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<!-- Generated by pkgdown: do not edit by hand --><html lang="en"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><meta charset="utf-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Event handler — observeEvent • shiny</title><!-- jquery --><script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.7.1/jquery.min.js" integrity="sha512-v2CJ7UaYy4JwqLDIrZUI/4hqeoQieOmAZNXBeQyjo21dadnwR+8ZaIJVT8EE2iyI61OV8e6M8PP2/4hpQINQ/g==" crossorigin="anonymous" referrerpolicy="no-referrer"></script><!-- Bootstrap --><link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.4.1/css/bootstrap.min.css" integrity="sha256-bZLfwXAP04zRMK2BjiO8iu9pf4FbLqX6zitd+tIvLhE=" crossorigin="anonymous"><script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.4.1/js/bootstrap.min.js" integrity="sha256-nuL8/2cJ5NDSSwnKD8VqreErSWHtnEP9E7AySL+1ev4=" crossorigin="anonymous"></script><!-- bootstrap-toc --><link rel="stylesheet" href="../bootstrap-toc.css"><script src="../bootstrap-toc.js"></script><!-- Font Awesome icons --><link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.12.1/css/all.min.css" integrity="sha256-mmgLkCYLUQbXn0B1SRqzHar6dCnv9oZFPEC1g1cwlkk=" crossorigin="anonymous"><link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.12.1/css/v4-shims.min.css" integrity="sha256-wZjR52fzng1pJHwx4aV2AO3yyTOXrcDW7jBpJtTwVxw=" crossorigin="anonymous"><!-- clipboard.js --><script src="https://cdnjs.cloudflare.com/ajax/libs/clipboard.js/2.0.6/clipboard.min.js" integrity="sha256-inc5kl9MA1hkeYUt+EC3BhlIgyp/2jDIyBLS6k3UxPI=" crossorigin="anonymous"></script><!-- headroom.js --><script src="https://cdnjs.cloudflare.com/ajax/libs/headroom/0.11.0/headroom.min.js" integrity="sha256-AsUX4SJE1+yuDu5+mAVzJbuYNPHj/WroHuZ8Ir/CkE0=" crossorigin="anonymous"></script><script src="https://cdnjs.cloudflare.com/ajax/libs/headroom/0.11.0/jQuery.headroom.min.js" integrity="sha256-ZX/yNShbjqsohH1k95liqY9Gd8uOiE1S4vZc+9KQ1K4=" crossorigin="anonymous"></script><!-- pkgdown --><link href="../pkgdown.css" rel="stylesheet"><script src="../pkgdown.js"></script><meta property="og:title" content="Event handler — observeEvent"><meta property="og:description" content='Respond to "event-like" reactive inputs, values, and expressions. As of Shiny
1.6.0, we recommend using bindEvent() instead of eventReactive() and
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<h1>Event handler</h1>
<small class="dont-index">Source: <a href="https://github.com/rstudio/shiny/blob/rc-v1.12.1/R/reactives.R" class="external-link"><code>R/reactives.R</code></a></small>
<div class="hidden name"><code>observeEvent.Rd</code></div>
</div>
<div class="ref-description">
<p>Respond to "event-like" reactive inputs, values, and expressions. As of Shiny
1.6.0, we recommend using <code><a href="bindEvent.html">bindEvent()</a></code> instead of <code>eventReactive()</code> and
<code>observeEvent()</code>. This is because <code><a href="bindEvent.html">bindEvent()</a></code> can be composed with
<code><a href="bindCache.html">bindCache()</a></code>, and because it can also be used with <code>render</code> functions (like
<code><a href="renderPrint.html">renderText()</a></code> and <code><a href="renderPlot.html">renderPlot()</a></code>).</p>
</div>
<div id="ref-usage">
<div class="sourceCode"><pre class="sourceCode r"><code><span><span class="fu">observeEvent</span><span class="op">(</span></span>
<span> <span class="va">eventExpr</span>,</span>
<span> <span class="va">handlerExpr</span>,</span>
<span> event.env <span class="op">=</span> <span class="fu"><a href="https://rdrr.io/r/base/sys.parent.html" class="external-link">parent.frame</a></span><span class="op">(</span><span class="op">)</span>,</span>
<span> event.quoted <span class="op">=</span> <span class="cn">FALSE</span>,</span>
<span> handler.env <span class="op">=</span> <span class="fu"><a href="https://rdrr.io/r/base/sys.parent.html" class="external-link">parent.frame</a></span><span class="op">(</span><span class="op">)</span>,</span>
<span> handler.quoted <span class="op">=</span> <span class="cn">FALSE</span>,</span>
<span> <span class="va">...</span>,</span>
<span> label <span class="op">=</span> <span class="cn">NULL</span>,</span>
<span> suspended <span class="op">=</span> <span class="cn">FALSE</span>,</span>
<span> priority <span class="op">=</span> <span class="fl">0</span>,</span>
<span> domain <span class="op">=</span> <span class="fu"><a href="domains.html">getDefaultReactiveDomain</a></span><span class="op">(</span><span class="op">)</span>,</span>
<span> autoDestroy <span class="op">=</span> <span class="cn">TRUE</span>,</span>
<span> ignoreNULL <span class="op">=</span> <span class="cn">TRUE</span>,</span>
<span> ignoreInit <span class="op">=</span> <span class="cn">FALSE</span>,</span>
<span> once <span class="op">=</span> <span class="cn">FALSE</span></span>
<span><span class="op">)</span></span>
<span></span>
<span><span class="fu">eventReactive</span><span class="op">(</span></span>
<span> <span class="va">eventExpr</span>,</span>
<span> <span class="va">valueExpr</span>,</span>
<span> event.env <span class="op">=</span> <span class="fu"><a href="https://rdrr.io/r/base/sys.parent.html" class="external-link">parent.frame</a></span><span class="op">(</span><span class="op">)</span>,</span>
<span> event.quoted <span class="op">=</span> <span class="cn">FALSE</span>,</span>
<span> value.env <span class="op">=</span> <span class="fu"><a href="https://rdrr.io/r/base/sys.parent.html" class="external-link">parent.frame</a></span><span class="op">(</span><span class="op">)</span>,</span>
<span> value.quoted <span class="op">=</span> <span class="cn">FALSE</span>,</span>
<span> <span class="va">...</span>,</span>
<span> label <span class="op">=</span> <span class="cn">NULL</span>,</span>
<span> domain <span class="op">=</span> <span class="fu"><a href="domains.html">getDefaultReactiveDomain</a></span><span class="op">(</span><span class="op">)</span>,</span>
<span> ignoreNULL <span class="op">=</span> <span class="cn">TRUE</span>,</span>
<span> ignoreInit <span class="op">=</span> <span class="cn">FALSE</span></span>
<span><span class="op">)</span></span></code></pre></div>
</div>
<div id="arguments">
<h2>Arguments</h2>
<dl><dt id="arg-eventexpr">eventExpr<a class="anchor" aria-label="anchor" href="#arg-eventexpr"></a></dt>
<dd><p>A (quoted or unquoted) expression that represents the event;
this can be a simple reactive value like <code>input$click</code>, a call to a
reactive expression like <code>dataset()</code>, or even a complex expression
inside curly braces</p></dd>
<dt id="arg-handlerexpr">handlerExpr<a class="anchor" aria-label="anchor" href="#arg-handlerexpr"></a></dt>
<dd><p>The expression to call whenever <code>eventExpr</code> is
invalidated. This should be a side-effect-producing action (the return
value will be ignored). It will be executed within an <code><a href="isolate.html">isolate()</a></code>
scope.</p></dd>
<dt id="arg-event-env">event.env<a class="anchor" aria-label="anchor" href="#arg-event-env"></a></dt>
<dd><p>The parent environment for the reactive expression. By default,
this is the calling environment, the same as when defining an ordinary
non-reactive expression. If <code>eventExpr</code> is a quosure and <code>event.quoted</code> is <code>TRUE</code>,
then <code>event.env</code> is ignored.</p></dd>
<dt id="arg-event-quoted">event.quoted<a class="anchor" aria-label="anchor" href="#arg-event-quoted"></a></dt>
<dd><p>If it is <code>TRUE</code>, then the <code><a href="https://rdrr.io/r/base/substitute.html" class="external-link">quote()</a></code>ed value of <code>eventExpr</code>
will be used when <code>eventExpr</code> is evaluated. If <code>eventExpr</code> is a quosure and you
would like to use its expression as a value for <code>eventExpr</code>, then you must set
<code>event.quoted</code> to <code>TRUE</code>.</p></dd>
<dt id="arg-handler-env">handler.env<a class="anchor" aria-label="anchor" href="#arg-handler-env"></a></dt>
<dd><p>The parent environment for the reactive expression. By default,
this is the calling environment, the same as when defining an ordinary
non-reactive expression. If <code>handlerExpr</code> is a quosure and <code>handler.quoted</code> is <code>TRUE</code>,
then <code>handler.env</code> is ignored.</p></dd>
<dt id="arg-handler-quoted">handler.quoted<a class="anchor" aria-label="anchor" href="#arg-handler-quoted"></a></dt>
<dd><p>If it is <code>TRUE</code>, then the <code><a href="https://rdrr.io/r/base/substitute.html" class="external-link">quote()</a></code>ed value of <code>handlerExpr</code>
will be used when <code>handlerExpr</code> is evaluated. If <code>handlerExpr</code> is a quosure and you
would like to use its expression as a value for <code>handlerExpr</code>, then you must set
<code>handler.quoted</code> to <code>TRUE</code>.</p></dd>
<dt id="arg--">...<a class="anchor" aria-label="anchor" href="#arg--"></a></dt>
<dd><p>Currently not used.</p></dd>
<dt id="arg-label">label<a class="anchor" aria-label="anchor" href="#arg-label"></a></dt>
<dd><p>A label for the observer or reactive, useful for debugging.</p></dd>
<dt id="arg-suspended">suspended<a class="anchor" aria-label="anchor" href="#arg-suspended"></a></dt>
<dd><p>If <code>TRUE</code>, start the observer in a suspended state. If
<code>FALSE</code> (the default), start in a non-suspended state.</p></dd>
<dt id="arg-priority">priority<a class="anchor" aria-label="anchor" href="#arg-priority"></a></dt>
<dd><p>An integer or numeric that controls the priority with which
this observer should be executed. An observer with a given priority level
will always execute sooner than all observers with a lower priority level.
Positive, negative, and zero values are allowed.</p></dd>
<dt id="arg-domain">domain<a class="anchor" aria-label="anchor" href="#arg-domain"></a></dt>
<dd><p>See <a href="domains.html">domains</a>.</p></dd>
<dt id="arg-autodestroy">autoDestroy<a class="anchor" aria-label="anchor" href="#arg-autodestroy"></a></dt>
<dd><p>If <code>TRUE</code> (the default), the observer will be
automatically destroyed when its domain (if any) ends.</p></dd>
<dt id="arg-ignorenull">ignoreNULL<a class="anchor" aria-label="anchor" href="#arg-ignorenull"></a></dt>
<dd><p>Whether the action should be triggered (or value
calculated, in the case of <code>eventReactive</code>) when the input event expression
is <code>NULL</code>. See Details.</p></dd>
<dt id="arg-ignoreinit">ignoreInit<a class="anchor" aria-label="anchor" href="#arg-ignoreinit"></a></dt>
<dd><p>If <code>TRUE</code>, then, when this <code>observeEvent</code> is
first created/initialized, ignore the <code>handlerExpr</code> (the second
argument), whether it is otherwise supposed to run or not. The default is
<code>FALSE</code>. See Details.</p></dd>
<dt id="arg-once">once<a class="anchor" aria-label="anchor" href="#arg-once"></a></dt>
<dd><p>Whether this <code>observeEvent</code> should be immediately destroyed
after the first time that the code in <code>handlerExpr</code> is run. This
pattern is useful when you want to subscribe to a event that should only
happen once.</p></dd>
<dt id="arg-valueexpr">valueExpr<a class="anchor" aria-label="anchor" href="#arg-valueexpr"></a></dt>
<dd><p>The expression that produces the return value of the
<code>eventReactive</code>. It will be executed within an <code><a href="isolate.html">isolate()</a></code>
scope.</p></dd>
<dt id="arg-value-env">value.env<a class="anchor" aria-label="anchor" href="#arg-value-env"></a></dt>
<dd><p>The parent environment for the reactive expression. By default,
this is the calling environment, the same as when defining an ordinary
non-reactive expression. If <code>valueExpr</code> is a quosure and <code>value.quoted</code> is <code>TRUE</code>,
then <code>value.env</code> is ignored.</p></dd>
<dt id="arg-value-quoted">value.quoted<a class="anchor" aria-label="anchor" href="#arg-value-quoted"></a></dt>
<dd><p>If it is <code>TRUE</code>, then the <code><a href="https://rdrr.io/r/base/substitute.html" class="external-link">quote()</a></code>ed value of <code>valueExpr</code>
will be used when <code>valueExpr</code> is evaluated. If <code>valueExpr</code> is a quosure and you
would like to use its expression as a value for <code>valueExpr</code>, then you must set
<code>value.quoted</code> to <code>TRUE</code>.</p></dd>
</dl></div>
<div id="value">
<h2>Value</h2>
<p><code>observeEvent</code> returns an observer reference class object (see
<code><a href="observe.html">observe()</a></code>). <code>eventReactive</code> returns a reactive expression
object (see <code><a href="reactive.html">reactive()</a></code>).</p>
</div>
<div id="details">
<h2>Details</h2>
<p>Shiny's reactive programming framework is primarily designed for calculated
values (reactive expressions) and side-effect-causing actions (observers)
that respond to <em>any</em> of their inputs changing. That's often what is
desired in Shiny apps, but not always: sometimes you want to wait for a
specific action to be taken from the user, like clicking an
<code><a href="actionButton.html">actionButton()</a></code>, before calculating an expression or taking an
action. A reactive value or expression that is used to trigger other
calculations in this way is called an <em>event</em>.</p>
<p>These situations demand a more imperative, "event handling" style of
programming that is possible–but not particularly intuitive–using the
reactive programming primitives <code><a href="observe.html">observe()</a></code> and
<code><a href="isolate.html">isolate()</a></code>. <code>observeEvent</code> and <code>eventReactive</code> provide
straightforward APIs for event handling that wrap <code>observe</code> and
<code>isolate</code>.</p>
<p>Use <code>observeEvent</code> whenever you want to <em>perform an action</em> in
response to an event. (Note that "recalculate a value" does not generally
count as performing an action–see <code>eventReactive</code> for that.) The first
argument is the event you want to respond to, and the second argument is a
function that should be called whenever the event occurs. Note that
<code>observeEvent()</code> is equivalent to using <code>observe() %&gt;% bindEvent()</code> and as of
Shiny 1.6.0, we recommend the latter.</p>
<p>Use <code>eventReactive</code> to create a <em>calculated value</em> that only
updates in response to an event. This is just like a normal
<a href="reactive.html">reactive expression</a> except it ignores all the usual
invalidations that come from its reactive dependencies; it only invalidates
in response to the given event. Note that
<code>eventReactive()</code> is equivalent to using <code>reactive() %&gt;% bindEvent()</code> and as of
Shiny 1.6.0, we recommend the latter.</p>
</div>
<div id="ignorenull-and-ignoreinit">
<h2>ignoreNULL and ignoreInit</h2>
<p>Both <code>observeEvent</code> and <code>eventReactive</code> take an <code>ignoreNULL</code>
parameter that affects behavior when the <code>eventExpr</code> evaluates to
<code>NULL</code> (or in the special case of an <code><a href="actionButton.html">actionButton()</a></code>,
<code>0</code>). In these cases, if <code>ignoreNULL</code> is <code>TRUE</code>, then an
<code>observeEvent</code> will not execute and an <code>eventReactive</code> will raise a
silent <a href="validate.html">validation</a> error. This is useful behavior if you
don't want to do the action or calculation when your app first starts, but
wait for the user to initiate the action first (like a "Submit" button);
whereas <code>ignoreNULL=FALSE</code> is desirable if you want to initially perform
the action/calculation and just let the user re-initiate it (like a
"Recalculate" button).</p>
<p>Likewise, both <code>observeEvent</code> and <code>eventReactive</code> also take in an
<code>ignoreInit</code> argument. By default, both of these will run right when they
are created (except if, at that moment, <code>eventExpr</code> evaluates to <code>NULL</code>
and <code>ignoreNULL</code> is <code>TRUE</code>). But when responding to a click of an action
button, it may often be useful to set <code>ignoreInit</code> to <code>TRUE</code>. For
example, if you're setting up an <code>observeEvent</code> for a dynamically created
button, then <code>ignoreInit = TRUE</code> will guarantee that the action (in
<code>handlerExpr</code>) will only be triggered when the button is actually clicked,
instead of also being triggered when it is created/initialized. Similarly,
if you're setting up an <code>eventReactive</code> that responds to a dynamically
created button used to refresh some data (then returned by that <code>eventReactive</code>),
then you should use <code>eventReactive([...], ignoreInit = TRUE)</code> if you want
to let the user decide if/when they want to refresh the data (since, depending
on the app, this may be a computationally expensive operation).</p>
<p>Even though <code>ignoreNULL</code> and <code>ignoreInit</code> can be used for similar
purposes they are independent from one another. Here's the result of combining
these:</p>
<dl><dt><code>ignoreNULL = TRUE</code> and <code>ignoreInit = FALSE</code></dt>
<dd><p>This is the default. This combination means that <code>handlerExpr</code>/
<code>valueExpr</code> will run every time that <code>eventExpr</code> is not
<code>NULL</code>. If, at the time of the creation of the
<code>observeEvent</code>/<code>eventReactive</code>, <code>eventExpr</code> happens
to <em>not</em> be <code>NULL</code>, then the code runs.</p></dd>
<dt><code>ignoreNULL = FALSE</code> and <code>ignoreInit = FALSE</code></dt>
<dd><p>This combination means that <code>handlerExpr</code>/<code>valueExpr</code> will
run every time no matter what.</p></dd>
<dt><code>ignoreNULL = FALSE</code> and <code>ignoreInit = TRUE</code></dt>
<dd><p>This combination means that <code>handlerExpr</code>/<code>valueExpr</code> will
<em>not</em> run when the <code>observeEvent</code>/<code>eventReactive</code> is
created (because <code>ignoreInit = TRUE</code>), but it will run every
other time.</p></dd>
<dt><code>ignoreNULL = TRUE</code> and <code>ignoreInit = TRUE</code></dt>
<dd><p>This combination means that <code>handlerExpr</code>/<code>valueExpr</code> will
<em>not</em> run when the <code>observeEvent</code>/<code>eventReactive</code> is
created (because <code>ignoreInit = TRUE</code>). After that,
<code>handlerExpr</code>/<code>valueExpr</code> will run every time that
<code>eventExpr</code> is not <code>NULL</code>.</p></dd>
</dl></div>
<div id="see-also">
<h2>See also</h2>
<div class="dont-index"><p><code><a href="actionButton.html">actionButton()</a></code></p></div>
</div>
<div id="ref-examples">
<h2>Examples</h2>
<div class="sourceCode"><pre class="sourceCode r"><code><span class="r-in"><span><span class="co">## Only run examples in interactive R sessions</span></span></span>
<span class="r-in"><span><span class="kw">if</span> <span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/interactive.html" class="external-link">interactive</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span> <span class="op">{</span></span></span>
<span class="r-in"><span></span></span>
<span class="r-in"><span> <span class="co">## App 1: Sample usage</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="shinyApp.html">shinyApp</a></span><span class="op">(</span></span></span>
<span class="r-in"><span> ui <span class="op">=</span> <span class="fu"><a href="fluidPage.html">fluidPage</a></span><span class="op">(</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="column.html">column</a></span><span class="op">(</span><span class="fl">4</span>,</span></span>
<span class="r-in"><span> <span class="fu"><a href="numericInput.html">numericInput</a></span><span class="op">(</span><span class="st">"x"</span>, <span class="st">"Value"</span>, <span class="fl">5</span><span class="op">)</span>,</span></span>
<span class="r-in"><span> <span class="fu"><a href="https://rstudio.github.io/htmltools/reference/builder.html" class="external-link">br</a></span><span class="op">(</span><span class="op">)</span>,</span></span>
<span class="r-in"><span> <span class="fu"><a href="actionButton.html">actionButton</a></span><span class="op">(</span><span class="st">"button"</span>, <span class="st">"Show"</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">)</span>,</span></span>
<span class="r-in"><span> <span class="fu"><a href="column.html">column</a></span><span class="op">(</span><span class="fl">8</span>, <span class="fu"><a href="renderTable.html">tableOutput</a></span><span class="op">(</span><span class="st">"table"</span><span class="op">)</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">)</span>,</span></span>
<span class="r-in"><span> server <span class="op">=</span> <span class="kw">function</span><span class="op">(</span><span class="va">input</span>, <span class="va">output</span><span class="op">)</span> <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="co"># Take an action every time button is pressed;</span></span></span>
<span class="r-in"><span> <span class="co"># here, we just print a message to the console</span></span></span>
<span class="r-in"><span> <span class="fu">observeEvent</span><span class="op">(</span><span class="va">input</span><span class="op">$</span><span class="va">button</span>, <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="https://rdrr.io/r/base/cat.html" class="external-link">cat</a></span><span class="op">(</span><span class="st">"Showing"</span>, <span class="va">input</span><span class="op">$</span><span class="va">x</span>, <span class="st">"rows\n"</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">}</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="co"># The observeEvent() above is equivalent to:</span></span></span>
<span class="r-in"><span> <span class="co"># observe({</span></span></span>
<span class="r-in"><span> <span class="co"># cat("Showing", input$x, "rows\n")</span></span></span>
<span class="r-in"><span> <span class="co"># }) %&gt;%</span></span></span>
<span class="r-in"><span> <span class="co"># bindEvent(input$button)</span></span></span>
<span class="r-in"><span></span></span>
<span class="r-in"><span> <span class="co"># Take a reactive dependency on input$button, but</span></span></span>
<span class="r-in"><span> <span class="co"># not on any of the stuff inside the function</span></span></span>
<span class="r-in"><span> <span class="va">df</span> <span class="op">&lt;-</span> <span class="fu">eventReactive</span><span class="op">(</span><span class="va">input</span><span class="op">$</span><span class="va">button</span>, <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="https://rdrr.io/r/utils/head.html" class="external-link">head</a></span><span class="op">(</span><span class="va">cars</span>, <span class="va">input</span><span class="op">$</span><span class="va">x</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">}</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="va">output</span><span class="op">$</span><span class="va">table</span> <span class="op">&lt;-</span> <span class="fu"><a href="renderTable.html">renderTable</a></span><span class="op">(</span><span class="op">{</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="https://rdrr.io/r/stats/Fdist.html" class="external-link">df</a></span><span class="op">(</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">}</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">}</span></span></span>
<span class="r-in"><span> <span class="op">)</span></span></span>
<span class="r-in"><span></span></span>
<span class="r-in"><span> <span class="co">## App 2: Using `once`</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="shinyApp.html">shinyApp</a></span><span class="op">(</span></span></span>
<span class="r-in"><span> ui <span class="op">=</span> <span class="fu"><a href="bootstrapPage.html">basicPage</a></span><span class="op">(</span> <span class="fu"><a href="actionButton.html">actionButton</a></span><span class="op">(</span><span class="st">"go"</span>, <span class="st">"Go"</span><span class="op">)</span><span class="op">)</span>,</span></span>
<span class="r-in"><span> server <span class="op">=</span> <span class="kw">function</span><span class="op">(</span><span class="va">input</span>, <span class="va">output</span>, <span class="va">session</span><span class="op">)</span> <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="fu">observeEvent</span><span class="op">(</span><span class="va">input</span><span class="op">$</span><span class="va">go</span>, <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="https://rdrr.io/r/base/print.html" class="external-link">print</a></span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/paste.html" class="external-link">paste</a></span><span class="op">(</span><span class="st">"This will only be printed once; all"</span>,</span></span>
<span class="r-in"><span> <span class="st">"subsequent button clicks won't do anything"</span><span class="op">)</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">}</span>, once <span class="op">=</span> <span class="cn">TRUE</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="co"># The observeEvent() above is equivalent to:</span></span></span>
<span class="r-in"><span> <span class="co"># observe({</span></span></span>
<span class="r-in"><span> <span class="co"># print(paste("This will only be printed once; all",</span></span></span>
<span class="r-in"><span> <span class="co"># "subsequent button clicks won't do anything"))</span></span></span>
<span class="r-in"><span> <span class="co"># }) %&gt;%</span></span></span>
<span class="r-in"><span> <span class="co"># bindEvent(input$go, once = TRUE)</span></span></span>
<span class="r-in"><span> <span class="op">}</span></span></span>
<span class="r-in"><span> <span class="op">)</span></span></span>
<span class="r-in"><span></span></span>
<span class="r-in"><span> <span class="co">## App 3: Using `ignoreInit` and `once`</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="shinyApp.html">shinyApp</a></span><span class="op">(</span></span></span>
<span class="r-in"><span> ui <span class="op">=</span> <span class="fu"><a href="bootstrapPage.html">basicPage</a></span><span class="op">(</span><span class="fu"><a href="actionButton.html">actionButton</a></span><span class="op">(</span><span class="st">"go"</span>, <span class="st">"Go"</span><span class="op">)</span><span class="op">)</span>,</span></span>
<span class="r-in"><span> server <span class="op">=</span> <span class="kw">function</span><span class="op">(</span><span class="va">input</span>, <span class="va">output</span>, <span class="va">session</span><span class="op">)</span> <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="fu">observeEvent</span><span class="op">(</span><span class="va">input</span><span class="op">$</span><span class="va">go</span>, <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="insertUI.html">insertUI</a></span><span class="op">(</span><span class="st">"#go"</span>, <span class="st">"afterEnd"</span>,</span></span>
<span class="r-in"><span> <span class="fu"><a href="actionButton.html">actionButton</a></span><span class="op">(</span><span class="st">"dynamic"</span>, <span class="st">"click to remove"</span><span class="op">)</span><span class="op">)</span></span></span>
<span class="r-in"><span></span></span>
<span class="r-in"><span> <span class="co"># set up an observer that depends on the dynamic</span></span></span>
<span class="r-in"><span> <span class="co"># input, so that it doesn't run when the input is</span></span></span>
<span class="r-in"><span> <span class="co"># created, and only runs once after that (since</span></span></span>
<span class="r-in"><span> <span class="co"># the side effect is remove the input from the DOM)</span></span></span>
<span class="r-in"><span> <span class="fu">observeEvent</span><span class="op">(</span><span class="va">input</span><span class="op">$</span><span class="va">dynamic</span>, <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="insertUI.html">removeUI</a></span><span class="op">(</span><span class="st">"#dynamic"</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">}</span>, ignoreInit <span class="op">=</span> <span class="cn">TRUE</span>, once <span class="op">=</span> <span class="cn">TRUE</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">}</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">}</span></span></span>
<span class="r-in"><span> <span class="op">)</span></span></span>
<span class="r-in"><span><span class="op">}</span></span></span>
</code></pre></div>
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