Page class member variable scope

I hope I selected a valid subject !

When I handle a VALUE_CHANGED event for a Select component. I receive the
current value for the Select component as an event handler method parameter.

I use this parameter to set a member variable in the page's class.

@OnEvent(component = "parameterType", value = EventConstants.VALUE_CHANGED)

In another part of the page, I use a Delegate to conditionally display a
block.

public Block getPairParameterInputBlock() {

What I'm facing there is that *this.parameterType* is always null and never
sees the value set to it in the Select component's event handler method !

I understand there could be other ways to achieve the same behavior but why
isn't this behaving as I expect ?

*---------------------*
*Muhammad Gelbana*
http://www.linkedin.com/in/mgelbana