Observer evaluation too eager?

[Norman Packard]

Hi!

I am misunderstanding something about observe().

In the example below, I am expecting the contents of observe() to be executed only when (and immediately when) input$bar changes, i.e., when the actionButton is pushed.

Instead, the contents are executed immediately.  In the warning message (seen in the log file), I see that input$bar=0, its initial value. 

Shouldn't observe() wait to get executed until the button is pushed??

shinyUI(

    pageWithSidebar(

        headerPanel('Foo'),

        sidebarPanel(

            actionButton('bar',"Bar")

            ),

        mainPanel(            )

        )

    )

shinyServer(function(input, output,session) {

    observe({

        input$bar

        warning(paste('input$bar value = ',input$bar))

        warning("shouldn't get here without pushing 'bar' button")

    })

})

[Sigbert Klinke]

I guess that shiny must run once through the anonymous server function at the beginning. In observe could be something that is necessary to generate some initial output.

[David Wahman]

I believe the following code gets you the desired result.  This forces the update to wait until the button has been pushed at least once to move it from 0 to 1.

shinyServer(function(input, output, session) {

  observe({

    if(input$bar == 0) return(NULL)

    warning(paste('input$bar value = ',input$bar))

    warning("shouldn't get here without pushing 'bar' button")

  })

})

[Norman Packard]

Sigbert, David,

Thanks very much for your replies.  Sigbert, I guess you are right, but this behavior seems to introduce an unintended behavior.  I guess I was hoping to hear from a developer why it might make sense.  David, your work-around is very clean, and I will use it.

[Joe Cheng]

This is the intended behavior and David's suggestion is the idiom we recommend. Observers (and reactive expressions for that matter) can't know what their dependencies are without running.

I understand that this isn't the most intuitive behavior if you're expecting an observer to work like an "event handler" in most other languages or UI frameworks; we have a pull request that's been open forever that adds a facade over observer() to provide an API that's more event handler-like.

This is the basic idiom for event handlers in Shiny:

observe({

  if (input$button == 0) return()  # or however else you detect the starting state

  isolate({

    # Your event handling logic here

  })

})

See http://shiny.rstudio.com/articles/isolation.html for more details.

On Wed, Aug 27, 2014 at 7:16 PM, Norman Packard <nhpa...@gmail.com> wrote:

Sigbert, David,

Thanks very much for your replies.  Sigbert, I guess you are right, but this behavior seems to introduce an unintended behavior.  I guess I was hoping to hear from a developer why it might make sense.  David, your work-around is very clean, and I will use it.

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[Norman Packard]

On Thursday, August 28, 2014 6:16:41 PM UTC-7, Joe Cheng [RStudio] wrote:

This is the intended behavior and David's suggestion is the idiom we recommend. Observers (and reactive expressions for that matter) can't know what their dependencies are without running.

I understand that this isn't the most intuitive behavior if you're expecting an observer to work like an "event handler" in most other languages or UI frameworks; we have a pull request that's been open forever that adds a facade over observer() to provide an API that's more event handler-like.

This is the basic idiom for event handlers in Shiny:

observe({

  if (input$button == 0) return()  # or however else you detect the starting state

  isolate({

    # Your event handling logic here

  })

})

See http://shiny.rstudio.com/articles/isolation.html for more details.

Joe,

Thanks very much for your elucidation.