Reactive R/Shiny 示例在参数化一个变量后失败

Question

我有以下简单且有效的 shiny-Rmarkdown 文档。它产生一个滑块并在下面输出一个日期，该日期根据滑块输入进行调整。

---
runtime: shiny
output: html_document
---

```{r}
library(rmarkdown); library(ggvis); library(shiny);

sliderInput("sliderDays", "Days forward from 1/1/2014: ", min=0, max=45, value=1, step=1)

graphDate = reactive({as.Date("2014-01-01") + input$sliderDays})
renderPrint(graphDate()) 
```

但是，如果我将 graphDate = reactive({as.Date("2014-01-01") + input$sliderDays}) 替换为

startDate <- as.Date("2014-04-01")
graphDate <- reactive({startDate + input$SliderDays})

然后 renderPrint(graphDate()) 的输出来自

1 "2014-04-01" [this changes on slider input, but is always a date]

到

character(0)

我不明白为什么必须在 graphDate 反应函数中创建和使用 startDate，而不是在其上方定义并在反应函数中引用，这样才能起作用。

sessionInfo()
## R version 3.2.0 (2015-04-16)
## Platform: i386-w64-mingw32/i386 (32-bit)
## Running under: Windows 7 x64 (build 7601) Service Pack 1
## 
## locale:
## [1] LC_COLLATE=English_United States.1252 
## [2] LC_CTYPE=English_United States.1252   
## [3] LC_MONETARY=English_United States.1252
## [4] LC_NUMERIC=C                          
## [5] LC_TIME=English_United States.1252    
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
## [1] DT_0.0.52         shiny_0.12.0      knitr_1.10.5      ggvis_0.4.1.9000 
## [5] rmarkdown_0.6.2.2
## 
## loaded via a namespace (and not attached):
##  [1] Rcpp_0.11.6      digest_0.6.8     dplyr_0.4.1      assertthat_0.1  
##  [5] mime_0.3         R6_2.0.1         xtable_1.7-4     jsonlite_0.9.16 
##  [9] DBI_0.3.1        formatR_1.2      magrittr_1.5     evaluate_0.7    
## [13] stringi_0.4-1    rstudioapi_0.3.1 tools_3.2.0      stringr_1.0.0   
## [17] htmlwidgets_0.4  yaml_2.1.13      httpuv_1.3.2     parallel_3.2.0  
## [21] htmltools_0.2.6

Answer 1

似乎对我有用。我没看错问题吗？

版本信息和会话信息：

Reactive R/Shiny 示例在参数化一个变量后失败

Reactive R/Shiny Example Fails after Parameterizing One Variable

r

r-markdown

shiny