| <!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="Abstracting over accessing parts of stored value."><meta name="keywords" content="rust, rustlang, rust-lang, access"><title>arc_swap::access - 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="../../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"><!--[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="../../arc_swap/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="../../arc_swap/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><h2 class="location"><a href="#">Module access</a></h2><div class="sidebar-elems"><section><ul class="block"><li><a href="#structs">Structs</a></li><li><a href="#traits">Traits</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">Module <a href="../index.html">arc_swap</a>::<wbr><a class="mod" href="#">access</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/arc_swap/access.rs.html#1-544">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>Abstracting over accessing parts of stored value.</p> |
| <p>Sometimes, there’s a big globalish data structure (like a configuration for the whole program). |
| Then there are parts of the program that need access to up-to-date version of their <em>part</em> of |
| the configuration, but for reasons of code separation and reusability, it is not desirable to |
| pass the whole configuration to each of the parts.</p> |
| <p>This module provides means to grant the parts access to the relevant subsets of such global |
| data structure while masking the fact it is part of the bigger whole from the component.</p> |
| <p>Note that the <a href="../cache/index.html" title="crate::cache"><code>cache</code></a> module has its own <a href="../cache/trait.Access.html" title="crate::cache::Access"><code>Access</code></a> trait |
| that serves a similar purpose, but with cached access. The signatures are different, therefore |
| an incompatible trait.</p> |
| <h2 id="the-general-idea"><a href="#the-general-idea">The general idea</a></h2> |
| <p>Each part of the code accepts generic <a href="trait.Access.html" title="Access"><code>Access<T></code></a> for the <code>T</code> of its interest. This |
| provides means to load current version of the structure behind the scenes and get only the |
| relevant part, without knowing what the big structure is.</p> |
| <p>For technical reasons, the <a href="trait.Access.html" title="Access"><code>Access</code></a> trait is not object safe. If type erasure is desired, it |
| is possible use the <a href="trait.DynAccess.html" title="crate::access::DynAccess"><code>DynAccess</code></a> instead, which is object safe, but |
| slightly slower.</p> |
| <p>For some cases, it is possible to use <a href="../struct.ArcSwapAny.html#method.map" title="ArcSwapAny::map"><code>ArcSwapAny::map</code></a>. If that is not flexible enough, the |
| <a href="struct.Map.html" title="Map"><code>Map</code></a> type can be created directly.</p> |
| <p>Note that the <a href="trait.Access.html" title="Access"><code>Access</code></a> trait is also implemented for <a href="../struct.ArcSwapAny.html" title="ArcSwapAny"><code>ArcSwapAny</code></a> itself. Additionally, |
| there’s the <a href="struct.Constant.html" title="crate::access::Constant"><code>Constant</code></a> helper type, which is useful mostly for |
| testing (it doesn’t allow reloading).</p> |
| <h2 id="performance"><a href="#performance">Performance</a></h2> |
| <p>In general, these utilities use <a href="../struct.ArcSwapAny.html#method.load" title="ArcSwapAny::load"><code>ArcSwapAny::load</code></a> internally and then apply the provided |
| transformation. This has several consequences:</p> |
| <ul> |
| <li>Limitations of the <a href="../struct.ArcSwapAny.html#method.load" title="ArcSwapAny::load"><code>load</code></a> apply ‒ including the recommendation to not |
| hold the returned guard object for too long, but long enough to get consistency.</li> |
| <li>The transformation should be cheap ‒ optimally just borrowing into the structure.</li> |
| </ul> |
| <h2 id="examples"><a href="#examples">Examples</a></h2> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Arc; |
| <span class="kw">use </span>std::thread::{<span class="self">self</span>, JoinHandle}; |
| <span class="kw">use </span>std::time::Duration; |
| |
| <span class="kw">use </span>arc_swap::ArcSwap; |
| <span class="kw">use </span>arc_swap::access::{Access, Constant, Map}; |
| |
| <span class="kw">fn </span>work_with_usize<A: Access<usize> + Send + <span class="lifetime">'static</span>>(a: A) -> JoinHandle<()> { |
| thread::spawn(<span class="kw">move </span>|| { |
| <span class="kw">let </span><span class="kw-2">mut </span>value = <span class="number">0</span>; |
| <span class="kw">while </span>value != <span class="number">42 </span>{ |
| <span class="kw">let </span>guard = a.load(); |
| value = <span class="kw-2">*</span>guard; |
| <span class="macro">println!</span>(<span class="string">"{}"</span>, value); |
| <span class="comment">// Not strictly necessary, but dropping the guard can free some resources, like |
| // slots for tracking what values are still in use. We do it before the sleeping, |
| // not at the end of the scope. |
| </span>drop(guard); |
| thread::sleep(Duration::from_millis(<span class="number">50</span>)); |
| } |
| }) |
| } |
| |
| <span class="comment">// Passing the whole thing directly |
| // (If we kept another Arc to it, we could change the value behind the scenes) |
| </span>work_with_usize(Arc::new(ArcSwap::from_pointee(<span class="number">42</span>))).join().unwrap(); |
| |
| <span class="comment">// Passing a subset of a structure |
| </span><span class="kw">struct </span>Cfg { |
| value: usize, |
| } |
| |
| <span class="kw">let </span>cfg = Arc::new(ArcSwap::from_pointee(Cfg { value: <span class="number">0 </span>})); |
| <span class="kw">let </span>thread = work_with_usize(Map::new(Arc::clone(<span class="kw-2">&</span>cfg), |cfg: <span class="kw-2">&</span>Cfg| <span class="kw-2">&</span>cfg.value)); |
| cfg.store(Arc::new(Cfg { value: <span class="number">42 </span>})); |
| thread.join().unwrap(); |
| |
| <span class="comment">// Passing a constant that can't change. Useful mostly for testing purposes. |
| </span>work_with_usize(Constant(<span class="number">42</span>)).join().unwrap();</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.AccessConvert.html" title="arc_swap::access::AccessConvert struct">AccessConvert</a></div><div class="item-right docblock-short"><a href="trait.DynAccess.html" title="DynAccess">DynAccess</a> to <a href="trait.Access.html" title="Access">Access</a> wrapper.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Constant.html" title="arc_swap::access::Constant struct">Constant</a></div><div class="item-right docblock-short">Access to an constant.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Map.html" title="arc_swap::access::Map struct">Map</a></div><div class="item-right docblock-short">An adaptor to provide access to a part of larger structure.</div></div></div><h2 id="traits" class="small-section-header"><a href="#traits">Traits</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="trait" href="trait.Access.html" title="arc_swap::access::Access trait">Access</a></div><div class="item-right docblock-short">Abstracts over ways code can get access to a value of type <code>T</code>.</div></div><div class="item-row"><div class="item-left module-item"><a class="trait" href="trait.DynAccess.html" title="arc_swap::access::DynAccess trait">DynAccess</a></div><div class="item-right docblock-short">An object-safe version of the <a href="trait.Access.html" title="Access"><code>Access</code></a> trait.</div></div></div></section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="arc_swap" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
