New in Flex 3 is the vaguely named, but oh-so cool Advanced Constraints feature. Constraints was originally an SDK feature built atop the existing Flex 2 constraint-based layout system. Remember that in Flex 2 you could use the constraint styles (left/right/top/bottom/verticalCenter/horizontalCenter) to constrain controls to the edges of their parent container’s border. You could use constraint-based layout in any container that supported absolute positioning, like Canvas, Panel and Application. For Flex 3, the Advanced Constraints feature introduces the concept of ConstraintColumn and ConstraintRow objects that can be used to partition up absolute positioning containers. We broadened the constraint syntax to allow for controls to be constrained to these columns and rows and using this mechanism you can do really cool things like sibling relative constraints and better re-flowing layouts. This functionality is still in the Flex 3 SDK and you can definitely hand code these layouts relatively easily. The FlexBuilder 3 code editor gives code hints and code editing help for the new Advanced Constraints syntax out of the box. Check out the documentation, feature writeup and sample code on labs for more in-depth examples of syntax and usage and how to leverage Advanced Constraints in your layouts.
Once the SDK implementation was done (somewhere after Flex 3 beta 1), we embarked on building a really robust authoring UI in FlexBuilder 3 DesignView to help create and visualize these new constraints. It was an ambitious (and fun!) feature. However, as we got closer to feature lockdown, we realized there were still too many open issues and didn’t feel comfortable with where the UI needed to go. It was a hard decision for all, but we decided to pull the UI from FlexBuilder 3 and fully intend to have a similar authoring UI for constraints in Thermo and future versions of FlexBuilder.
For everyone who really liked the UI and got used to that workflow, worry not, it will come back. You’ll just have to wait a little bit longer, but there are so many Flex goodies to play with in the meantime!