Using the Visual State Manager

Part of the job of the designer or creative that you typically see being done in Expression Blend is skinning and templating controls, views, and other objects. For the most part, all controls have built-in templates, and in Visual Studio it is very difficult to get at these as they are part of the framework and not exposed. Expression Blend has a great tool to help you get at the templates by creating copies of templates for any control, putting them into your code and allowing you to edit them in the Designer using the Visual State Manager area labeled States that was mentioned earlier.

This States tab, and moreover the entire Visual State Manager infrastructure, was built as part of Expression Blend but has been added to the underlying framework as of Silverlight 4. You can tweak the code in Visual Studio, but the VSM was designed for use in Expression Blend or specifically to make it easier for designers to work with visual states of objects. Here, you will use the VSM to create a custom skin or template and then use the VSM to help build or change the default animations and transitions between states.
Start by creating your custom control template.

Creating a Control Template(s)

Since control templates are baked-in, sometimes creating a control template can be very difficult to extrapolate independently without Expression Blend. In Expression Blend it's really simple.

1. Start by selecting the control. At the top of the design surface, you see a breadcrumb that shows the control in question as the root item. If you select the breadcrumb, you will get a drop-down menu that lets you select either Edit Template or Edit Additional Template.

2. Normally you select Edit Template, which displays an additional menu consisting of Edit Current, which will be disabled; Add Resource, which will also be disabled; as well as Edit a Copy or Create Empty. You will also generally want to select Edit a Copy, at least until you understand the templates enough to build them from scratch and know what "states" are available. For most people, it is easier to just edit a copy of the default template.

3. When you select Edit a Copy, the Control Template dialog appears. Because a control template is a style resource, a box comes up so you can give the resource a proper name. All the correct settings are there by default. You do have the option to have the new control template put into the application or you can also create a new resource dictionary and have the control template go there.

4. Once you select the settings you want, click the OK button. All of the underlying code required to support this new custom template (that is a copy of the one baked in) is added to your project; in addition, the control is bound to this new custom template. Now you click the States tab.

All the states and transitions to each state are divided into what is a tree. When you select any of these, that state is applied visually in the design surface. In addition, the state properties show in the Properties pane so that they can be customized as needed visually without going into code. However, you can open the project in Visual Studio and edit the template if you want to. You will also note that the Visual Tree shows all the elements of the template for you to select as well, and you can remove any element you do not need to complete your custom control template.

Note: The Visual Tree is a representation of your UI and how it is rendered visually and how the underlying engine renders elements to the screen. Some things might not be in the Visual Tree because they are in a collapsed state. In other words, Visibility='Collapsed' rather than Opacity='0' where the Opacity='0' is considered to still be in the Visual Tree.

Customizing Visual States

When customizing elements of your control template, you generally are going to use the VSM States tab and the Visual Tree in the Objects and Timeline viewer, as well as the design surface and Properties pane. (You can also edit it in the raw XAML if you are comfortable with that but, keep in mind that some default templates can be very complex.) Often when you are customizing the control, the entire look and feel needs to change. When you select your control, you can go to the Visual Tree and delete anything you do not want or entirely replace it by pasting in whatever it is you want or editing it on the design surface.

To change the state, select the correct state in the VSM State tab, which turns on the State Recorder, and change the control however you like. For example, if you select the state of a button, that state is shown on the control on the design surface. You then can select the element of the control in the object tree of the control that you want customized and edit it - say, change the color or add a transform.

There is also in the VSM tab on each element, a Transitions drop-down that you can select to add specific transitions to one state or the other as needed. Right on the transition, you can set the timing of the transition as well; however, the transition details are edited elsewhere. The last important element of working with transitions that you will need to know is that when you select a transition, you get the Timeline view next to the object tree. This allows you to do custom key frame animations.

note: this article is an edited version of a a section of a chapter I wrote for the WROX title: Silverlight 4 Professional, check it out on: http://www.wrox.com/WileyCDA/WroxTitle/Professional-Silverlight-4.productCd-0470650923.html )