Reduce instances when dplyr::lag() warning is shown

The warning was shown whenever it detected dplyr is installed, even if
the user wasn't actively using dplyr. That caused an excessive amount
of noise when other packages attached xts (e.g. quantmod).

Thanks to Duncan Murdoch for the report and suggested fix!

Fixes #393.

DESCRIPTION

@@ -1,7 +1,7 @@
 Package: xts
 Type: Package
 Title: eXtensible Time Series
-Version: 0.13.0.1
+Version: 0.13.0.2
 Authors@R: c(
   person(given=c("Jeffrey","A."), family="Ryan", role=c("aut","cph")),
   person(given=c("Joshua","M."), family="Ulrich", role=c("cre","aut"), email="josh.m.ulrich@gmail.com"),

R/zzz.R

@@ -64,42 +64,26 @@ function(pkg, generic, class, fun = NULL)
   }
 
   if (warn_dplyr_lag && dplyr_will_mask_lag) {
-
-    dplyr_path <- find.package("dplyr", quiet = TRUE, verbose = FALSE)
-
-    is_dplyr_installed <- (length(dplyr_path) > 0)
-    is_dplyr_attached <- "package:dplyr" %in% search()
-
-    if (is_dplyr_installed) {
-
-      msg <- "
-################################### WARNING ###################################
-# We noticed you have dplyr installed. The dplyr lag() function breaks how    #
-# base R's lag() function is supposed to work, which breaks lag(my_xts).      #"
-
-      if (is_dplyr_attached) { msg <- paste0(msg, "
+      ugly_message <- "
+######################### Warning from 'xts' package ##########################
 #                                                                             #
-# Calls to lag(my_xts) that you enter or source() into this session won't     #
-# work correctly.                                                             #")
-      } else {
-          msg <- paste0(msg, "
+# The dplyr lag() function breaks how base R's lag() function is supposed to  #
+# work, which breaks lag(my_xts). Calls to lag(my_xts) that you type or       #
+# source() into this session won't work correctly.                            #
 #                                                                             #
-# If you call library(dplyr) later in this session, then calls to lag(my_xts) #
-# that you enter or source() into this session won't work correctly.          #")
-      }
-
-      msg <- paste0(msg, "
-#                                                                             #
-# All package code is unaffected because it is protected by the R namespace   #
-# mechanism.                                                                  #
+# Use stats::lag() to make sure you're not using dplyr::lag(), or you can add #
+# conflictRules('dplyr', exclude = 'lag') to your .Rprofile to stop           #
+# dplyr from breaking base R's lag() function.                                #
 #                                                                             #
+# Code in packages is not affected. It's protected by R's namespace mechanism #
 # Set `options(xts.warn_dplyr_breaks_lag = FALSE)` to suppress this warning.  #
 #                                                                             #
-# You can use stats::lag() to make sure you're not using dplyr::lag(), or you #
-# can add conflictRules('dplyr', exclude = 'lag') to your .Rprofile to stop   #
-# dplyr from breaking base R's lag() function.                                #
-################################### WARNING ###################################")
-      packageStartupMessage(msg)
+###############################################################################"
+    if ("package:dplyr" %in% search()) {
+      packageStartupMessage(ugly_message)
+    } else {
+      setHook(packageEvent("dplyr", "attach"),
+              function(...) packageStartupMessage(ugly_message))
     }
   }
 }