Fundamental understanding of reactive lists - r

Question

I wrote a little standard application in R that is running pretty fine.
It scans the *.png files in a folder and extracts some statistics on pixel granularity and colours.
Now it is my idea to transfer this to shiny. Although I once wrote some smaller Shiny-Apps, i never used reactivity intensely.
The basic idea of the here presented code is:
to choose png files from a folder
show the number of chosen images
initiate some calculations on the chosen files after clicking a button
displaying the extracted parameters
I now have critical problems with the basic design of the reactive variables, obviously as my construct in mind is not suitable.
Is there someone who could advise me the basic outline of the shiny construct?
I tried to reduce the code to the bare minimum. I hope one can follow my thoughts.
library (shiny)
options (shiny.maxRequestSize = 250 * 1024^2)
# increase maximum data upload to 250MB
const_Number_of_Variables = 5
# just to keep the number of extracted parameters
# up to date when new parameters will be included
ui <- fluidPage(
# just a simple UI with some basic elements
fileInput ("ui_IMGfiles",
"Choose PNG files",
multiple = TRUE,
accept = c ("image/png",
".png")
),
tag$hr,
tableOutput ("ui_IMGfilelistcontents"),
tag$hr,
textOutput ("imagecount"),
tag$hr,
actionButton ("ui_btn_CalcFeatures", "Calculate image features", class = "btn-warn"),
tag$hr,
tableOutput('IMG_featuretable')
)
IMG_filelist <- reactiveValues (data = NULL)
IMG_features <- reactiveValues (val1 = NULL, val2 = NULL, val3 = NULL, val4= NULL, val5 = NULL, val6 = NULL)
IMG_data <- reactiveValues (data = NULL)
server <- function(input, output) {
observeEvent (input$ui_IMGfiles, {
# input$ui_IMGfiles will be NULL initially. After the user selects
# and uploads files, it will be a data frame with 'name',
# 'size', 'type', and 'datapath' columns. The 'datapath'
# column will contain the local filenames where the data can
# be found.
IMG_filelist$data <- input$ui_IMGfiles
})
output$ui_IMGfilelistcontents <- renderTable({
# if file list is not initialised, return NULL
if (is.null (IMG_filelist$data () ))
return(NULL)
#finally display the list of images
IMG_filelist$data ()
})
output$imagecount <- renderText({
# whenever the file list changes, identify
# the number of elements from the length of list entries
paste0 ("# of files: ", length (IMG_filelist$data () ))
}
)
IMG_data <- observeEvent (input$ui_btn_CalcFeatures, {
# check whether some images are loaded
if (is.null (IMG_filelist)) {IMG_data = NULL}
IMG_files_count = length (IMG_filelist)
# prepare the feature list that will be
# overwritten every time the button is pressed
IMG_data$data = matrix (NaN, nrow = 1, ncol = const_Number_of_Variables)
for (i in 1:IMG_files_count) {
# now read the i'th image from the list and pass it to the evaluation function
aSourceImage = readPNG (IMG_filelist$data [i]$datapath, native = FALSE, info = TRUE)
# extract the desired parameters and attach them to the list
IMG_data$data = rbind ( c ("val1", "Val2", "val3", "Val4", "etc"),
#fn_ExtractTileInformation (aSourceImage, aFileFullPath, aFileName, 128),
IMG_data$data ())
}
})
output$IMG_featuretable <- renderDataTable (img_data$data ())
}
shinyApp (ui, server)