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<div class="page-header">
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<h1>Create a reactive observer</h1>
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<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>
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<div class="hidden name"><code>observe.Rd</code></div>
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</div>
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<div class="ref-description">
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<p>Creates an observer from the given expression.</p>
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</div>
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<div id="ref-usage">
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<div class="sourceCode"><pre class="sourceCode r"><code><span><span class="fu">observe</span><span class="op">(</span></span>
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<span> <span class="va">x</span>,</span>
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<span> 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>
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<span> quoted <span class="op">=</span> <span class="cn">FALSE</span>,</span>
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<span> <span class="va">...</span>,</span>
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<span> label <span class="op">=</span> <span class="cn">NULL</span>,</span>
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<span> suspended <span class="op">=</span> <span class="cn">FALSE</span>,</span>
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<span> priority <span class="op">=</span> <span class="fl">0</span>,</span>
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<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>
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<span> autoDestroy <span class="op">=</span> <span class="cn">TRUE</span>,</span>
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<span> ..stacktraceon <span class="op">=</span> <span class="cn">TRUE</span></span>
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<span><span class="op">)</span></span></code></pre></div>
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</div>
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<div id="arguments">
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<h2>Arguments</h2>
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<dl><dt id="arg-x">x<a class="anchor" aria-label="anchor" href="#arg-x"></a></dt>
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<dd><p>An expression (quoted or unquoted). Any return value will be
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ignored.</p></dd>
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<dt id="arg-env">env<a class="anchor" aria-label="anchor" href="#arg-env"></a></dt>
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<dd><p>The parent environment for the reactive expression. By default,
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this is the calling environment, the same as when defining an ordinary
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non-reactive expression. If <code>x</code> is a quosure and <code>quoted</code> is <code>TRUE</code>,
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then <code>env</code> is ignored.</p></dd>
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<dt id="arg-quoted">quoted<a class="anchor" aria-label="anchor" href="#arg-quoted"></a></dt>
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<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>x</code>
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will be used when <code>x</code> is evaluated. If <code>x</code> is a quosure and you
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would like to use its expression as a value for <code>x</code>, then you must set
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<code>quoted</code> to <code>TRUE</code>.</p></dd>
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<dt id="arg--">...<a class="anchor" aria-label="anchor" href="#arg--"></a></dt>
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<dd><p>Not used.</p></dd>
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<dt id="arg-label">label<a class="anchor" aria-label="anchor" href="#arg-label"></a></dt>
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<dd><p>A label for the observer, useful for debugging.</p></dd>
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<dt id="arg-suspended">suspended<a class="anchor" aria-label="anchor" href="#arg-suspended"></a></dt>
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<dd><p>If <code>TRUE</code>, start the observer in a suspended state. If
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<code>FALSE</code> (the default), start in a non-suspended state.</p></dd>
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<dt id="arg-priority">priority<a class="anchor" aria-label="anchor" href="#arg-priority"></a></dt>
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<dd><p>An integer or numeric that controls the priority with which
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this observer should be executed. A higher value means higher priority: an
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observer with a higher priority value will execute before all observers
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with lower priority values. Positive, negative, and zero values are
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allowed.</p></dd>
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<dt id="arg-domain">domain<a class="anchor" aria-label="anchor" href="#arg-domain"></a></dt>
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<dd><p>See <a href="domains.html">domains</a>.</p></dd>
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<dt id="arg-autodestroy">autoDestroy<a class="anchor" aria-label="anchor" href="#arg-autodestroy"></a></dt>
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<dd><p>If <code>TRUE</code> (the default), the observer will be
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automatically destroyed when its domain (if any) ends.</p></dd>
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<dt id="arg--stacktraceon">..stacktraceon<a class="anchor" aria-label="anchor" href="#arg--stacktraceon"></a></dt>
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<dd><p>Advanced use only. For stack manipulation purposes; see
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<code><a href="stacktrace.html">stacktrace()</a></code>.</p></dd>
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</dl></div>
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<div id="value">
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<h2>Value</h2>
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<p>An observer reference class object. This object has the following
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methods:</p><dl><dt><code>suspend()</code></dt>
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<dd><p>Causes this observer to stop scheduling flushes (re-executions) in
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response to invalidations. If the observer was invalidated prior to
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this call but it has not re-executed yet then that re-execution will
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still occur, because the flush is already scheduled.</p></dd>
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<dt><code>resume()</code></dt>
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<dd><p>Causes this observer to start re-executing in response to
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invalidations. If the observer was invalidated while suspended, then it
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will schedule itself for re-execution.</p></dd>
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<dt><code>destroy()</code></dt>
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<dd><p>Stops the observer from executing ever again, even if it is currently
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scheduled for re-execution.</p></dd>
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<dt><code>setPriority(priority = 0)</code></dt>
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<dd><p>Change this observer's priority. Note that if the observer is currently
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invalidated, then the change in priority will not take effect until the
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next invalidation–unless the observer is also currently suspended, in
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which case the priority change will be effective upon resume.</p></dd>
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<dt><code>setAutoDestroy(autoDestroy)</code></dt>
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<dd><p>Sets whether this observer should be automatically destroyed when its
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domain (if any) ends. If autoDestroy is TRUE and the domain already
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ended, then destroy() is called immediately."</p></dd>
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<dt><code>onInvalidate(callback)</code></dt>
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<dd><p>Register a callback function to run when this observer is invalidated.
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No arguments will be provided to the callback function when it is
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invoked.</p></dd>
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</dl></div>
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<div id="details">
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<h2>Details</h2>
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<p>An observer is like a reactive expression in that it can read reactive values
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and call reactive expressions, and will automatically re-execute when those
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dependencies change. But unlike reactive expressions, it doesn't yield a
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result and can't be used as an input to other reactive expressions. Thus,
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observers are only useful for their side effects (for example, performing
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I/O).</p>
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<p>Another contrast between reactive expressions and observers is their
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execution strategy. Reactive expressions use lazy evaluation; that is, when
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their dependencies change, they don't re-execute right away but rather wait
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until they are called by someone else. Indeed, if they are not called then
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they will never re-execute. In contrast, observers use eager evaluation; as
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soon as their dependencies change, they schedule themselves to re-execute.</p>
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<p>Starting with Shiny 0.10.0, observers are automatically destroyed by default
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when the <a href="domains.html">domain</a> that owns them ends (e.g. when a Shiny
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session ends).</p>
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</div>
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<div id="ref-examples">
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<h2>Examples</h2>
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<div class="sourceCode"><pre class="sourceCode r"><code><span class="r-in"><span><span class="va">values</span> <span class="op"><-</span> <span class="fu"><a href="reactiveValues.html">reactiveValues</a></span><span class="op">(</span>A<span class="op">=</span><span class="fl">1</span><span class="op">)</span></span></span>
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<span class="r-in"><span></span></span>
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<span class="r-in"><span><span class="va">obsB</span> <span class="op"><-</span> <span class="fu">observe</span><span class="op">(</span><span class="op">{</span></span></span>
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<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="va">values</span><span class="op">$</span><span class="va">A</span> <span class="op">+</span> <span class="fl">1</span><span class="op">)</span></span></span>
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<span class="r-in"><span><span class="op">}</span><span class="op">)</span></span></span>
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<span class="r-in"><span></span></span>
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<span class="r-in"><span><span class="co"># To store expressions for later conversion to observe, use rlang::quo()</span></span></span>
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<span class="r-in"><span><span class="va">myquo</span> <span class="op"><-</span> <span class="fu">rlang</span><span class="fu">::</span><span class="fu"><a href="https://rlang.r-lib.org/reference/defusing-advanced.html" class="external-link">quo</a></span><span class="op">(</span><span class="op">{</span> <span class="fu"><a href="https://rdrr.io/r/base/print.html" class="external-link">print</a></span><span class="op">(</span><span class="va">values</span><span class="op">$</span><span class="va">A</span> <span class="op">+</span> <span class="fl">3</span><span class="op">)</span> <span class="op">}</span><span class="op">)</span></span></span>
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<span class="r-in"><span><span class="va">obsC</span> <span class="op"><-</span> <span class="fu">rlang</span><span class="fu">::</span><span class="fu"><a href="https://rlang.r-lib.org/reference/inject.html" class="external-link">inject</a></span><span class="op">(</span><span class="fu">observe</span><span class="op">(</span><span class="op">!</span><span class="op">!</span><span class="va">myquo</span><span class="op">)</span><span class="op">)</span></span></span>
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<span class="r-in"><span></span></span>
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<span class="r-in"><span><span class="co"># (Legacy) Can use quoted expressions</span></span></span>
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<span class="r-in"><span><span class="va">obsD</span> <span class="op"><-</span> <span class="fu">observe</span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/substitute.html" class="external-link">quote</a></span><span class="op">(</span><span class="op">{</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="va">values</span><span class="op">$</span><span class="va">A</span> <span class="op">+</span> <span class="fl">2</span><span class="op">)</span> <span class="op">}</span><span class="op">)</span>, quoted <span class="op">=</span> <span class="cn">TRUE</span><span class="op">)</span></span></span>
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<span class="r-in"><span></span></span>
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<span class="r-in"><span><span class="co"># In a normal Shiny app, the web client will trigger flush events. If you</span></span></span>
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<span class="r-in"><span><span class="co"># are at the console, you can force a flush with flushReact()</span></span></span>
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<span class="r-in"><span><span class="fu">shiny</span><span class="fu">:::</span><span class="fu">flushReact</span><span class="op">(</span><span class="op">)</span></span></span>
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<span class="r-out co"><span class="r-pr">#></span> [1] -30</span>
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<span class="r-out co"><span class="r-pr">#></span> [1] 2</span>
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<span class="r-out co"><span class="r-pr">#></span> [1] 4</span>
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<span class="r-out co"><span class="r-pr">#></span> [1] 3</span>
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</code></pre></div>
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<p></p><p>Developed by Winston Chang, Joe Cheng, JJ Allaire, Carson Sievert, Barret Schloerke, Garrick Aden-Buie, Yihui Xie, Jeff Allen, Jonathan McPherson, Alan Dipert, Barbara Borges, Posit Software, PBC.</p>
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