UI Composition

Last updated 7 months ago

UI Composition

Radical offers a fully flagged UI Composition engine based on the concept of regions.

A UI Composition sample is available in the Radical-Samples repository.


A region is a named injectable portion of the UI where other components can inject their on content. A region is attached to a DependencyObject on the UI, depending on the type of the object the region is attached to the region behavior changes. Radical has 3 different main region types:

  • IContentRegion<T>: a content region is thought for a ContentPresenter or a ContentControl UIElement, it can host one single content at a time and each time a new content is set the previous one will be removed;

  • IElementsRegion<T>: an elements region can host multiple contents at a time, it is thought for a Panel UIElement, so each WPF control that inherits from panel, such as the StackPanel, can be used with an IElementsRegion; Content from an IElementsRegion can be added or removed and will be available depending on the logic implemented by the underlying UIElement;

  • ISwitchingElementsRegion<T>: a switching elements region is an element region that, other than being able to host multiple elements at a time, has also the concept of an active element that can change over time; a typical sample is a TabControl where each TabItem can be seen;

Note: each time a content is removed from a region its lifecycle is managed as every View/ViewModel:

  • View and ViewModel will be released;

  • If View or ViewModel implements IDisposable they will be disposed;

  • If View or ViewModel implements IExpectViewClosedCallback they will receive a callback notification;

Each region is characterized by 2 main attributes:

  • is owned by a Region Manager, an IRegionManager implementation;

  • has a unique name in the set of regions owned by the same Region Manager;

A RegionManager is automatically created by the UI Composition engine as soon as a region is added to a View, a RegionManager is bound to a WPF Window instance.


Regions can be nested as preferred, a Window can contain a region that at runtime will contain another region and so on without limitations. For example the following is a valid logical tree:

-> Grid
-> ContentPresenter
-> IContentRegion<ContentPresenter>
-> UserControl
-> Grid
-> StackPanel
-> IElementsRegion<StackPanel>

In the above sample one single RegionManager will be created at runtime.

Region Setup

Region markup definition

First define a region in the XAML where is needed and attach it to the UIElement that requires injection:

<Window x:Class="Samples.Presentation.MyView"
d:DesignHeight="300" d:DesignWidth="300">
<ContentPresenter rg:RegionService.Region="{rg:ContentPresenterRegion Name=MyRegion}" />


  • The rg namespace declaration pointing to the Radical region namespace http://schemas.topics.it/wpf/radical/windows/presentation/regions;

  • A region is attached to a UIElement via the Region attached property of the RegionService element;

  • A region is declared as a markup extension whose primary role is to define the region type and the region name;

As soon as we define a region the UI Composition engine, at runtime, will create a RegionManager to host the region, RegionManager whose lifecycle is bound to the lifecycle of the hosting Window. If the region is defined in a UserControl the RegionManager lifecycle will be bound to the lifecycle of the Window hosting the UserControl.

Region Injection

Once a region is defined in the XAML we need to inject some content, we can inject content in a region in 3 different ways: manually, using partial views or using a declarative approach.

Manual injection

Once a View contains a region each time the View is loaded a ViewLoaded message is broadcasted to notify that the View has been loaded:

class MyViewLoadedHandler : MessageHandler<ViewLoaded>, INeedSafeSubscription
public IViewResolver ViewResolver{ get; set; }
public IConventionsHandler Conventions{ get; set; }
public IRegionService RegionService{ get; set; }
protected override bool OnShouldHandle( ViewLoaded message )
return message.View is Samples.Presentation.MyView;
public override void Handle( ViewLoaded message )
if ( this.RegionService.HoldsRegionManager( message.View ) )
var view = this.viewResolver.GetView<MyRegionView>();
var region = this.RegionService.GetRegionManager( message.View )
.GetRegion<IContentRegion>( "MyRegion" );
region.Content = view;

In the above sample we are defining a message handler to handle the ViewLoaded message, overriding the OnShouldHandle method to define a rule to handle only the ViewLoaded event related to the View we are interested in.

In the Handle method we utilize:

  • the RegionService to determine is the View has a RegionManager;

  • if the View has a region manager

    • we resolve the content to inject;

    • retrieve a reference to the region manager and to the region;

    • inject the content;


Automatic (aka Partial regions)

Radical UI Composition engine has a concept called partial view, a partial view is a View, and if defined its ViewModel, that can be automatically picked up and injected based on a set of conventions:

  • Given a region, as in the previous XAML sample named MyRegion;

  • Given a View, and an optional ViewModel, that lives in a namespace that matches *.Presentation.Partial.*;

  • Where the last segment of the View/ViewModel namespace is the region name, in our sample MyRegion;

The View will be resolved, as usual, and injected into the expected region. Given the following namespace structure:


The MySampleView and it ViewModel, MySampleViewModel, will be automatically injected into the MyRegion region.


The last option, to inject a View in a specific region, is to decorate the View class with the InjectViewInRegionAttribute:

[InjectViewInRegion( Named = "MyRegion" )]
class MyUserControlView : UserControl

In the above sample, at runtime, the UI Composition engine will inject an instance of the MyUserControlView into the region named "MyRegion".

Region implementations

As previously said Radical has 3 different region types: IContentRegion<T>, IElementsRegion<T> and ISwitchingElementsRegion<T>. Each region type has a default implementation.


A ContentPresenterRegion is a IContentRegion<ContentPresenter> that can be applied to a ContentPresenter UIElement.


A PanelRegion is a IElementsRegion<Panel>, given that a Panel is an abstract class, this region can be used with any UIElement that inherits from Panel, such as a StackPanel.


A TabControlRegion is an implementation of the ISwitchingElementsRegion<TabControl> and can be used with a TabControl.