I have been using the ‘attached behaviors’ pattern more and more in my adoption of WPF. In general, I spurn significant ‘code behind’ in my windows/ controls, etc – unless absolutely necessary. Using attached properties in WPF, you can actually build up a library of behaviors that you can attach to various objects in your application. I have a large # of these behaviors in my current project – especially for Xceed’s WPF DataGridControl.

I will use this simple example to illustrate the concepts.

The goal is to allow a window to resize when a specific tab is focused. I have a window with a number of tabs, and for most tabs it is appropriate to have the same size window (relatively small). There is one tab however that is too squashed to look at in this small-ish size. What I decided to do was to save the users from manually resizing the window each time they went to it. The result was this simple ‘attached behavior’.

Attached properties in WPF are properties that you can attach arbitrarily to any dependency object. In this example, I simply attached my TabBehavior to the TabItem like this in XAML:

<TabItem  Behaviors:TabBehaviors.ResizeOnFocus=”805″ … />

The underlined bit is my attached property. It is simply giving the size I want to resize the parent window to when this tab gets focus.

So – how does this work? Well the effect of adding an attached property is to pass the value in the argument + the object the attached property is decorated on… So you get a call with the TabItem and the 805 value passed into your code.

The 1st step is to declare a public static class called TabBehaviors, next we declare our 1st attached property; ResizeOnFocus – like we see above:

This is very much like a normal Dependecy

Property in WPF. Note the last bit – the callback OnResizeFocusChanged; this is what we get when the property is set on an object.

The OnResizeFocusChanged method is where the magic starts (I have renamed that method after I took my screenshots ). The 1st argument passed in is the object the attached property is attached to – the TabItem in our example. The 2nd argument has the NewValue – the 805 value in our current example. The next bit – the DependencyPropertyDescriptorFromProperty is a way to add a ‘changed’ handler to the IsSelected property of the tab item. This is sort of like using a .net event, but it is just the WPF Dependency Property way to do so. 

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