Building and Using Expression Blend Behaviors

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);

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