250
Event Debit: -150