Extract all of the ggplot2 plots from a list (including sub-lists)

extract_plots_from_list.R

#' @title Extract ggplot2 plots from a list
#' @description Takes a list (potentially containing sublists) and extracts all of the ggplot2 'plot-type' objects from that list into a simple list of 'plot-type' objects.
#' @param x A list object, potentially containing sublists.
#' @return Returns a 'flat', single-level list of all the ggplot2 'plot-type' objects from within `x`, reaching recursively into sub-lists as needed. If there are no 'plot-type' objects, returns an empty list.
#' @note Whether an object is a ggplot2 'plot-type' object is defined here as an object with classes 'gg', 'gTree', or 'gtable'.
#' @export
#' @examples
#' 
#' library(ggplot2)
#' 
#' # Create a list of lists, containing multiple plots
#' generic_plot <- ggplot(mtcars) + geom_point(aes(x = wt, y = mpg))
#' 
#' summary_plots <- list() 
#' summary_plots$Demographics$Socioeconomic$Income <- generic_plot 
#' summary_plots$Demographics$Socioeconomic$Education <- generic_plot 
#' summary_plots$Demographics$Age <- generic_plot 
#' summary_plots$Product_Usage$Purchase_Frequency <- generic_plot
#' 
#' # Call the extraction function
#' extract_plots_from_list(summary_plots)
#' 
extract_plots_from_list <- function(x) {
  
  if (!is.list(x)) {
    stop("`x` must be a list.")
  }
  
  if (length(x) < 1) {
    return(x)
  }
  
  more_lists <- sapply(x, function(object_to_check) {
    'list' %in% class(object_to_check) & !(any(c('gg', "gTree", "gtable") %in% class(object_to_check)))
  })
  
  result <- c(x[!more_lists],
              unlist(x[more_lists],
                     recursive = FALSE))
  
  if (sum(more_lists)) { 
    
    Recall(result)
    
  } else {
    
    is_plot_obj <- sapply(result, function(result_object) {
      any(c('gg', "gTree", "gtable") %in% class(result_object))
    })
    
    result <- result[is_plot_obj]
    return(result)
  }
}