Thursday, January 26, 2012

ASP.NET MVC AsyncController Helper

The AsyncController is a really nice feature introduced in ASP.NET MVC 2. It allows you to process your actions  asynchronously increasing the throughput of your web server.
Although implementing it is not that hard, I’m not a big fan of the current syntax(The new async api in ASP.NET MVC 4 looks promising).
In the meanwhile I have written a small helper class to simplify the usage of the AsyncController.
public static class AsyncManagerExtensions
   {
       public static void RegisterTask<T>(this AsyncManager asyncManager, Task<T> task, Func<T, object> func)
       {
           asyncManager.OutstandingOperations.Increment();
           task.ContinueWith(task2 =>
           {
               object propertyObject = null;

               if (!task2.IsFaulted)
               {
                   //invoke the provided function with the
                   //result of running the task
                   propertyObject = func(task2.Result);
               }
               else
               {
                   propertyObject = func(default(T));
               }


               //use reflection to set asyncManager.Parameters
               //for the returned object's fields and properties
               var ty = propertyObject.GetType();

               foreach (var f in ty.GetFields())
               {
                   asyncManager.Parameters[f.Name] = f.GetValue(propertyObject);
               }

               foreach (var p in ty.GetProperties())
               {
                   var v = p.GetGetMethod().Invoke(propertyObject, null);
                   asyncManager.Parameters[p.Name] = v;
               }

               asyncManager.OutstandingOperations.Decrement();
           });
       }
   }

Using it is simple. I create a controller that inherits from AsyncController:

public class PrintController : AsyncController
{
    private readonly IReportService _reportService;
        
    public PrintController(IReportService reportService)
    {
        _reportService = reportService;
    }
}


I add a method that returns the Task I want to execute and and the Action Controller method that will be called by the View(in this case through /Print/Pdf) . In this Action Controller method I call the RegisterTask<T> extension method and pass 2 properties:

  • The method that needs to be executed 
  • An expression that takes the output of the task and maps it to the parameters of the Completed method(based on the parameter name).
public void PdfAsync(string reportName)
{
 AsyncManager.RegisterTask<byte[]>(RenderReport(reportName)), renderReportResult=> new { data = renderReportResult, reportName = reportName });
}

private Task<byte[]> RenderReport(string reportName)
{
 return  Task.Factory.StartNew(()=>_reportService.RenderReport(reportName));
}

public ActionResult PdfCompleted(byte[] data, string reportName)
{
 var fileName = reportName + ".pdf";
 var fileContentResult = File(data, "application/pdf", fileName);
 Response.AppendHeader("Content-Disposition", string.Format("inline; filename={0}", fileName));
 return fileContentResult;
}

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