如何捕捉S4方法选择过程中发送

Question

警告警告时产生的S4泛型函数的评价参数不能使用withCallingHandlers被抓（）。

插图withCallingHandlers的正常行为：

### simple function that sends a warning 
send_warning <- function() { 
    warning('send_warning') 
} 
send_warning() 
# Warning message: 
# In send_warning() : send_warning 

### the warning can be caught by withCallingHandlers 
withCallingHandlers(send_warning(), warning = function(w) 
    { stop('got warning:', w) }) 
# Error in (function (w) : 
# got warning:simpleWarning in send_warning(): send_warning 

现在，让我们做一个S4调度过程中发出的警告：

### simplest method ever 
setGeneric('my_method', function(x) standardGeneric('my_method')) 
setMethod('my_method', 'ANY', function(x) str(x)) 

### call it with an argument that produces a warning: seems to work 
my_method(warning('argh')) 
# chr "argh" 
# Warning message: 
# argh 

### !!! BUT it is not caught !!!!!!!!!!!!!!!!!!: 
withCallingHandlers(my_method(warning('argh')), warning = function(w) 
    { stop('got warning:', w) }) 
# chr "argh" 
# Warning message: 
# argh 

最后一个例子表明，这期间不会发生S4方法调用：

setGeneric('my_method2', function(x) standardGeneric('my_method2')) 
setMethod('my_method2', 'ANY', function(x) warning('my_method2')) 
my_method2() 
# Warning message: 
# In my_method2() : my_method2 

### warning is caught 
withCallingHandlers(my_method2(), warning = function(w) 
    { stop('got warning:', w) }) 
# Error in (function (w) : 
# got warning:simpleWarning in my_method2(): my_method2 

从我所了解的情况来看，似乎有在S4派遣期间发出的警告的特殊行为。我想知道为什么以及如何捕捉它们。

Answer 1

我最初的猜测是，这是因为方法分派围巾警告

f = function(x) withCallingHandlers(x, warning=function(w) { 
    cat('muffled\n') 
    invokeRestart("muffleWarning") 
}) 

然后

> withCallingHandlers(f(warning("f")), warning=function(w) message("caught")) 
muffled 

挖越深，实际的报警电话是在这个C堆栈跟踪

#0 do_warning (call=0x3d4ad50, op=0x9c4bf0, args=0x45b9968, rho=0x45b9c40) at /home/mtmorgan/src/R-devel/src/main/errors.c:1140 
#1 0x00000000004b4198 in bcEval (body=<optimized out>, rho=<optimized out>, useCache=<optimized out>) at /home/mtmorgan/src/R-devel/src/main/eval.c:4700 
#2 0x00000000004bff40 in Rf_eval (e=0x3d45618, rho=0x45b9c40) at /home/mtmorgan/src/R-devel/src/main/eval.c:554 
#3 0x00000000004c4e4d in Rf_applyClosure (call=0x45b8630, op=0x3d45730, arglist=<optimized out>, rho=0x9e7638, suppliedenv=0x9e7670) at /home/mtmorgan/src/R-devel/src/main/eval.c:1033 
#4 0x00000000004c0005 in Rf_eval (e=0x45b8630, rho=0x9e7638) at /home/mtmorgan/src/R-devel/src/main/eval.c:670 
#5 0x00000000004c0695 in forcePromise (e=0x45b9410) at /home/mtmorgan/src/R-devel/src/main/eval.c:458 
#6 0x00000000004c0462 in Rf_eval (e=0xa1d338, rho=0x45b9368) at /home/mtmorgan/src/R-devel/src/main/eval.c:577 
#7 0x000000000046fbfb in protectedEval (d=0x7fffffffc7f0) at /home/mtmorgan/src/R-devel/src/main/context.c:750 
#8 0x0000000000470d48 in R_ToplevelExec (fun=0x46fbe0 <protectedEval>, data=0x7fffffffc7f0) at /home/mtmorgan/src/R-devel/src/main/context.c:705 
#9 0x0000000000470de7 in R_tryEval (e=<optimized out>, env=<optimized out>, ErrorOccurred=0x7fffffffc89c) at /home/mtmorgan/src/R-devel/src/main/context.c:764 
#10 0x0000000000470e26 in R_tryEvalSilent (e=<optimized out>, env=<optimized out>, ErrorOccurred=<optimized out>) at /home/mtmorgan/src/R-devel/src/main/context.c:787 
#11 0x00007ffff49230b9 in R_dispatchGeneric (fname=0x44b37e8, ev=0x45b9368, fdef=0x45b92f8) at /home/mtmorgan/src/R-devel/src/library/methods/src/methods_list_dispatch.c:993 
#12 0x00000000004f5337 in do_standardGeneric (call=<optimized out>, op=<optimized out>, args=<optimized out>, env=0x45b9368) at /home/mtmorgan/src/R-devel/src/main/objects.c:1167 
.... 

其中在R_dispatchGeneric方法调度的C代码中是tryin摹通过评估找出类的参数，它

989   if(arg_sym == R_dots) { 
990   thisClass = dots_class(ev, &check_err); 
991   } 
992   else { 
993   PROTECT(arg = R_tryEvalSilent(arg_sym, ev, &check_err)); 
994   if(!check_err) 
995    thisClass = R_data_class(arg, TRUE); 
996   UNPROTECT(1); /* for arg */ 
997   } 

这似乎是做来实现C级错误处理程序：

998   if(check_err) 
999   error(_("error in evaluating the argument '%s' in selecting a method for function '%s': %s"), 
1000     CHAR(PRINTNAME(arg_sym)),CHAR(asChar(fname)), 
1001     R_curErrorBuf()); 

Rf_tryEvalSilent评估在顶层，像在没有建立调用处理程序的命令提示符;你可以在C代码，其中处理器堆栈设置为NULL

686 Rboolean R_ToplevelExec(void (*fun)(void *), void *data) 
687 { 
688  RCNTXT thiscontext; 
689  RCNTXT * volatile saveToplevelContext; 
690  volatile SEXP topExp, oldHStack; 
691  Rboolean result; 
692 
693 
694  PROTECT(topExp = R_CurrentExpr); 
695  PROTECT(oldHStack = R_HandlerStack); 
696  R_HandlerStack = R_NilValue; 
697  saveToplevelContext = R_ToplevelContext; 

看到这一点，因为参数评测启动，但不处理，一个变通办法可能是

my_method(withCallingHandlers(warning('arrgh'), warning=function(w) ...)) 

但这可能是不实际的。