[FEATURE] display console messages in shiny app

[kevinrue]

Display console messages in a modal while downloading/loading a data set.

The idea is to give users an idea of what is going on in the R session, especially when data sets are being downloaded for the first time, as this can take some time, during which users may think that the app has frozen.

A progress bar was added in #4 but it is unable to report whether a data set is being downloaded or loaded from cache, as this is all handled within a single call to the ExperimentHub.

Attempt 1

I've used capture.output(..., type = 'message', file = NULL), stored the result to rObjects and used verbatimTextOutput() to display it in a modal. However, file=NULL does not return anything until the command has completed, which is useless in terms of interactivity.

In future attempts, try using file=tempfile(), and invalidateLater() to regularly check whether the file is being updated. If so, the contents of the file could be displayed in the modal as the file is being written/updated.

[kevinrue]

Attempt 2

See branch kra-capture-output for an implementation that progressively captures console output to a file.

However, the single-threaded nature of R and the priority of reactive events in Shiny also results in the app displaying the final contents of the file after the download has completed.

[kevinrue]

the in-app progress bar is just fine