Snapshot function errors when the output itself is error

Plotly uses snapshotPreprocessOutput to remove any output values that are randomly generated. This is great.

However, if the render function outputs an error, the result is not a valid JSON. Because of this, the snapshotPreprocessOutput function errors out and frameworks like shinytest error out as well.

Example code:

library(plotly)
ui <- fluidPage(
  checkboxInput("go", label="Render plot"),
  plotlyOutput("plot")
)
server <- function(input, output, session) {
  output$plot_plotly <- renderPlotly({
    shiny::req(input$go) #induce a silent.error if input$go is FALSE
    ggplot(data.frame(x=rnorm(1000))) + 
      geom_histogram(aes(x=x))
  })
}
shinyApp(ui=ui, server=server)

Test code:

library(shinytest)
app <- ShinyDriver$new(path="tmp/")
app$getAllValues()

app$getAllValues() reports an error, because fetching the snapshot URL reports an error 500.

The issue can be easily solved in
https://github.com/ropensci/plotly/blob/bcae42bbeea8d8368fb3ca881243e6acd0c661b4/R/shiny.R#L58
I suggest you check if value is a character string before parsing as JSON.

从 R/shiny.R 大约第 58 行开始，这里 snapshotPreprocessOutput 会解析 snapshot 值。使用提供的 shinytest 示例重现该问题，包括返回错误的 renderPlotly 输出，并验证当 snapshot 输出不是有效 JSON 时，app$getAllValues() 不再失败。