print barcode labels in c# Part II The Core of ASP.NET MVC in Visual Basic .NET

Encoding Quick Response Code in Visual Basic .NET Part II The Core of ASP.NET MVC

Part II The Core of ASP.NET MVC
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Async Actions in ASP.NET MVC
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As Figure 4-13 shows, in an ASP.NET MVC request the async point is placed between the ActionExecuting and ActionExecuted events. The action invoker is responsible for orchestrating the various steps.
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Initial thread
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Executing action Async point Action Method
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Final thread
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Executed action
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Executing result Action Result Executed result
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FIGuRE 4-13 Mechanics of an asynchronous action
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When the action invoker sends a notification that it is about to execute the action, the thread engaged is still the original ASP.NET thread that picked up the request from the Web server queue. The code running at this point is the trigger method, usually in the form of an xxxAsync method, as the following code shows:
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public void PerformLengthTaskAsync(SomeData data) { // Process input ... // Post a work item to a component that can result // in a lengthy operation (for example, invoke a Web service) ... // That's all for now the action is being executed elsewhere. // All that remains to be done is wait for it to terminate; // for this task, we don't want to squander an ASP.NET thread. return; }
When the trigger method returns, the lengthy action is running in the care of some other thread, possibly on some other process. The asynchronous action invoker manages to sync up with the ASP.NET runtime so that a completion port is used to monitor the completion of the operation. When this happens, the ASP.NET runtime puts the requests back in circulation with a special flag that indicates it only needs to complete its second half. The first available ASP.NET thread picks up the request and begins processing it.
4 Inside Controllers
In ASP.NET MVC, this means that the action is executed and the finalizer method is invoked. Here s the typical structure of a finalizer:
public ActionResult PerformLengthTaskCompleted(SomeResponse data) { // Manage the model state (if any) ... // Prepare and render the view ... }
The finalizer receives a custom object (or a multitude of parameters) that contains the data it is expected to process and pass on to the view object. However, the signature of the finalizer must be known in some way to the trigger. Let s find out the details.
Designing Asynchronous Action Methods
Is there any difference between synchronous and asynchronous routes In ASP.NET MVC 2, no distinction exists at the route level. You still use the MapRoute method to define both. (I ll cover routes in detail in 8.)
routes.MapRoute( "Default", "{controller}/{action}/{id}", new { controller = "Home", action = "Index", id = "" } );
The URL of the request is therefore processed as usual to find out the name of the controller class. A controller that exposes asynchronous methods is expected to derive from the new AsyncController class.
public class ServerFacadeController : AsyncController { ... }
Note that an AsyncController class can serve both synchronous and asynchronous requests. The name of the method conventionally indicates how the method has to be processed. You must be careful to avoid any ambiguity when you name your methods in an AsyncController class. Let s consider the following example that has a synchronous method and an asynchronous method:
public class ServerFacadeController : AsyncController { public ActionResult PerformTask(SomeData data) { ... }
Part II The Core of ASP.NET MVC
public void PerformTaskAsync(SomeData data) { ... } public ActionResult PerformTaskCompleted(SomeResponse data) { ... } }
The preceding code will throw an exception, as shown in Figure 4-14.
FIGuRE 4-14 Ambiguous references in the name of the action
An async action is identified by name, and the expected pattern is xxxAsync where xxx indicates the default name of the action to execute. Clearly, if another method named xxx exists and is not disambiguated using attributes, an exception is thrown. The word Async is considered as a suffix and the URL required to invoke the PerformTaskAsync method will contain only the prefix PerformTask. For example, the following URL will invoke the method PerformTaskAsync passing a value of 2 as a route parameter:
http://myserver/serverfacade/performtask/2
Whether it will be resolved as a synchronous or asynchronous action depends on the methods you actually have in the AsyncController class: for a given action name, you can only have either a synchronous or an asynchronous method match.
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