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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="This function will attempt to pull items from both streams. You provide a closure to tell `SelectWithStrategy` which stream to poll. The closure can store state on `SelectWithStrategy` to which it will receive a `&#38;mut` on every invocation. This allows basing the strategy on prior choices."><title>select_with_strategy in futures_util::stream - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../static.files/rustdoc-b0742ba02757f159.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="futures_util" data-themes="" data-resource-suffix="" data-rustdoc-version="1.83.0 (90b35a623 2024-11-26)" data-channel="1.83.0" data-search-js="search-f0d225181b97f9a4.js" data-settings-js="settings-805db61a62df4bd2.js" ><script src="../../static.files/storage-1d39b6787ed640ff.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../static.files/main-f070b9041d14864c.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-0111fcff984fae8f.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc fn"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../futures_util/index.html">futures_<wbr>util</a><span class="version">0.3.31</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">select_<wbr>with_<wbr>strategy</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#examples" title="Examples">Examples</a><ul><li><a href="#priority" title="Priority">Priority</a></li><li><a href="#round-robin" title="Round Robin">Round Robin</a></li></ul></li></ul></section><div id="rustdoc-modnav"><h2><a href="index.html">In futures_<wbr>util::<wbr>stream</a></h2></div></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><span class="rustdoc-breadcrumbs"><a href="../index.html">futures_util</a>::<wbr><a href="index.html">stream</a></span><h1>Function <span class="fn">select_with_strategy</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/futures_util/stream/select_with_strategy.rs.html#139-157">source</a> </span></div><pre class="rust item-decl"><code>pub fn select_with_strategy&lt;St1, St2, Clos, State&gt;(
stream1: St1,
stream2: St2,
which: Clos,
) -&gt; <a class="struct" href="struct.SelectWithStrategy.html" title="struct futures_util::stream::SelectWithStrategy">SelectWithStrategy</a>&lt;St1, St2, Clos, State&gt;<div class="where">where
St1: <a class="trait" href="trait.Stream.html" title="trait futures_util::stream::Stream">Stream</a>,
St2: <a class="trait" href="trait.Stream.html" title="trait futures_util::stream::Stream">Stream</a>&lt;Item = St1::<a class="associatedtype" href="trait.Stream.html#associatedtype.Item" title="type futures_util::stream::Stream::Item">Item</a>&gt;,
Clos: <a class="trait" href="https://doc.rust-lang.org/1.83.0/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(<a class="primitive" href="https://doc.rust-lang.org/1.83.0/core/primitive.reference.html">&amp;mut State</a>) -&gt; <a class="enum" href="enum.PollNext.html" title="enum futures_util::stream::PollNext">PollNext</a>,
State: <a class="trait" href="https://doc.rust-lang.org/1.83.0/core/default/trait.Default.html" title="trait core::default::Default">Default</a>,</div></code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>This function will attempt to pull items from both streams. You provide a
closure to tell <a href="struct.SelectWithStrategy.html" title="struct futures_util::stream::SelectWithStrategy"><code>SelectWithStrategy</code></a> which stream to poll. The closure can
store state on <code>SelectWithStrategy</code> to which it will receive a <code>&amp;mut</code> on every
invocation. This allows basing the strategy on prior choices.</p>
<p>After one of the two input streams completes, the remaining one will be
polled exclusively. The returned stream completes when both input
streams have completed.</p>
<p>Note that this function consumes both streams and returns a wrapped
version of them.</p>
<h3 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h3><h4 id="priority"><a class="doc-anchor" href="#priority">§</a>Priority</h4>
<p>This example shows how to always prioritize the left stream.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::stream::{ repeat, select_with_strategy, PollNext, StreamExt };
<span class="kw">let </span>left = repeat(<span class="number">1</span>);
<span class="kw">let </span>right = repeat(<span class="number">2</span>);
<span class="comment">// We don't need any state, so let's make it an empty tuple.
// We must provide some type here, as there is no way for the compiler
// to infer it. As we don't need to capture variables, we can just
// use a function pointer instead of a closure.
</span><span class="kw">fn </span>prio_left(<span class="kw">_</span>: <span class="kw-2">&amp;mut </span>()) -&gt; PollNext { PollNext::Left }
<span class="kw">let </span><span class="kw-2">mut </span>out = select_with_strategy(left, right, prio_left);
<span class="kw">for _ in </span><span class="number">0</span>..<span class="number">100 </span>{
<span class="comment">// Whenever we poll out, we will alwas get `1`.
</span><span class="macro">assert_eq!</span>(<span class="number">1</span>, out.select_next_some().<span class="kw">await</span>);
}</code></pre></div>
<h4 id="round-robin"><a class="doc-anchor" href="#round-robin">§</a>Round Robin</h4>
<p>This example shows how to select from both streams round robin.
Note: this special case is provided by [<code>futures-util::stream::select</code>].</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::stream::{ repeat, select_with_strategy, PollNext, StreamExt };
<span class="kw">let </span>left = repeat(<span class="number">1</span>);
<span class="kw">let </span>right = repeat(<span class="number">2</span>);
<span class="kw">let </span>rrobin = |last: <span class="kw-2">&amp;mut </span>PollNext| last.toggle();
<span class="kw">let </span><span class="kw-2">mut </span>out = select_with_strategy(left, right, rrobin);
<span class="kw">for _ in </span><span class="number">0</span>..<span class="number">100 </span>{
<span class="comment">// We should be alternating now.
</span><span class="macro">assert_eq!</span>(<span class="number">1</span>, out.select_next_some().<span class="kw">await</span>);
<span class="macro">assert_eq!</span>(<span class="number">2</span>, out.select_next_some().<span class="kw">await</span>);
}</code></pre></div>
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