Flex 4 provides new containers that you should use instead of the previous HBox and VBox available in Flex 2 and 3 and the Spark Group is one of the most used to display visual childs.
This container allow you to set the layout property, used to define the rules to display these visual elements and their properties (i.e. gap, alignments, padding and so on).

In Flex 2 and 3, with Binding, you could automatically copies the value of a property of a source object to a property of a destination object when the source property changed.
Flex 4 has introduced the Bidirectional Binding (also know as “two-way” binding”) that allows to set two objects as the source and the destination for each other.

In this article we’ll build an XML-driven video gallery exploring a lot of new Flex 4 features (out in spring 2010): skinning spark components, spark containers, new states and transitions; but we’ll also use some “old” concepts like itemRenderers, custom events dispatch, load data from an xml file (using the Singleton design pattern), value objects, embedding fonts, css and a lot of other useful stuff when you develop Flex applications.

I’m really happy to see that in Flash Builder 4, the next release of Flex out in spring 2010, a complete video player component will be finally available with a lot of integrated features: play/pause, progress bar, time and fullScreen button too.

See it in action (click on buttons to play videos)