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122 lines
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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="A multi-producer, multi-consumer channel that only retains the last sent value."><title>tokio::sync::watch - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Italic-81dc35de.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-MediumItalic-ccf7e434.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2"href="../../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../../../static.files/rustdoc-ca0dd0c4.css"><meta name="rustdoc-vars" data-root-path="../../../" data-static-root-path="../../../static.files/" data-current-crate="tokio" data-themes="" data-resource-suffix="" data-rustdoc-version="1.93.1 (01f6ddf75 2026-02-11) (Arch Linux rust 1:1.93.1-1)" data-channel="1.93.1" data-search-js="search-9e2438ea.js" data-stringdex-js="stringdex-a3946164.js" data-settings-js="settings-c38705f0.js" ><script src="../../../static.files/storage-e2aeef58.js"></script><script defer src="../sidebar-items.js"></script><script defer src="../../../static.files/main-a410ff4d.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-263c88ec.css"></noscript><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-32x32-eab170b8.png"><link rel="icon" type="image/svg+xml" href="../../../static.files/favicon-044be391.svg"></head><body class="rustdoc mod"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><rustdoc-topbar><h2><a href="#">Module watch</a></h2></rustdoc-topbar><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../../tokio/index.html">tokio</a><span class="version">1.49.0</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Module watch</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#usage" title="Usage">Usage</a><ul><li><a href="#change-notifications" title="Change notifications">Change notifications</a></li><li><a href="#changed-versus-has_changed" title="`changed` versus `has_changed`"><code>changed</code> versus <code>has_changed</code></a></li><li><a href="#borrow_and_update-versus-borrow" title="`borrow_and_update` versus `borrow`"><code>borrow_and_update</code> versus <code>borrow</code></a></li></ul></li><li><a href="#examples" title="Examples">Examples</a></li><li><a href="#closing" title="Closing">Closing</a></li><li><a href="#thread-safety" title="Thread safety">Thread safety</a></li></ul><h3><a href="#modules">Module Items</a></h3><ul class="block"><li><a href="#modules" title="Modules">Modules</a></li><li><a href="#structs" title="Structs">Structs</a></li><li><a href="#functions" title="Functions">Functions</a></li></ul></section><div id="rustdoc-modnav"><h2><a href="../index.html">In tokio::<wbr>sync</a></h2></div></div></nav><div class="sidebar-resizer" title="Drag to resize sidebar"></div><main><div class="width-limiter"><section id="main-content" class="content"><div class="main-heading"><div class="rustdoc-breadcrumbs"><a href="../../index.html">tokio</a>::<wbr><a href="../index.html">sync</a></div><h1>Module <span>watch</span> <button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../../src/tokio/sync/watch.rs.html#1-1556">Source</a> </span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>A multi-producer, multi-consumer channel that only retains the <em>last</em> sent
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value.</p>
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<p>This channel is useful for watching for changes to a value from multiple
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points in the code base, for example, changes to configuration values.</p>
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<h2 id="usage"><a class="doc-anchor" href="#usage">§</a>Usage</h2>
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<p><a href="fn.channel.html" title="fn tokio::sync::watch::channel"><code>channel</code></a> returns a <a href="struct.Sender.html" title="struct tokio::sync::watch::Sender"><code>Sender</code></a> / <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a> pair. These are the producer
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and consumer halves of the channel. The channel is created with an initial
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value.</p>
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<p>Each <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a> independently tracks the last value <em>seen</em> by its caller.</p>
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<p>To access the <strong>current</strong> value stored in the channel and mark it as <em>seen</em>
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by a given <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a>, use <a href="struct.Receiver.html#method.borrow_and_update" title="method tokio::sync::watch::Receiver::borrow_and_update"><code>Receiver::borrow_and_update()</code></a>.</p>
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<p>To access the current value <strong>without</strong> marking it as <em>seen</em>, use
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<a href="struct.Receiver.html#method.borrow" title="method tokio::sync::watch::Receiver::borrow"><code>Receiver::borrow()</code></a>. (If the value has already been marked <em>seen</em>,
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<a href="struct.Receiver.html#method.borrow" title="method tokio::sync::watch::Receiver::borrow"><code>Receiver::borrow()</code></a> is equivalent to <a href="struct.Receiver.html#method.borrow_and_update" title="method tokio::sync::watch::Receiver::borrow_and_update"><code>Receiver::borrow_and_update()</code></a>.)</p>
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<p>For more information on when to use these methods, see
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<a href="#borrow_and_update-versus-borrow">here</a>.</p>
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<h3 id="change-notifications"><a class="doc-anchor" href="#change-notifications">§</a>Change notifications</h3>
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<p>The <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a> half provides an asynchronous <a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a> method. This
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method is ready when a new, <em>unseen</em> value is sent via the <a href="struct.Sender.html" title="struct tokio::sync::watch::Sender"><code>Sender</code></a> half.</p>
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<ul>
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<li><a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>Receiver::changed()</code></a> returns:
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<ul>
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<li><code>Ok(())</code> on receiving a new value.</li>
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<li><code>Err(</code><a href="error/struct.RecvError.html" title="struct tokio::sync::watch::error::RecvError"><code>RecvError</code></a><code>)</code> if the
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channel has been closed <strong>AND</strong> the current value is <em>seen</em>.</li>
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</ul>
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</li>
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<li>If the current value is <em>unseen</em> when calling <a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a>, then
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<a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a> will return immediately. If the current value is <em>seen</em>, then
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it will sleep until either a new message is sent via the <a href="struct.Sender.html" title="struct tokio::sync::watch::Sender"><code>Sender</code></a> half,
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or the <a href="struct.Sender.html" title="struct tokio::sync::watch::Sender"><code>Sender</code></a> is dropped.</li>
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<li>On completion, the <a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a> method marks the new value as <em>seen</em>.</li>
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<li>At creation, the initial value is considered <em>seen</em>. In other words,
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<a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>Receiver::changed()</code></a> will not return until a subsequent value is sent.</li>
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<li>New <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a> instances can be created with <a href="struct.Sender.html#method.subscribe" title="method tokio::sync::watch::Sender::subscribe"><code>Sender::subscribe()</code></a>.
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The current value at the time the <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a> is created is considered
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<em>seen</em>.</li>
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</ul>
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<h3 id="changed-versus-has_changed"><a class="doc-anchor" href="#changed-versus-has_changed">§</a><a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a> versus <a href="struct.Receiver.html#method.has_changed" title="method tokio::sync::watch::Receiver::has_changed"><code>has_changed</code></a></h3>
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<p>The <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a> half provides two methods for checking for changes
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in the channel, <a href="struct.Receiver.html#method.has_changed" title="method tokio::sync::watch::Receiver::has_changed"><code>has_changed</code></a> and <a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a>.</p>
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<ul>
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<li>
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<p><a href="struct.Receiver.html#method.has_changed" title="method tokio::sync::watch::Receiver::has_changed"><code>has_changed</code></a> is a <em>synchronous</em> method that checks whether the current
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value is seen or not and returns a boolean. This method does <strong>not</strong> mark the
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value as seen.</p>
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</li>
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<li>
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<p><a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a> is an <em>asynchronous</em> method that will return once an unseen
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value is in the channel. This method does mark the value as seen.</p>
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</li>
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</ul>
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<p>Note there are two behavioral differences on when these two methods return
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an error.</p>
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<ul>
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<li><a href="struct.Receiver.html#method.has_changed" title="method tokio::sync::watch::Receiver::has_changed"><code>has_changed</code></a> errors if and only if the channel is closed.</li>
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<li><a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a> errors if the channel has been closed <strong>AND</strong>
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the current value is seen.</li>
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</ul>
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<p>See the example below that shows how these methods have different fallibility.</p>
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<h3 id="borrow_and_update-versus-borrow"><a class="doc-anchor" href="#borrow_and_update-versus-borrow">§</a><a href="struct.Receiver.html#method.borrow_and_update" title="method tokio::sync::watch::Receiver::borrow_and_update"><code>borrow_and_update</code></a> versus <a href="struct.Receiver.html#method.borrow" title="method tokio::sync::watch::Receiver::borrow"><code>borrow</code></a></h3>
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<p>If the receiver intends to await notifications from <a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a> in a loop,
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<a href="struct.Receiver.html#method.borrow_and_update" title="method tokio::sync::watch::Receiver::borrow_and_update"><code>Receiver::borrow_and_update()</code></a> should be preferred over
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<a href="struct.Receiver.html#method.borrow" title="method tokio::sync::watch::Receiver::borrow"><code>Receiver::borrow()</code></a>. This avoids a potential race where a new value is
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sent between <a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a> being ready and the value being read. (If
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<a href="struct.Receiver.html#method.borrow" title="method tokio::sync::watch::Receiver::borrow"><code>Receiver::borrow()</code></a> is used, the loop may run twice with the same value.)</p>
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<p>If the receiver is only interested in the current value, and does not intend
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to wait for changes, then <a href="struct.Receiver.html#method.borrow" title="method tokio::sync::watch::Receiver::borrow"><code>Receiver::borrow()</code></a> can be used. It may be more
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convenient to use <a href="struct.Receiver.html#method.borrow" title="method tokio::sync::watch::Receiver::borrow"><code>borrow</code></a> since it’s an <code>&self</code>
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method—<a href="struct.Receiver.html#method.borrow_and_update" title="method tokio::sync::watch::Receiver::borrow_and_update"><code>borrow_and_update</code></a> requires <code>&mut self</code>.</p>
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<h2 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h2>
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<p>The following example prints <code>hello! world! </code>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::watch;
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<span class="kw">use </span>tokio::time::{Duration, sleep};
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<span class="kw">let </span>(tx, <span class="kw-2">mut </span>rx) = watch::channel(<span class="string">"hello"</span>);
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tokio::spawn(<span class="kw">async move </span>{
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<span class="comment">// Use the equivalent of a "do-while" loop so the initial value is
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// processed before awaiting the `changed()` future.
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</span><span class="kw">loop </span>{
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<span class="macro">println!</span>(<span class="string">"{}! "</span>, <span class="kw-2">*</span>rx.borrow_and_update());
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<span class="kw">if </span>rx.changed().<span class="kw">await</span>.is_err() {
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<span class="kw">break</span>;
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}
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}
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});
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sleep(Duration::from_millis(<span class="number">100</span>)).<span class="kw">await</span>;
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tx.send(<span class="string">"world"</span>)<span class="question-mark">?</span>;</code></pre></div>
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<p>Difference on fallibility of <a href="struct.Receiver.html#method.changed" title="method tokio::sync::watch::Receiver::changed"><code>changed</code></a> versus <a href="struct.Receiver.html#method.has_changed" title="method tokio::sync::watch::Receiver::has_changed"><code>has_changed</code></a>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::watch;
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<span class="attr">#[tokio::main(flavor = <span class="string">"current_thread"</span>)]
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</span><span class="kw">let </span>(tx, <span class="kw-2">mut </span>rx) = watch::channel(<span class="string">"hello"</span>);
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tx.send(<span class="string">"goodbye"</span>).unwrap();
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drop(tx);
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<span class="comment">// `has_changed` does not mark the value as seen and errors
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// since the channel is closed.
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</span><span class="macro">assert!</span>(rx.has_changed().is_err());
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<span class="comment">// `changed` returns Ok since the value is not already marked as seen
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// even if the channel is closed.
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</span><span class="macro">assert!</span>(rx.changed().<span class="kw">await</span>.is_ok());
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<span class="comment">// The `changed` call above marks the value as seen.
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// The next `changed` call now returns an error as the channel is closed
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// AND the current value is seen.
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</span><span class="macro">assert!</span>(rx.changed().<span class="kw">await</span>.is_err());</code></pre></div><h2 id="closing"><a class="doc-anchor" href="#closing">§</a>Closing</h2>
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<p><a href="struct.Sender.html#method.is_closed" title="method tokio::sync::watch::Sender::is_closed"><code>Sender::is_closed</code></a> and <a href="struct.Sender.html#method.closed" title="method tokio::sync::watch::Sender::closed"><code>Sender::closed</code></a> allow the producer to detect
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when all <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a> handles have been dropped. This indicates that there
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is no further interest in the values being produced and work can be stopped.</p>
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<p>The value in the channel will not be dropped until all senders and all
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receivers have been dropped.</p>
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<h2 id="thread-safety"><a class="doc-anchor" href="#thread-safety">§</a>Thread safety</h2>
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<p>Both <a href="struct.Sender.html" title="struct tokio::sync::watch::Sender"><code>Sender</code></a> and <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a> are thread safe. They can be moved to other
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threads and can be used in a concurrent environment. Clones of <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a>
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handles may be moved to separate threads and also used concurrently.</p>
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</div></details><h2 id="modules" class="section-header">Modules<a href="#modules" class="anchor">§</a></h2><dl class="item-table"><dt><a class="mod" href="error/index.html" title="mod tokio::sync::watch::error">error</a></dt><dd>Watch error types.</dd></dl><h2 id="structs" class="section-header">Structs<a href="#structs" class="anchor">§</a></h2><dl class="item-table"><dt><a class="struct" href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver">Receiver</a></dt><dd>Receives values from the associated <a href="struct.Sender.html" title="struct tokio::sync::watch::Sender"><code>Sender</code></a>.</dd><dt><a class="struct" href="struct.Ref.html" title="struct tokio::sync::watch::Ref">Ref</a></dt><dd>Returns a reference to the inner value.</dd><dt><a class="struct" href="struct.Sender.html" title="struct tokio::sync::watch::Sender">Sender</a></dt><dd>Sends values to the associated <a href="struct.Receiver.html" title="struct tokio::sync::watch::Receiver"><code>Receiver</code></a>.</dd></dl><h2 id="functions" class="section-header">Functions<a href="#functions" class="anchor">§</a></h2><dl class="item-table"><dt><a class="fn" href="fn.channel.html" title="fn tokio::sync::watch::channel">channel</a></dt><dd>Creates a new watch channel, returning the “send” and “receive” handles.</dd></dl></section></div></main></body></html> |