Function to create a function to use as a taskCallBack to monitor changes in objects easily

monitor_object.R

#' Monitor objects using output
#' 
#' This creates a function that is meant to be used as a taskCallBack.
#' You can call the resulting function directly yourself when you want
#' to monitor changes but that isn't the intended use.
#' 
#' @param object_name Character string. The object name you want to montior
#' @param FUN function - The function used to monitor the object for changes.
#'                       The only time notifications will be messaged is when
#'                       the output from FUN changes.
#' @examples 
#' my_monitor <- monitor_object("mtcars", dim)
#' my_monitor()
#' mtcars <- mtcars[1:5,]
#' my_monitor()
#' mtcars <- mtcars[T, ] # no change
#' my_monitor
#' rm(mtcars)
#' 
#' tc <- addTaskCallback(monitor_object("mtcars", dim))
#' mtcars <- mtcars[1:5,]
#' removeTaskCallback(tc)
monitor_object <- function(object_name, FUN = dim){
    
    fun_name <- deparse(substitute(FUN))
    print(fun_name)
    object_dim <- NULL
    if(exists(object_name)){
        object_output <- FUN(get(object_name))
    }
    
    callback_function <- function(...){
        new_output <- NULL
        if(exists(object_name)){
            new_output <- FUN(get(object_name))
        }
        if(!identical(new_output, object_output)){
            
            
            msg <- paste0(object_name, " changed. Summary function: ", fun_name, 
                         "\nOld output: ", paste(object_output, collapse = " "),
                         "\nNew output: ", paste(new_output, collapse = " "))
            object_output <<- new_output
            message(msg)
        }
        return(TRUE)
    }
    
    return(callback_function)
}

# tc <- addTaskCallback(monitor_object("mtcars", nrow))
# removeTaskCallback(tc)