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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>Reactive polling — reactivePoll • 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="Reactive polling — reactivePoll"><meta property="og:description" content="Used to create a reactive data source, which works by periodically polling a
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<h1>Reactive polling</h1>
<small class="dont-index">Source: <a href="https://github.com/rstudio/shiny/blob/main/R/reactives.R" class="external-link"><code>R/reactives.R</code></a></small>
<div class="hidden name"><code>reactivePoll.Rd</code></div>
</div>
<div class="ref-description">
<p>Used to create a reactive data source, which works by periodically polling a
non-reactive data source.</p>
</div>
<div id="ref-usage">
<div class="sourceCode"><pre class="sourceCode r"><code><span><span class="fu">reactivePoll</span><span class="op">(</span><span class="va">intervalMillis</span>, <span class="va">session</span>, <span class="va">checkFunc</span>, <span class="va">valueFunc</span><span class="op">)</span></span></code></pre></div>
</div>
<div id="arguments">
<h2>Arguments</h2>
<dl><dt id="arg-intervalmillis">intervalMillis<a class="anchor" aria-label="anchor" href="#arg-intervalmillis"></a></dt>
<dd><p>Approximate number of milliseconds to wait between
calls to <code>checkFunc</code>. This can be either a numeric value, or a
function that returns a numeric value.</p></dd>
<dt id="arg-session">session<a class="anchor" aria-label="anchor" href="#arg-session"></a></dt>
<dd><p>The user session to associate this file reader with, or
<code>NULL</code> if none. If non-null, the reader will automatically stop when
the session ends.</p></dd>
<dt id="arg-checkfunc">checkFunc<a class="anchor" aria-label="anchor" href="#arg-checkfunc"></a></dt>
<dd><p>A relatively cheap function whose values over time will be
tested for equality; inequality indicates that the underlying value has
changed and needs to be invalidated and re-read using <code>valueFunc</code>. See
Details.</p></dd>
<dt id="arg-valuefunc">valueFunc<a class="anchor" aria-label="anchor" href="#arg-valuefunc"></a></dt>
<dd><p>A function that calculates the underlying value. See
Details.</p></dd>
</dl></div>
<div id="value">
<h2>Value</h2>
<p>A reactive expression that returns the result of <code>valueFunc</code>,
and invalidates when <code>checkFunc</code> changes.</p>
</div>
<div id="details">
<h2>Details</h2>
<p><code>reactivePoll</code> works by pairing a relatively cheap "check" function with
a more expensive value retrieval function. The check function will be
executed periodically and should always return a consistent value until the
data changes. When the check function returns a different value, then the
value retrieval function will be used to re-populate the data.</p>
<p>Note that the check function doesn't return <code>TRUE</code> or <code>FALSE</code> to
indicate whether the underlying data has changed. Rather, the check function
indicates change by returning a different value from the previous time it was
called.</p>
<p>For example, <code>reactivePoll</code> is used to implement
<code>reactiveFileReader</code> by pairing a check function that simply returns the
last modified timestamp of a file, and a value retrieval function that
actually reads the contents of the file.</p>
<p>As another example, one might read a relational database table reactively by
using a check function that does <code>SELECT MAX(timestamp) FROM table</code> and
a value retrieval function that does <code>SELECT * FROM table</code>.</p>
<p>The <code>intervalMillis</code>, <code>checkFunc</code>, and <code>valueFunc</code> functions
will be executed in a reactive context; therefore, they may read reactive
values and reactive expressions.</p>
</div>
<div id="see-also">
<h2>See also</h2>
<div class="dont-index"><p><code><a href="reactiveFileReader.html">reactiveFileReader()</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="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></span>
<span class="r-in"><span> <span class="va">data</span> <span class="op">&lt;-</span> <span class="fu">reactivePoll</span><span class="op">(</span><span class="fl">1000</span>, <span class="va">session</span>,</span></span>
<span class="r-in"><span> <span class="co"># This function returns the time that log_file was last modified</span></span></span>
<span class="r-in"><span> checkFunc <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="kw">if</span> <span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/files.html" class="external-link">file.exists</a></span><span class="op">(</span><span class="va">log_file</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/base/file.info.html" class="external-link">file.info</a></span><span class="op">(</span><span class="va">log_file</span><span class="op">)</span><span class="op">$</span><span class="va">mtime</span><span class="op">[</span><span class="fl">1</span><span class="op">]</span></span></span>
<span class="r-in"><span> <span class="kw">else</span></span></span>
<span class="r-in"><span> <span class="st">""</span></span></span>
<span class="r-in"><span> <span class="op">}</span>,</span></span>
<span class="r-in"><span> <span class="co"># This function returns the content of log_file</span></span></span>
<span class="r-in"><span> valueFunc <span class="op">=</span> <span class="kw">function</span><span class="op">(</span><span class="op">)</span> <span class="op">{</span></span></span>
<span class="r-in"><span> <span class="fu"><a href="https://rdrr.io/r/utils/read.table.html" class="external-link">read.csv</a></span><span class="op">(</span><span class="va">log_file</span><span class="op">)</span></span></span>
<span class="r-in"><span> <span class="op">}</span></span></span>
<span class="r-in"><span> <span class="op">)</span></span></span>
<span class="r-in"><span></span></span>
<span class="r-in"><span> <span class="va">output</span><span class="op">$</span><span class="va">dataTable</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/utils/data.html" class="external-link">data</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-out co"><span class="r-pr">#&gt;</span> function (input, output, session) </span>
<span class="r-out co"><span class="r-pr">#&gt;</span> {</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> data &lt;- reactivePoll(1000, session, checkFunc = function() {</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> if (file.exists(log_file)) </span>
<span class="r-out co"><span class="r-pr">#&gt;</span> file.info(log_file)$mtime[1]</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> else ""</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> }, valueFunc = function() {</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> read.csv(log_file)</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> })</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> output$dataTable &lt;- renderTable({</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> data()</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> })</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> }</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> &lt;environment: 0x55afd5fc3f10&gt;</span>
</code></pre></div>
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