Colour gradient input for R-Shiny

A colour gradient picker for Shiny, implemented as a shiny module. This can be used, for example, in ggplots with the scale_color_gradientn() function. This can most likely be improved upon by making it a proper input instead of a module--left as an exercise for the reader :)

The UI function is gradientInputUI() and has two parameters:

• width: The width of the input (either in pixels or in percent) (default: 300px)
• resource_path: The path where the JavaScript and CSS files are located.

The server function is gradientInput() and has 4 parameters:

• init_cols: The initial state of the colour gradient. There are 4 different values that are accepted:
  1. If a single integer N is provided (eg. 5), then the input is initialized with N colours. The colours are random and their positions are evenly distributed. If nothing is provided, the default behaviour is to initialize the input with 2 colours.
  2. If a vector of N integers is provided (each number must be between 0 and 100) (eg. c(10, 40, 90)), the input is initialized with N different colours and each number in the vector corresponds to the position of one colour. The colours are random.
  3. If a vector of N colours is provided (a colour can be any R colour name or a HEX string) (eg. c("red", "blue", "#00FF00")), the input is initialized with these N colours. The positions of the colours are distributed evenly.
  4. If a dataframe with 2 columns "col" (any R colour) and "position" (number between 0 and 100) and N rows is provided, the input is initialized with N colours. Each colour uses the "col" column as its initial colour and "position" as its initial position.
• allow_modify: Whether or not the user can add, delete, and change positions of colours. (default: true)
• col_expand: Whether or not the colour input can expand into a full colour picker text box that lets the user write colour names in English. (default: false)

The return value is a list with two items:

• result: a dataframe with 2 columns: position (the left position, between 0 and 100) and col (the colour hex string).
• reset: a function that will reset the gradient input back to its original state.

Example usage:

library(shiny)
source("gradientInput.R")

ui <- fluidPage(
  gradientInputUI("cols"),
  tableOutput("result"),
  actionButton("reset", "Reset")
)
server <- function(input, output, session) {
  gradient <- callModule(gradientInput, "cols", init_cols = c(10, 50, 70))
  output$result <- renderTable(gradient$result())
  
  observeEvent(input$reset, {
    gradient$reset()
  })
}
shinyApp(ui, server)

Required packages: glue, colourpicker, shinyjs, shinyjqui, uuid, dplyr