| <!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 Futures channel-like utility to signal when a value is wanted."><meta name="keywords" content="rust, rustlang, rust-lang, want"><title>want - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="../crates.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.svg"></head><body class="rustdoc mod crate"><!--[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">☰</button><a class="sidebar-logo" href="../want/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../want/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2 class="location"><a href="#">Crate want</a></h2><div class="sidebar-elems"><ul class="block"><li class="version">Version 0.3.0</li><li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#structs">Structs</a></li><li><a href="#functions">Functions</a></li></ul></section></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Crate <a class="mod" href="#">want</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../src/want/lib.rs.html#1-585">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>A Futures channel-like utility to signal when a value is wanted.</p> |
| <p>Futures are supposed to be lazy, and only starting work if <code>Future::poll</code> |
| is called. The same is true of <code>Stream</code>s, but when using a channel as |
| a <code>Stream</code>, it can be hard to know if the receiver is ready for the next |
| value.</p> |
| <p>Put another way, given a <code>(tx, rx)</code> from <code>futures::sync::mpsc::channel()</code>, |
| how can the sender (<code>tx</code>) know when the receiver (<code>rx</code>) actually wants more |
| work to be produced? Just because there is room in the channel buffer |
| doesn’t mean the work would be used by the receiver.</p> |
| <p>This is where something like <code>want</code> comes in. Added to a channel, you can |
| make sure that the <code>tx</code> only creates the message and sends it when the <code>rx</code> |
| has <code>poll()</code> for it, and the buffer was empty.</p> |
| <h2 id="example"><a href="#example">Example</a></h2><div class="example-wrap"><pre class="language-nightly"><code>extern crate want; |
| |
| |
| // Some message that is expensive to produce. |
| struct Expensive; |
| |
| // Some futures-aware MPSC channel... |
| let (mut tx, mut rx) = mpsc_channel(); |
| |
| // And our `want` channel! |
| let (mut gv, mut tk) = want::new(); |
| |
| |
| // Our receiving task... |
| spawn(async move { |
| // Maybe something comes up that prevents us from ever |
| // using the expensive message. |
| // |
| // Without `want`, the "send" task may have started to |
| // produce the expensive message even though we wouldn't |
| // be able to use it. |
| if !we_still_want_message() { |
| return; |
| } |
| |
| // But we can use it! So tell the `want` channel. |
| tk.want(); |
| |
| match rx.recv().await { |
| Some(_msg) => println!("got a message"), |
| None => println!("DONE"), |
| } |
| }); |
| |
| // Our sending task |
| spawn(async move { |
| // It's expensive to create a new message, so we wait until the |
| // receiving end truly *wants* the message. |
| if let Err(_closed) = gv.want().await { |
| // Looks like they will never want it... |
| return; |
| } |
| |
| // They want it, let's go! |
| tx.send(Expensive); |
| }); |
| </code></pre></div></div></details><h2 id="structs" class="small-section-header"><a href="#structs">Structs</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Closed.html" title="want::Closed struct">Closed</a></div><div class="item-right docblock-short">The <code>Taker</code> has canceled its interest in a value.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Giver.html" title="want::Giver struct">Giver</a></div><div class="item-right docblock-short">An entity that gives a value when wanted.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.SharedGiver.html" title="want::SharedGiver struct">SharedGiver</a></div><div class="item-right docblock-short">A cloneable <code>Giver</code>.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Taker.html" title="want::Taker struct">Taker</a></div><div class="item-right docblock-short">An entity that wants a value.</div></div></div><h2 id="functions" class="small-section-header"><a href="#functions">Functions</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.new.html" title="want::new fn">new</a></div><div class="item-right docblock-short">Create a new <code>want</code> channel.</div></div></div></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="want" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
