问题
At http://shiny.rstudio.com/articles/scoping.html the rules for scoping in shiny are well explained. There are 3 environments or levels nested inside each other: objects available within a function, within a session and within all sessions. Using <- will change the object within the environment you are in and <<- will change it globally i.e. for all sessions.
What if I define a variable within the session but want to change it from within a function?
<- will just change it within the function so not readable by other functions and <<- will change it for all sessions. Is there nothing inbetween? Like "just one level up"?
回答1:
Thanks for that reference Stephane. If an object is defined before the shinyServer() then using <<- anywhere within shinyServer() will change the value for all instances of the app. If the object is defined within shinyServer() then <<- (inside or outside a function) will only change the value for that instance of the app.
I put together a little app with a counter and instance ids to test this. Running two instances of the app and switching between them increasing the count demonstrates the effect of <<-
ui.r
library(shiny)
shinyUI(pageWithSidebar(
headerPanel("Testing Environments"),
sidebarPanel(
actionButton("increment_counter", "Increase Count")
),
mainPanel(
tabsetPanel(
tabPanel("Print", verbatimTextOutput("text1"))
))
))
server.r
instance_id<-1000
shinyServer(function(input, output, session) {
instance_id<<-instance_id+1
this_instance<-instance_id
counter<-0
edit_counter<-reactive({
if(input$increment_counter>counter){
counter<<-counter+1
}
list(counter=counter)
})
output$text1 <- renderPrint({
cat(paste("Session ID: ",Sys.getpid()," \n"))
cat(paste("Global Instance ID: ",instance_id," \n"))
cat(paste("This Instance ID: ",this_instance," \n"))
cat(paste("Button Value: ",input$increment_counter," \n"))
cat(paste("Counter Value: ",edit_counter()$counter," \n"))
})
}) # end server function
来源:https://stackoverflow.com/questions/23409267/environments-in-r-shiny