Shiny allows you to build a graphical user interface for your R script. The so-called Shiny Apps are specific R scripts that run on special servers. The script is split into two parts: a part that does all the heavy duty calculations on the server; and a part that generates a graphical user interface to interact with the user. I don't intend to explain Shiny in detail in this post. You can consult the Shiny page, which offers excellent information and tutorials.
In this post I will focus on a specific issue with Shiny. Information thrown from the user interface can be caught by ‘event observers’. For instance, when a user clicks a button on the interface this can be detected on the server, which then executes a specific part of the script. Shiny offers many standard user interface elements, but custom elements can also be created.
You can for instance create a link that can be clicked, which then sends information to the server. The problem is that such ‘events’ (clicks) will only be detected by the Shiny server when the information that is being send changes. So, when you create a custom interface element that should send the same information each time it is clicked, it will not be detected.
In the example below I show how you can work around this issue. I solved it by negating (i.e., multiplying the information with minus one) each time the custom interface element is clicked. This way, it will get detected each time by the server. On the server side you only need to take the absolute value of the data sent by the interface.
Below you will find a fully commented source code showing the work around described in this post. It also shows you the App actually running on the shinyapps.io server.
Note: the app runs under my free account at shinyapps.io. So, when it is not working, I probably ran out of my monthly server-time. In that case please come back later and try again. Or get the source code from my Gist:
