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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>Add callbacks for Shiny session events — onFlush • 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="Add callbacks for Shiny session events — onFlush"><meta property="og:description" content="These functions are for registering callbacks on Shiny session events.
onFlush registers a function that will be called before Shiny flushes the
reactive system. onFlushed registers a function that will be called after
Shiny flushes the reactive system. onUnhandledError registers a function to
be called when an unhandled error occurs before the session is closed.
onSessionEnded registers a function to be called after the client has
disconnected.
These functions should be called within the application's server function.
All of these functions return a function which can be called with no
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<h1>Add callbacks for Shiny session events</h1>
<small class="dont-index">Source: <a href="https://github.com/rstudio/shiny/blob/main/R/shiny.R" class="external-link"><code>R/shiny.R</code></a></small>
<div class="hidden name"><code>onFlush.Rd</code></div>
</div>
<div class="ref-description">
<p>These functions are for registering callbacks on Shiny session events.
<code>onFlush</code> registers a function that will be called before Shiny flushes the
reactive system. <code>onFlushed</code> registers a function that will be called after
Shiny flushes the reactive system. <code>onUnhandledError</code> registers a function to
be called when an unhandled error occurs before the session is closed.
<code>onSessionEnded</code> registers a function to be called after the client has
disconnected.</p>
<p>These functions should be called within the application's server function.</p>
<p>All of these functions return a function which can be called with no
arguments to cancel the registration.</p>
</div>
<div id="ref-usage">
<div class="sourceCode"><pre class="sourceCode r"><code><span><span class="fu">onFlush</span><span class="op">(</span><span class="va">fun</span>, once <span class="op">=</span> <span class="cn">TRUE</span>, session <span class="op">=</span> <span class="fu"><a href="domains.html">getDefaultReactiveDomain</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span></span>
<span></span>
<span><span class="fu">onFlushed</span><span class="op">(</span><span class="va">fun</span>, once <span class="op">=</span> <span class="cn">TRUE</span>, session <span class="op">=</span> <span class="fu"><a href="domains.html">getDefaultReactiveDomain</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span></span>
<span></span>
<span><span class="fu">onSessionEnded</span><span class="op">(</span><span class="va">fun</span>, session <span class="op">=</span> <span class="fu"><a href="domains.html">getDefaultReactiveDomain</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span></span>
<span></span>
<span><span class="fu">onUnhandledError</span><span class="op">(</span><span class="va">fun</span>, session <span class="op">=</span> <span class="fu"><a href="domains.html">getDefaultReactiveDomain</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span></span></code></pre></div>
</div>
<div id="arguments">
<h2>Arguments</h2>
<dl><dt id="arg-fun">fun<a class="anchor" aria-label="anchor" href="#arg-fun"></a></dt>
<dd><p>A callback function.</p></dd>
<dt id="arg-once">once<a class="anchor" aria-label="anchor" href="#arg-once"></a></dt>
<dd><p>Should the function be run once, and then cleared, or should it
re-run each time the event occurs. (Only for <code>onFlush</code> and
<code>onFlushed</code>.)</p></dd>
<dt id="arg-session">session<a class="anchor" aria-label="anchor" href="#arg-session"></a></dt>
<dd><p>A shiny session object.</p></dd>
</dl></div>
<div id="unhandled-errors">
<h2>Unhandled Errors</h2>
<p>Unhandled errors are errors that aren't otherwise handled by Shiny or by the
application logic. In other words, they are errors that will either cause the
application to crash or will result in "Error" output in the UI.</p>
<p>You can use <code>onUnhandledError()</code> to register a function that will be called
when an unhandled error occurs. This function will be called with the error
object as its first argument. If the error is fatal and will result in the
session closing, the error condition will have the <code>shiny.error.fatal</code> class.</p>
<p>Note that the <code>onUnhandledError()</code> callbacks cannot be used to prevent the
app from closing or to modify the error condition. Instead, they are intended
to give you an opportunity to log the error or perform other cleanup
operations.</p>
</div>
<div id="see-also">
<h2>See also</h2>
<div class="dont-index"><p><code><a href="onStop.html">onStop()</a></code> for registering callbacks that will be
invoked when the application exits, or when a session ends.</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="kw">if</span> <span class="op">(</span><span class="cn">FALSE</span><span class="op">)</span> <span class="op">{</span> <span class="co"># interactive()</span></span></span>
<span class="r-in"><span><span class="kw"><a href="https://rdrr.io/r/base/library.html" class="external-link">library</a></span><span class="op">(</span><span class="va"><a href="https://shiny.posit.co/" class="external-link">shiny</a></span><span class="op">)</span></span></span>
<span class="r-in"><span></span></span>
<span class="r-in"><span><span class="va">ui</span> <span class="op">&lt;-</span> <span class="fu"><a href="fixedPage.html">fixedPage</a></span><span class="op">(</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="markdown.html">markdown</a></span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/c.html" class="external-link">c</a></span><span class="op">(</span></span></span>
<span class="r-in"><span> <span class="st">"Set the number to 8 or higher to cause an error"</span>,</span></span>
<span class="r-in"><span> <span class="st">"in the `renderText()` output."</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="fu"><a href="sliderInput.html">sliderInput</a></span><span class="op">(</span><span class="st">"number"</span>, <span class="st">"Number"</span>, <span class="fl">0</span>, <span class="fl">10</span>, <span class="fl">4</span><span class="op">)</span>,</span></span>
<span class="r-in"><span> <span class="fu"><a href="textOutput.html">textOutput</a></span><span class="op">(</span><span class="st">"text"</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">hr</a></span><span class="op">(</span><span class="op">)</span>,</span></span>
<span class="r-in"><span> <span class="fu"><a href="markdown.html">markdown</a></span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/c.html" class="external-link">c</a></span><span class="op">(</span></span></span>
<span class="r-in"><span> <span class="st">"Click the button below to crash the app with an unhandled error"</span>,</span></span>
<span class="r-in"><span> <span class="st">"in an `observe()` block."</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="fu"><a href="actionButton.html">actionButton</a></span><span class="op">(</span><span class="st">"crash"</span>, <span class="st">"Crash the app!"</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="va">log_event</span> <span class="op">&lt;-</span> <span class="kw">function</span><span class="op">(</span><span class="va">level</span>, <span class="va">...</span><span class="op">)</span> <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="va">ts</span> <span class="op">&lt;-</span> <span class="fu"><a href="https://rdrr.io/r/base/strptime.html" class="external-link">strftime</a></span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/Sys.time.html" class="external-link">Sys.time</a></span><span class="op">(</span><span class="op">)</span>, <span class="st">" [%F %T] "</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="https://rdrr.io/r/base/message.html" class="external-link">message</a></span><span class="op">(</span><span class="va">level</span>, <span class="va">ts</span>, <span class="va">...</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="va">server</span> <span class="op">&lt;-</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">log_event</span><span class="op">(</span><span class="st">"INFO"</span>, <span class="st">"Session started"</span><span class="op">)</span></span></span>
<span class="r-in"><span></span></span>
<span class="r-in"><span> <span class="fu">onUnhandledError</span><span class="op">(</span><span class="kw">function</span><span class="op">(</span><span class="va">err</span><span class="op">)</span> <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="co"># log the unhandled error</span></span></span>
<span class="r-in"><span> <span class="va">level</span> <span class="op">&lt;-</span> <span class="kw">if</span> <span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/class.html" class="external-link">inherits</a></span><span class="op">(</span><span class="va">err</span>, <span class="st">"shiny.error.fatal"</span><span class="op">)</span><span class="op">)</span> <span class="st">"FATAL"</span> <span class="kw">else</span> <span class="st">"ERROR"</span></span></span>
<span class="r-in"><span> <span class="fu">log_event</span><span class="op">(</span><span class="va">level</span>, <span class="fu"><a href="https://rdrr.io/r/base/conditions.html" class="external-link">conditionMessage</a></span><span class="op">(</span><span class="va">err</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></span>
<span class="r-in"><span> <span class="fu"><a href="onStop.html">onStop</a></span><span class="op">(</span><span class="kw">function</span><span class="op">(</span><span class="op">)</span> <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="fu">log_event</span><span class="op">(</span><span class="st">"INFO"</span>, <span class="st">"Session ended"</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></span>
<span class="r-in"><span> <span class="fu"><a href="observeEvent.html">observeEvent</a></span><span class="op">(</span><span class="va">input</span><span class="op">$</span><span class="va">crash</span>, <span class="kw"><a href="https://rdrr.io/r/base/stop.html" class="external-link">stop</a></span><span class="op">(</span><span class="st">"Oops, an unhandled error happened!"</span><span class="op">)</span><span class="op">)</span></span></span>
<span class="r-in"><span></span></span>
<span class="r-in"><span> <span class="va">output</span><span class="op">$</span><span class="va">text</span> <span class="op">&lt;-</span> <span class="fu"><a href="renderPrint.html">renderText</a></span><span class="op">(</span><span class="op">{</span></span></span>
<span class="r-in"><span> <span class="kw">if</span> <span class="op">(</span><span class="va">input</span><span class="op">$</span><span class="va">number</span> <span class="op">&gt;</span> <span class="fl">7</span><span class="op">)</span> <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="kw"><a href="https://rdrr.io/r/base/stop.html" class="external-link">stop</a></span><span class="op">(</span><span class="st">"that's too high!"</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">}</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="https://rdrr.io/r/base/sprintf.html" class="external-link">sprintf</a></span><span class="op">(</span><span class="st">"You picked number %d."</span>, <span class="va">input</span><span class="op">$</span><span class="va">number</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></span>
<span class="r-in"><span><span class="fu"><a href="shinyApp.html">shinyApp</a></span><span class="op">(</span><span class="va">ui</span>, <span class="va">server</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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