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Wed, 15 Feb 2006

This is an archived blog post. I've switched to using WordPress as my blogging software and have not migrated all my old posts. I believe strongly in not letting an old link die, so this page continues to work. Please do visit mikemason.ca/blog to read newer posts.

Most people are familiar with the Model, View, Controller pattern (MVC) for separating business logic and presentational logic within an application. MVC is implemented in a number of Java web frameworks, such as Struts and Spring. A more recent pattern–Model, View, Presenter–can be applied in contexts where there is no central “controller” for the application. One such framework is ASP.NET.

My team recently built an MVP-based application on ASP.NET 2.0 and had great success with highly testable presenters and a highly adaptable presentation layer. In fact, we chose to switch from creating custom web controls to using simple .aspx pages and didn’t have to change our presenters at all–it’s always nice to validate those previously abstrct design decisions!

The MVP pattern separates three elements. The model is one or more domain-specific objects representing the current state of the system, the information we’re trying to display, etc. The view is how we present that information to the user and handle input, usually a particular screen or web page. The presenter is the logic that ties together the model and the view, handles navigation, business logic requests, and model updates.

For MVP in ASP.NET 2.0, we use an .aspx page–or more precisely, the code behind partial class–as the view, custom domain objects as the model, and a Plain Old C# Object (can I steal the term POCO?) as the presenter. Let’s assume we’re creating a page to list customers in our application. Our ListCustomers.aspx page might look like this:

<asp:Content ContentPlaceHolderID="Main" Runat="Server">
    <h1>Customer List</h1>
    <asp:GridView ID="customerGridView" AutoGenerateColumns="false" runat="server" SkinID="CustomerGrid">
        <Columns>
            <asp:BoundField HeaderText="Customer" DataField="Name" />
        </Columns>
    </asp:GridView>
</asp:Content>

Nothing special here, we’ve just defined a GridView which will list the customers and display a title. What’s interesting is the code-behind:

public partial class ListCustomers : System.Web.UI.Page, IListCustomersView
{
    protected void Page_Load(object sender, EventArgs e)
    {
        ICustomerService customerService = ServiceRegistry.GetService();
        ListCustomersPresenter presenter = new ListCustomersPresenter(this, customerService);
        presenter.PageLoading();
    }

    public List<Customer> Customers
    {
        set
        {
            customerGridView.DataSource = value;
            customerGridView.DataBind();
        }
    }
}

The code-behind partial class implements IListCustomersView, which we’ll see in a moment. When the page is loaded we create a new ListCustomersPresenter, passing it the view (this) and anything else it requires (in this case, an ICustomerService). We then call the presenter’s PageLoading() method. What’s happening is that the code-behind is making no decisions about what to display on the page, it simply delegates to the presenter for any non-display-related business logic.

The IListCustomersView interface defines how the presenter can interact with the ASP page. There’s just one settable property, Customers:

public interface IListCustomersView
{
    List<Customer> Customers { set; }
}

Looking back up at the code-behind, you can see that the implementation of the Customers property sets the grid view’s datasource to the list of customers and then calls DataBind() to populate the grid.

Let’s take a look at the final piece of the puzzle, the presenter:

public class ListCustomersPresenter
{
    private readonly IListCustomersView view;
    private readonly ICustomerService customerService;

    public ListCustomersPresenter(IListCustomersView view, ICustomerService customerService)
    {
        this.view = view;
        this.customerService = customerService;
    }

    public void PageLoad()
    {
        List<Customer> customers = customerService.GetAllCustomers();
        view.Customers = customers;
    }
}

When our presenter is constructed the two things it depends on, the view and the customer service, are passed to it (in this case, by the code-behind). This is known as constructor dependency injection. In the PageLoad() method the presenter simply accesses the customer service to load customers and sets this information on the view. This simple example can be extended to include input (the view can have read-only properties that correspond to text boxes, etc) and action (add a button to your web page and in the code-behind for its click call an action method on the presenter, like AddCustomerClicked()).

So why is this useful? Why not just have the code-behind access the CustomerService to load the customers? One of the main benefits is that the difficult business logic is captured in the presenter and can be more easily tested. Because we’re using dependency injection we can instantiate the presenter in an NUnit test, mock-out the view and service, and check the presenter does the right thing. The “load customers” example isn’t very hard but you can imagine logic that needed to take a set of user input and perform something more complex, such as placing an order. A second benefit is it’s very easy to see how the presenter can interact with the view–it can only use methods and properties on the IListCustomersView interface, which means it’s much easier for us to see the logical interface between the UI and the business layer. Finally, it’s possible to test drive your presenters and views, which tends to lead to simpler, more modular design for the system.

I’ve simplified some of the other stuff you’ll need to do in a real application. Screen flow and input validation are good examples. We solved the flow issue by having presenters able to return a “presenter result” object, indicating whether the ASP.NET framework should redirect to another page, stay on the current page, go to a login screen, etc. With input validation and error conditions, we add properties on the view so that the presenter can instruct the view to show a particular error message. You can still use an ASP.NET validator for client-side validation, but the presenter needs to be able to toggle it visible too.

We started our application using MVP, implementing the view using custom controls coded in C#. We did this mostly because we envisaged our application as a series of reusable controls, some of which (for example a Wiki control) should be embeddable in other applications. Ultimately we went a bit too far with this and coded some simple screenflow, which wasn’t really reusable, with the same C# controls. We found that layout and other tweaks became fairly onerous and decided to switch to implementing the view using traditional .aspx pages and code-behind. We found that our presenters required no changes at all to be able to accomodate the new view–excellent validation of the MVP design pattern.

Posted 22:01, 15 Feb 2006.  

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