mirror of
https://github.com/rstudio/shiny.git
synced 2026-01-30 09:18:33 -05:00
110 lines
3.8 KiB
Markdown
110 lines
3.8 KiB
Markdown
## Isolation: avoiding dependency
|
|
|
|
Sometimes it's useful for an observer/endpoint to access a reactive value or expression, but not to take a dependency on it. For example, if the observer performs a long calculation or downloads large data set, you might want it to execute only when a button is clicked.
|
|
|
|
For this, we'll use `actionButton`. We'll define a `ui.R` that is a slight modification of the one from 01_hello -- the only difference is that it has an actionButton labeled "Go!". You can see it in action at [http://glimmer.rstudio.com/winston/actionbutton/](http://glimmer.rstudio.com/winston/actionbutton/).
|
|
|
|
The actionButton includes some JavaScript code that sends numbers to the server. When the web browser first connects, it sends a value of 0, and on each click, it sends an incremented value: 1, 2, 3, and so on.
|
|
|
|
{% highlight r %}
|
|
shinyUI(pageWithSidebar(
|
|
headerPanel("Click the button"),
|
|
sidebarPanel(
|
|
sliderInput("obs", "Number of observations:",
|
|
min = 0, max = 1000, value = 500),
|
|
actionButton("goButton", "Go!")
|
|
),
|
|
mainPanel(
|
|
plotOutput("distPlot")
|
|
)
|
|
))
|
|
{% endhighlight %}
|
|
|
|
In our `server.R`, there are two changes to note. First, `output$distPlot` will take a dependency on `input$goButton`, simply by accessing it. When the button is clicked, the value of `input$goButton` increases, and so `output$distPlot` re-executes.
|
|
|
|
The second change is that the access to `input$obs` is wrapped with `isolate()`. This function takes an R expression, and it tells Shiny that the calling observer or reactive expression should not take a dependency on any reactive objects inside the expression.
|
|
|
|
{% highlight r %}
|
|
shinyServer(function(input, output) {
|
|
output$distPlot <- renderPlot({
|
|
# Take a dependency on input$goButton
|
|
input$goButton
|
|
|
|
# Use isolate() to avoid dependency on input$obs
|
|
dist <- isolate(rnorm(input$obs))
|
|
hist(dist)
|
|
})
|
|
})
|
|
{% endhighlight %}
|
|
|
|
|
|
The resulting graph looks like this:
|
|
|
|
|
|
|
|
|
|
And here's a walkthrough of the process when `input$obs` is set to 1000, and then the Go button is clicked:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
In the `actionButton` example, you might want to prevent it from returning a plot the first time, before the button has been clicked. Since the starting value of an `actionButton` is zero, this can be accomplished with the following:
|
|
|
|
{% highlight r %}
|
|
output$distPlot <- renderPlot({
|
|
if (input$goButton == 0)
|
|
return()
|
|
|
|
# plot-making code here
|
|
})
|
|
{% endhighlight %}
|
|
|
|
|
|
Reactive values are not the only things that can be isolated; reactive expressions can also be put inside an `isolate()`. Building off the Fibonacci example from above, this would calculate the _n_th value only when the button is clicked:
|
|
|
|
{% highlight r %}
|
|
output$nthValue <- renderText({
|
|
if (input$goButton == 0)
|
|
return()
|
|
|
|
isolate({ fib(as.numeric(input$n)) })
|
|
})
|
|
{% endhighlight %}
|
|
|
|
|
|
|
|
It's also possible to put multiple lines of code in `isolate()`. For example here are some blocks of code that have equivalent effect:
|
|
|
|
{% highlight r %}
|
|
# Separate calls to isolate -------------------------------
|
|
x <- isolate({ input$xSlider }) + 100
|
|
y <- isolate({ input$ySlider }) * 2
|
|
z <- x/y
|
|
|
|
# Single call to isolate ----------------------------------
|
|
isolate({
|
|
x <- input$xSlider + 100
|
|
y <- input$ySlider * 2
|
|
z <- x/y
|
|
})
|
|
|
|
# Single call to isolate, use return value ----------------
|
|
z <- isolate({
|
|
x <- input$xSlider + 100
|
|
y <- input$ySlider * 2
|
|
x/y
|
|
})
|
|
{% endhighlight %}
|
|
|
|
In all of these cases, the calling function won't take a reactive dependency on either of the `input` variables.
|