17 Print debugging

Published

2024-09-03

Warning

The contents for section are under development. Thank you for your patience.

Accessing applications

I’ve created the shinypak R package In an effort to make each section accessible and easy to follow:

Install shinypak using pak (or remotes):

# install.packages('pak')
pak::pak('mjfrigaard/shinypak')

Review the chapters in each section:

library(shinypak)
list_apps(regex = 'debug')
## # A tibble: 5 × 2
##   branch                   last_updated       
##   <chr>                    <dttm>             
## 1 15_debug-util-funs       2024-09-04 10:06:57
## 2 25.0_debug-error         2024-02-13 04:29:39
## 3 25.1_debug-selected_vars 2024-01-15 10:29:25
## 4 25.2_debug-var_inputs    2024-01-15 10:25:12
## 5 25.4_debug-print         2024-01-15 10:04:21

Launch the app:

launch(app = "23.1_debug-error")

Download the app:

get_app(app = "23.1_debug-error")

Launch app with the shinypak package:

launch('25.4_debug-print')

One of the best tried and tested methods of debugging is simply adding a cat() or print() call somewhere in your code to print variables or objects to the R console. This is a basic but effective way to track variable changes.

We can do something similar in Shiny applications by combining verbatimTextOutput(), renderPrint(), and reactiveValuesToList():

In mod_var_input:

Place a verbatimTextOutput() in the ui function.

    code("module reactive values"),
    verbatimTextOutput(outputId = ns("mod_vals"))
  )

1: Optional label
2: Include the ns() for the inputId

In a renderPrint(), use reactiveValuesToList() to gather the inputIds and pass them to print() (I’m actually using lobstr::tree() to give a clearer display).

  output$mod_vals <- renderPrint({
    lobstr::tree(
      reactiveValuesToList(
        x = input,
        all.names = TRUE
      )
    )
  })

1: Collect reactive values in module
2: Print these values to the UI
3: Include all reactive objects

Load the package and run the app:

Ctrl/Cmd + Shift + L

ℹ Loading sap

launch_app(options = list(test.mode = FALSE), run = 'p')

Now we can see the reactive values from our module in the application sidebar!

We can also use this ‘print’ method to explore reactive values at various locations in our application. For example, if we wanted to print the reactive values for multiple modules in an app, we can use these methods in the top level movies_ui() and movies_server() functions.

In the bslib portion of movies_ui():

Add verbatimTextOutput() with an optional label

code("reactive values"),
verbatimTextOutput(outputId = 'vals')
)

In movies_server():

Collect all the inputIds with reactiveValuesToList() and print with print() or lobstr::ast()

all_vals <- reactive({
  reactiveValuesToList(x = input, all.names = TRUE)
})

output$vals <- renderPrint({
  lobstr::tree(all_vals())
})

Load the package and run the app:

Ctrl/Cmd + Shift + L

ℹ Loading sap

launch_app(options = list(test.mode = FALSE), 
  run = 'p', bslib = TRUE)

(a) ‘Print’ in `launch_app(bslib = TRUE)`

Here we can see both levels of reactive values (from the module and the UI/server functions). The handy thing about this method is that the values change when we interact with the application:

(a) Changing values in `launch_app(bslib = TRUE)`