Event-Based Asynchronous Pattern(EAP)

Introduction

Sometime application need to perform long running tasks and yet it should be responsive. Or we need to execute multiple task simultaneously to boost the performance of the application. In window application <strong>BackgroundWorker</strong> provide straightforward solution. <strong>System.Threading</strong> class provides all tools we need to develop good multi-threaded application. Here I will implement Event-Base Asynchronous pattern in the class which will support Asynchronous version of method.

Event-Based Asynchronous Pattern(EAP)

The class that supports EAP will have Asynchronous version of method. Class will have more one or more methods named <em>MethodName</em>Async. It will also may have MethodNameCompleted event which will be raised when the asynchronous method complete processing. There can be corresponding cancel method <em> MethodName </em> Cancel to cancel the process in middle. This is an standard pattern which hide the complexity of asynchronous execution of any method. EAP will allow us to :

• Perform time-consuming task; downloads and database operations "background", without interrupting our application.
• Execute multiple operations simultaneously, receiving notification when each completes. It also allow same operation to run multiple time simultaneously.
• Wait for resources to become available without hanging the application.
• Communicate with Asynchronous operation using ever
• MethodNameCompleted is a type of MethodNameCompletedEventHandler

event, that except MethodNameCompletedEventArgs which is inherits AsyncCompletedEventArgs

Implementing EAP in a class

Class will have Synchronous(<strong>Method1</strong>) method and two overloaded Asynchronous(<strong>Method1Async</strong>) methods. Why two overloaded Asynchronous methods? If we need to call method multiple times for different values and there is only one aysnc method Method1Async( string message ), we can't call method second time until first asynchronous call completes. Otherwise it will throw InvalidOperationException. To avoid this situation we will create overloaded method which will except one more parameter calls Method1Async(string message, object userState). This method can be call multiple time with unique userState method and every call run asynchronously and independent to each other.

Create Completed Event Handler Delegate

First we have to create public Method1CompletedEventHandler delegate <strong>outside of the class</strong> in same name space. This will except Method1CompletedEventArgs as one event argument paramete

public delegate void Method1CompletedEventHandler( object sender, Method1CompletedEventArgs e );

public class Method1CompletedEventArgs : AsyncCompletedEventArgs

{

public Method1CompletedEventArgs(Exception ex, bool canceled, object userState)

: base(ex, canceled, userState)

{

}

As Method1CompletedEventArgs is inherited from AsynchCompletedEventArgs , constructor must have three minimum parameters

• Error (Exception): Any exception occurred during the operation.
• Canceled (boolean): Whether the operation canceled before it complete.
• userState (object): Any user object ( may be sum result expected from the method ).

It can have many other parameter as per requirement.

Creating Class

public class MyAsyncDemo

{

//delegate will execute main worker method asynchronously

private delegate void WorkerEventHandler(string message, AsyncOperation asyncOp);

//This delegate raise the event post completing the async operation.

private SendOrPostCallback onCompletedDelegate;

//To allow async method to call multiple time, We need to store tasks in the list

//so we can send back the proper value back to main thread

private HybridDictionary tasks = new HybridDictionary();

//Event will we captured by the main thread.

public event Method1CompletedEventHandler Method1Completed;

public MyAsyncDemo()

{

onCompletedDelegate = new SendOrPostCallback(CompletedDelegateFunc);

}

/// <summary>

/// This function will be called by SendOrPostCallback to raise Method1Completed Event

/// </summary>

/// <param name="operationState">Method1CompletedEventArgs object</param>

private void CompletedDelegateFunc(object operationState)

{

Method1CompletedEventArgs e = operationState as Method1CompletedEventArgs ;

if (Method1Completed != null)

{

Method1Completed(this, e);

}

}