shiny/inst/examples-shiny/03_reactivity

This example demonstrates a core feature of Shiny: reactivity. In the server function, a reactive called datasetInput is declared.

Notice that the reactive expression depends on the input expression input$dataset, and that it's used by two output expressions: output$summary and output$view. Try changing the dataset (using Choose a dataset) while looking at the reactive and then at the outputs; you will see first the reactive and then its dependencies flash.

Notice also that the reactive expression doesn't just update whenever anything changes--only the inputs it depends on will trigger an update. Change the "Caption" field and notice how only the output$caption expression is re-evaluated; the reactive and its dependents are left alone.