another attempt to close rstudio#249, using withCallingHandlers() ins…

…tead of modifying tryCatch()

also closes rstudio#217 if everyone agrees with this approach

R/imageutils.R

@@ -48,9 +48,8 @@ plotPNG <- function(func, filename=tempfile(fileext='.png'),
   }
 
   pngfun(filename=filename, width=width, height=height, res=res, ...)
-  tryCatch(
-    func(),
-    finally=dev.off())
+  dv <- dev.cur()
+  tryCatch(shinyCallingHandlers(func()), finally = dev.off(dv))
 
   filename
 }

R/reactives.R

@@ -354,7 +354,7 @@ Observable <- setRefClass(
       on.exit(.running <<- wasRunning)
 
       ctx$run(function() {
-        result <- withVisible(try(.func(), silent=FALSE))
+        result <- withVisible(try(shinyCallingHandlers(.func()), silent=FALSE))
         .visible <<- result$visible
         .value <<- result$value
       })

R/shiny.R

@@ -146,7 +146,7 @@ ShinySession <- setRefClass(
 
         obs <- observe({
 
-          value <- try(func(), silent=FALSE)
+          value <- try(shinyCallingHandlers(func()), silent=FALSE)
 
           .invalidatedOutputErrors$remove(name)
           .invalidatedOutputValues$remove(name)
@@ -1383,15 +1383,12 @@ runApp <- function(appDir=getwd(),
 
   .globals$retval <- NULL
   .globals$stopped <- FALSE
-  tryCatch(
+  tryCatch(shinyCallingHandlers(
     while (!.globals$stopped) {
       serviceApp()
       Sys.sleep(0.001)
-    },
-    finally = {
-      timerCallbacks$clear()
     }
-  )
+  ), finally = timerCallbacks$clear())
 
   return(.globals$retval)
 }

R/utils.R

@@ -249,6 +249,16 @@ parseQueryString <- function(str) {
   setNames(as.list(values), keys)
 }
 
+# decide what to do in case of errors; it is customizable using the shiny.error
+# option (e.g. we can set options(shiny.error = recover))
+shinyCallingHandlers <- function(expr) {
+  withCallingHandlers(expr, error = function(e) {
+    handle <- getOption('shiny.error')
+    if (is.null(handle) || !is.function(handle)) return()
+    if (length(formals(handle)) > 0) handle(e) else handle()
+  })
+}
+
 #' Print message for deprecated functions in Shiny
 #'
 #' To disable these messages, use \code{options(shiny.deprecation.messages=FALSE)}.