Inside Controllers in Visual Basic .NET

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4 Inside Controllers
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Passing Data to a View
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After the action method has executed any tasks associated with the request, it likely holds some fresh data to be integrated in the next view to be displayed. In ASP.NET MVC, the generation of the view is delegated to a distinct layer of code the view engine. Figure 4-5 shows the whole life cycle of an action method from processing the input data to delivery of view data to the rendering engine.
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Processing input data
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Connecting to the middletier gateway
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Output data View model (ViewData collection and/or Model)
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Middle tier fa ade
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Filling the view model
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View engine
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FIGuRE 4-5 The life cycle of an action method
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Because the view engine is distinct from the controller, it needs to receive any data required to generate the next browser view. Earlier in the chapter, while discussing the ControllerBase class (see Table 4-1), you briefly met a property named ViewData that is defined as follows:
public abstract class ControllerBase : IController { ... public ViewDataDictionary ViewData { get; set; } }
The ViewData property represents a built-in container used for passing data between a controller and a view. The property is of type ViewDataDictionary. The idea is that once the controller has executed a given action, it packs it into the ViewData container and gets any significant results to be shown to the user. The following code snippet, which is an extremely simple depiction, shows you what you get with any ASP.NET MVC project template:
public class HomeController : Controller { public ActionResult Index() { // Pack data for the view this.ViewData["Message"] = "Welcome to ASP.NET MVC!";
Part II The Core of ASP.NET MVC
// Tell the view to render return this.View(); } ... }
The ViewData dictionary is definitely the object that contains a valid representation of the view-model that is, any data being worked on in the view. You can add as many entries to the ViewData dictionary as you plan to consume from within the view class. From within a view class, you then retrieve the content of the ViewData dictionary using the same syntax as just shown. Here s an example:
<!-- Snippet taken from an ASPX template in the Views folder --> <%@ Page Language="C#" Inherits="System.Web.Mvc.ViewPage" %> <h2><%= Html.Encode(this.ViewData["Message"]) %></h2>
What s different in the two snippets, of course, is the type of the this object, which exposes the ViewData property. It is a Controller-derived class in the first snippet; it is a ViewPage-derived class in the snippet just shown. It s useful now to have a closer look at the type of the ViewData property the ViewDataDictionary type.
The View-Model Container
As noted in 3, the ViewDataDictionary type is a class that implements the IDictionary interface, and it looks and behaves like a classic name/value pair, enumerable dictionary. Any data you store in a dictionary is treated as a plain object and requires casting, boxing, or both to be worked on. (This is nothing new for ASP.NET developers because it is the same model that you still use for managing the global ASP.NET cache or the session state.) A dictionary is definitely not something you would call a strongly typed container. At the same time, though, a dictionary is straightforward to use and works just fine. The ViewDataDictionary class is special because it also features a Model property, as shown here:
public class ViewDataDictionary : IDictionary<string, object>, ICollection<KeyValuePair<string, object>>, IEnumerable<KeyValuePair<string, object>>, IEnumerable { public object Model { get; set; } ... }
The Model property is an alternative and object-oriented way of passing data to the view object. Instead of fitting flat data into a dictionary, you can shape up a custom object that faithfully represents the data the view expects. In other words, the Model property just represents your chance of creating a view-model object that is unique for each view.
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