Enhancing Abstraction in App Inventor with Generic Event Handlers

Work on code smells (undesirable programming patterns) in blocks languages has found that programmers often duplicate blocks code rather than abstracting over common patterns of computation using procedure-like features. For example, previous analyses of over a million MIT App Inventor projects have revealed that procedures are used surprisingly rarely in the wild and that many users miss opportunities for using procedural abstraction to avoid code duplication in their projects. In this work, we use data analysis to explain how particular features of App Inventor create barriers to abstracting over event handlers. In many cases, duplicated code in event handlers cannot be extracted into a procedure without using so-called generic blocks that abstract over a particular component (e.g., a label). Generic blocks are rarely used in practice, possibly because programmers do not know about them or find them difficult to use. But even proceduralization with generic blocks does not remove the need for duplicating the event handlers themselves. We address these issues with two enhancements to App Inventor. First, we add generic event handlers, a new form of abstraction that allows specifying a single handler for all components of a particular type. Second, we add a way to easily convert between specific and generic blocks to facilitate genericization, that is, abstracting actions over a particular component to apply to a group of components of that type. We also discuss related design choices and ways to encourage programmers to use the new features to avoid code duplication. Our work is an example of data-informed programming language design, in which the creation or modification of features is informed by the analysis of large datasets of programs from the language's target audience.