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Building and Using Expression Blend Behaviors

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Behaviors are a cool way of adding functionality to XAML design elements in Expression Blend. The idea is that some rich functionality that would be hard for a designer to do can be wrapped in a way that can then be used as a drag-and-drop feature to add that functionality to a XAML element in Expression Blend. A Behavior then is an ‘object’ that implements a certain base class and member(s) so that it can be easily consumed in Expression Blend as a drag-and-drop ‘behavior’ on the design surface. To build a new behavior you need to start in Visual Studio.

Implementing Behaviors

Implementing a Behavior is straightforward and can be as complicated or as simple as you like. To start with, you will need Expression Blend installed so you can test your behavior in Expression Blend. If you are already in Expression Blend, right-click the project and click "Open in Visual Studio," which this implies correctly that you need both Expression Blend AND Visual Studio installed to create and test a Behavior. Once the project is opened in Visual Studio, right-click and select "Add New." Then in the "Add New" dialog, select Class. Give the class a name, and then you need to get the Expression Blend Library into your project. To get the base class (and the associated name space), you must add a reference to the System.Windows.Interactivity name space that comes with the Silverlight 4 framework. Right click on references and select "Add Reference.” Once the namespace is included, you are ready to build out the class you created into a Behavior. You need to start by adding the namespace at the top like this:

using System.Windows.Interactivity;

This gets the base library (namespace) you need so you can inherit from the behavior base class. Next, of course, you need to set up your class to inherit from TargetedTriggerAction and make your class look in effect like this:

public class SomeBehavior : TargetedTriggerAction

TargetedTriggerAction is our base class, where you will be able to apply it to a class of type FrameworkElement. For the purposes of this example, the Behavior will also be targeted specifically at Shape objects. The next step is to implement Invoke, which is what is fired when the Behavior is applied to the target. Invoke needs to look like this block of code:

protected override void Invoke(object parameter)

From this point, you need to get a reference to the object that your behavior targets and do to the object whatever is necessary to make the object do what you want it to do (the ‘behavior’). In this case, you typically would add a member event handler to the targeted object event to the associated object, and you start be creating a location for the reference to the target object:

Shape TargetElementItem1;

Now when Invoke is called, you would get your reference, cast it to a Shape and place it into the member reference:

TargetElementItem1 = (Shape)(this.AssociatedObject);

This code then needs to be in the Invoke member. At this point, the implementation for each Behavior will be increasingly different for each Behavior that you build. This example changes the color back and forth between two colors when a user clicks on the shape. Next, you need to add these members to the Behavior class like this:

Brush Color1;
Brush Color2 = new SolidColorBrush(Color.FromArgb(0,0,0,0));

This gives you a color to switch to and the reference to the base color of the class. To populate Color1 with the base or start color of the object, add this second line to the Invoke method:

Color1 = (Brush)(TargetElementItem1.Fill);

Now that the Behavior has a reference to the colors and the Shape is typed and referenced, you can then add your behavior logic. In this example, add two event bindings to the Shape reference like this:

TargetElementItem1.MouseLeftButtonDown += new
TargetElementItem1.MouseLeftButtonUp += new
These lines won’t actually work until you add the two methods, which should look like this:
void TargetElementItem1_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
TargetElementItem1.Fill = Color1;
void TargetElementItem1_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
TargetElementItem1.Fill = Color2;

This completes the Behavior. You should now be able to use it in Expression Blend.

Consuming Behaviors

Besides visual behaviors, you can also add non-visual functionality as you might in a command. Therefore, if you are familiar with commanding, a good way to look at Behaviors is as "commands for designers in Expression Blend." To be able to use Expression Blend to work visually on an element, you need to be able to see the element that you want a Behavior to be applied to. For example, in the last section, you built out a simple behavior. Now you need a Shape to apply the Behavior to. You can start by dragging a Rectangle from the toolbar onto the design surface in Blend. Then you need to set the fill to a solid color brush using the Properties pane. The XAML code might look like this:

<rectangle fill="Green" />

Now you should open the Asset Explorer from the toolbar. On the left side of the Asset Explorer, select Behavior and you will see that your behavior is one of the Behaviors listed as well as others that are built in to Expression Blend. Select the Behavior you want and drag it onto the object, in this case Rectangle, and you are finished. The XAML code will appear something like this:

<rectangle fill="Green">
<local:SomeBehavior />

If you look at this closely, you will note that there is a couple namespaces referenced here. You will find these referenced at the top of the XAML document that were inserted by Expression Blend dynamically. A designer or creative is not going to care, but as a developer, it is important for you to realize this. Behaviors, as you can see, are a way to provide rich functionality that is bound to controls in XAML that also, and more importantly, is easy for designers and developers to use in building, maintaining, and customizing the UX/Design of views in Silverlight applications.

(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 )

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Published at DZone with permission of David Kelley. See the original article here.

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