print barcode labels in c# Part I The Programming Paradigm Browser MVC HTTP handler Controller Middle tier View in Visual Basic .NET

Print Quick Response Code in Visual Basic .NET Part I The Programming Paradigm Browser MVC HTTP handler Controller Middle tier View

Part I The Programming Paradigm Browser MVC HTTP handler Controller Middle tier View
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Invoke action Invoke method Fresh data
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FIGuRE 3-7 The sequence diagram for the ASP.NET MVC architecture
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You won t see any model actor in the figure. However, the model is essentially the container for the data labeled Fresh data. It represents the return values of any method invoked by the controller on the middle tier. This content is then forwarded to the view engine for the actual rendering of the HTML response. In ASP.NET MVC, the model actor might have a fixed and system-provided form a dictionary or it might become a strongly typed graph of objects under the total control of the developer.
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Implementation of the Controller Actor
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In ASP.NET MVC, you generally don t think in terms of pages to build, but rather in terms of actions to perform and subsequent views to create. The controller is a .NET class that exposes a bunch of public methods for the MVC HTTP handler to invoke in response to a request. Arguments for the controller s method are figured out from the request in a pure REST fashion. As you ll see in great detail in 4, Inside Controllers, the framework does a good job of guessing your intention and extracting values from either the query string or the body of the request to match any declared formal parameter on the selected method. Otherwise, the author of the controller s method can always extract input data directly from ASP.NET intrinsic collections such as Request.QueryString and Request.Form.
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3 The MVC Pattern and Beyond
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Any action method on a controller s method has three responsibilities: performing the action, populating the model with the results, and triggering the view engine. Typically, the controller invokes one endpoint in the service layer (the front of the middle tier) and gets some data back. Next, it massages this data into the model. This could be as easy as packing objects into a name/value dictionary or mapping values and instances to the properties of made-to-measure objects. Finally, the controller selects the next view for the user and orders the view engine to render it using the information stored in the model. Here s a quick but illustrative example of a controller method:
public ActionResult About() { // Populate the model this.LoadLocalizableInformationIntoModel(); // Next view string viewName = this.GetNextView("About"); // Trigger the next view return View(viewName); } private void LoadLocalizableInformationIntoModel() { // Load data into the built-in model actor. // Data is read from a global resource file named globals.resx. The item // read in this case is labeled WelcomeMessage. ViewData["WelcomeMessage"] = this.HttpContext.GetGlobalResourceObject( "globals", "WelcomeMessage"); ... } private string GetNextView (string currentViewName) { // Possible workflow implemented here to select next view // and assign its name to the returned view name variable... ... // Return next view name return currentViewName; }
The About method in the example doesn t really invoke any endpoint on the service layer. It s limited to extracting some information from the application s resource file and stuffing that into the model the ViewData collection. In addition, the controller selects the next view to render and orders its rendering via the View method.
Implementation of the View Actor
In this regard, a view is ultimately the response sent to the client browser. A view is identified by name and has content that, processed by a view engine, produces the response for the browser.
Part I The Programming Paradigm
As you ve seen, a controller method returns an object of type ActionResult. The ActionResult type is an internal framework type that encapsulates the result of an action method and represents the following step after the controller has completed its job. To be precise, the View method doesn t actually return ActionResult but a derived type ViewResult. Here s the complete type hierarchy:
public class ViewResult : ViewResultBase { ... } public abstract class ViewResultBase : ActionResult { ... }
ViewResultBase is the base class used to supply the model to the view and contains most of the code to trigger the rendering of the view to get some response for the browser. The ViewResult type customizes its base class by providing the logic to find the view content to pass to the selected engine. As it turns out, ViewResult isn t a simple container of data. Instead, it encapsulates all the logic necessary to produce a response for the user. The logic can be broken down into various steps. First, the ViewResult object retrieves the currently selected view engine. Next, it locates the source for a particular view and passes it to the engine. The view engine mixes the source code (for example, an HTML template) with any content in the model (for example, the ViewData collection) and returns a response. A default view engine is preregistered with any ASP.NET MVC application; however, you can create your own engines and add them programmatically to the application handling the Application_Start event in global.asax. If you want to use different view engines for different controller actions, you then set the view engine directly in the body of the controller method just before invoking the View method. (I ll return to the details of this in 5, Inside Views. ) Note The default view engine is a class named WebFormViewEngine, and it inherits from an abstract base class VirtualPathProviderViewEngine. The base class provides a basic implementation of the IViewEngine interface that characterizes a view engine. You can use the VirtualPathProviderViewEngine class as a starting point for building your own view engine, especially if your view engine needs to access disk files to read the source of the view. In fact, the VirtualPathProviderViewEngine class relies on the ASP.NET VirtualPathProvider class to access disk files on the server. The VirtualPathProviderViewEngine won t create a view object, but it delegates that work to any derived class currently, the sole WebFormViewEngine. As you might guess, this class retrieves and processes view sources in the form of ASPX and ASCX markup files.
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