Defining pages and view-models

Windows Store applications based upon the Okra App Framework generally consist of a number of interconnected pages starting from a central hub, or home page. The user navigates forward through the application flow by clicking images, buttons or links, and can also navigate backwards by using a back button that is placed in the top left hand corner of each page.

By default the Okra App Framework defines pages with a unique name using a declarative attribute-based approach (an alternative convention based approach is also possible). The framework will then use MEF (the built in .Net composition framework) to locate the view and view-model associated with this page name, create and initialise instances of these and wire them together as appropriate.

For the Okra App Framework to find a page it should be annotated with the PageExportAttribute,

[PageExport("MyPageName")]
public sealed partial class MyPage : Page
{
    public MyPage()
    {
        this.InitializeComponent();
    }
}

Simliarly the corresponding view-model is annotated with the ViewModelExportAttribute,

[ViewModelExport("MyPageName")]
public class MyViewModel : MyViewModelBase
{
    ...
}

Special page names

The Okra App Framework also provides a set of predefined page names as part of the Okra.Navigation.SpecialPageNames class. These are used as default page names for a number of the services provided as part of the framework. The SpecialPageNames values can be used anywhere that a string-based page name is used. For example to define the home page for an application,

[PageExport(SpecialPageNames.Home)]
public sealed partial class MyHomePage : Page
{
    ...
}

The provided values are detailed below,

Using types as page names

As an alternative to string-based page names you can also use types for a strongly-typed approach. Note that the same type must be used for both the view and view-model.

[PageExport(typeof(MyViewModel))]
public sealed partial class MyPage : Page
{
    ...
}

[ViewModelExport(typeof(MyViewModel))]
public class MyViewModel : MyViewModelBase
{
    ...
}

In some parts of the framework overloads for type-based page names are not available. In these cases there is a PageName.FromType(…) helper method that can be used to convert to the corresponding string-based page name. For example,

NavigationManager.HomePageName = PageName.FromType(typeof(MyViewModel));