The MXML editor in Flex Builder lets you work in either Source or Design mode. You can also use Flex Builder to create a split view so that you can work in both Source and Design modes simultaneously.

View your file in Design mode

 Click Design at the top of the editor area.

View your file in Source mode

 Click Source at the top of the editor area.

Work in both Source and Design modes simultaneously

1. Right-click (Control-click on Mac OS) the editor's tab and select New Editor.

   You now have two editor tabs for the same file.

2. Drag one of the tabs to the right to position the editor windows side-by-side.
3. Set one of the editors to Design mode, and set the other editor to Source mode.

Switch between the Source and Design modes

 Press Control+`(Left Quote).