1. A method, comprising: providing a design interface by a computing system programmed to provide an application development environment, the design interface depicting a runtime appearance of a plurality of interface elements of an application under development, each of the interface elements defined in source code accessible by the application development environment, the source code including declarative expressions defining the interface elements, the source code expressed in a declarative markup language; receiving, through the design interface, data representing a selection of a plurality of the interface elements; identifying the expressions corresponding to the selected interface elements in one or more code segments of the source code for the application under development; defining a new declarative expression; generating at least one code segment using the identified expressions; and storing the generated code segment in a computer readable medium, the stored code segment associated with the new declarative expression.