##' identify node by interactive click
 ##'
 ##'
 ##' @rdname identify
 ##' @title identify
 ##' @param x tree view
 ##' @param ... additional parameters
 ##' @return node id
 ##' @importFrom grid convertX
 ##' @importFrom grid convertY
 ##' @importFrom grid pushViewport
 ##' @importFrom grid grid.locator
 ##' @importFrom grid unit
 ##' @importFrom grid dataViewport
 ##' @importFrom graphics identify
 ##' @method identify gg
 ##' @export
 ##' @author Guangchuang Yu
 identify.gg <- function(x, ...) {
     tree_view <- x
     ## not used, since if x=NULL, it will call graphics::identify
     ## tree_view %<>% get_tree_view 
 
     x <- tree_view$data$x
     y <- tree_view$data$y
 
     pushViewport(dataViewport(x, y))
     loc <- grid.locator('in') %>% as.numeric
 
     xx <- as.numeric(convertX( unit(x,'native'), 'in' ))
     yy <- as.numeric(convertY( unit(y,'native'), 'in' ))
 
     idx <- which.min( (xx-loc[1])^2 + (yy-loc[2])^2 )
     return(tree_view$data$node[idx])
 }