Non-standard evaluation of expressions in S4 conte

问题:

This is borrowed from shiny's function exprToFunction which is used in their reactive function.

Actual question

How can I delay the evaluation of an expression (e.g. specified via arg expr) in order to "capture" its content when calling a S4 method (as opposed to a standard R function)?

Example

Note that x_1 does not exist yet, that's why we want to delay the evaluation of expr and just "capture" its content.

Function captureExpression:

captureExpression <- function(
  expr,
  caller_offset = 1,
  brackets = TRUE
) {
  out <- eval(substitute(substitute(expr)), parent.frame(caller_offset))
  if (brackets && class(out) != "{") {
    out <- substitute({CODE}, list(CODE = out))
  }
  out
}

captureExpression(x_1 * 2)
# {
#     x_1 * 2
# }

Passing unevaluated expr through works when calling a standard R function:

foo <- function(expr) {
  captureExpression(expr = expr)
}

foo(x_1 * 2)
# {
#     x_1 * 2
# }

Passing unevaluated expr through does not work when calling an S4 method:

setGeneric(
  name = "bar",
  signature = c(
    "expr"
  ),
  def = function(
    expr,
    ...
  ) {
    standardGeneric("bar")       
  }
)
setMethod(
  f = "bar", 
  signature = signature(
    expr = "ANY"
  ), 
  definition = function(
    expr,
    ...
  ) {
  captureExpression(expr = expr, ...)    
})

Applying the S4 method:

bar(x_1 * 2)
# Error in bar(x_1 * 2) : 
#   error in evaluating the argument 'expr' in selecting a method for function 'bar': Error: 
# object 'x_1' not found

bar(x_1 * 2, caller_offset = 2)
# Error in bar(x_1 * 2, caller_offset = 2) :
#   error in evaluating the argument 'expr' in selecting a method for function 'bar': Error: 
# object 'x_1' not found

bar(x_1 * 2, caller_offset = 3)
# Error in bar(x_1 * 2, caller_offset = 3) :
#   error in evaluating the argument 'expr' in selecting a method for function 'bar': Error: 
# object 'x_1' not found

I guess this has to do with the way the method dispatch is actually carried out. However, maybe there's a way that this can be done anyways.