Export Error/Warning Messages from R console?

[YL]

Hi Joe and other Shiny developers, 

I have a question about how to export errors for Shiny app. Let's say I have an app doing mixed effect model, and drawing the fitted population mean plot in Shiny. When there are errors regarding the model fit (e.g., model cannot converge), we see relevant error or warning messages in R console. 

However, when it comes to Shiny, there is no plot generated. We will have to go back to R console in order to check if there is something wrong, which is totally fine for developers. But for users, they may be very confused with regard to why no plot showing up in Shiny.

Therefore, is there a good way to export some error/warning messages? 

(so generally, I want some 'important' error/warning messages, not ALL of them (printed in R console); In addition, we can either export the messages in Shiny or store them as a log in a temporary directory).

Any insight will be highly appreciated! Thank you.

[Joe Cheng]

Well an error ought to show up in Shiny, usually in bold red letters. You can try this by calling stop("boom!") from within a renderPlot, for example.

For warnings, they get printed to the console (and in Shiny server, a log file) but nothing else. You can change the handling yourself using either options(warn=[number]) to turn a warning to an error (see ?options for details) or you can use tryCatch/withCallingHandlers to do something else with warnings. http://adv-r.had.co.nz/Exceptions-Debugging.html You will still need some way of passing the message to the user; we don't have this as a feature of Shiny yet, maybe someone knows of an extension that lets you do this? Maybe in shinyjs?

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[YL]

Hi Joe, 

Thank you for your reply. 

I understand that any error messages will show up in Shiny (in bold red letters). However, some of them are not of interest. For example, when we create uiOutput(s) for users to select variables, I notice that there is often an error message saying 'Error: undefined columns selected' (before this variable gets selected). We do NOT want this type of error to show up to users; rather, we just want to show the modeling errors. I know that we can completely turn off error messages by specifying

tags$style(type = "text/css",

           ".shiny-output-error { visibility: hidden; }",

           ".shiny-output-error:before { visibility: hidden; }")                                                                                              

This will suppress ALL the error messages including any possible ones from modeling/analysis. Therefore, I wonder is there a way to control which error messages to show, and which to not show? Or alternatively, turn off error message for some pages, but turn it back on when needed (like, .shiny-output-error {visibility: show;} or something similar)? 

Sorry if this question seems to be dumb. but yea thank you so much!

[Joe Cheng]

You shouldn't ignore those warnings, but rather endeavor to make them go away. In the case of missing inputs, use the req() function to abort processing cleanly: http://shiny.rstudio.com/articles/req.html

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