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<!DOCTYPE html>
<html>
<head>
<title>How Do I Declare a Closure in Swift?</title>
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<br>
<h1>How Do I Declare a Closure in Swift?</h1>
<div>
<h2>As a <strong>variable</strong>:</h2><code><span class='return'>var</span> closureName: (<span class='parameter-types'>ParameterTypes</span>) -> <span class='return'>ReturnType</span> = { ... }</code>
</div>
<div>
<h2>As an <strong>optional variable</strong>:</h2><code><span class='return'>var</span> closureName: ((<span class='parameter-types'>ParameterTypes</span>) -> <span class='return'>ReturnType</span>)?</code>
</div>
<div>
<h2>As a <strong>type alias</strong>:</h2><code><span class='return'>typealias</span> <span class='closure-type'>ClosureType</span> = (<span class='parameter-types'>ParameterTypes</span>) -> <span class='return'>ReturnType</span></code>
</div>
<div>
<h2>As a <strong>constant</strong>:</h2><code><span class='return'>let</span> closureName: <span class='closure-type'>ClosureType</span> = { ... }</code>
</div>
<div>
<h2>As a <strong>parameter to another function</strong>:</h2><code><span class='name'>funcName</span>(parameter: (<span class='parameter-types'>ParameterTypes</span>) -> <span class='return'>ReturnType</span>)</code>
<p class='disclaimer'>Note: if the passed-in closure is going to outlive the scope of the method, e.g. if you are saving it to a property, it needs to be annotated with <span class='inline-code'>@escaping</span>.</p>
</div>
<div>
<h2>As an <strong>argument to a function call</strong>:</h2><code><span class='name'>funcName</span>({ (<span class='parameter-types'>ParameterTypes</span>) -> <span class='return'>ReturnType</span> in statements })</code>
</div>
<div>
<h2>As a <strong>function parameter</strong>:</h2><code><span class='name'>array</span>.<span class='func'>sorted</span>(<span class='func'>by:</span> { (item1: <span class='func'>Int</span>, item2: <span class='func'>Int</span>) -> Bool <span class='return'>in return</span> item1 < item2 })</code>
<h2>As a <strong>function parameter with implied types</strong>:</h2><code><span class='name'>array</span>.<span class='func'>sorted</span>(<span class='func'>by:</span> { (item1, item2) -> Bool <span class='return'>in return</span> item1 < item2 })</code>
<h2>As a <strong>function parameter with implied return type</strong>:</h2><code><span class='name'>array</span>.<span class='func'>sorted</span>(<span class='func'>by:</span> { (item1, item2) <span class='return'>in return</span> item1 < item2 })</code>
<h2>As the <strong>last function parameter</strong>:</h2><code><span class='name'>array</span>.<span class='func'>sorted</span> { (item1, item2) <span class='return'>in return</span> item1 < item2 }</code>
<h2>As the last parameter, <strong>using shorthand argument names</strong>:</h2><code><span class='name'>array</span>.<span class='func'>sorted</span> { <span class='return'>return</span> $0 < $1 }</code>
<h2>As the last parameter, <strong>with an implied return value</strong>:</h2><code><span class='name'>array</span>.<span class='func'>sorted</span> { $0 < $1 }</code>
<h2>As the last parameter, <strong>as a reference to an existing function</strong>:</h2><code><span class='name'>array</span>.<span class='func'>sorted</span>(<span class='func'>by:</span> <)</code>
<h2>As a function parameter <strong>with explicit capture semantics</strong>:</h2><code><span class='name'>array</span>.<span class='func'>sorted</span>(<span class='func'>by:</span> { [unowned self] (item1: <span class='func'>Int</span>, item2: <span class='func'>Int</span>) -> Bool <span class='return'>in return</span> item1 < item2 })</code>
<h2>As a function parameter <strong>with explicit capture semantics and inferred parameters / return type</strong>:</h2><code><span class='name'>array</span>.<span class='func'>sorted</span>(<span class='func'>by:</span> { [unowned self] <span class='return'>in return</span> $0 < $1 })</code>
</div>
<div class='disclaimer'>
This site is not intended to be an exhaustive list of all possible uses of closures.<br>
<br>
<br>
Unable to access this site due to the profanity in the URL? <strong><a href="http://goshdarnclosuresyntax.com">http://goshdarnclosuresyntax.com</a></strong> is a more work-friendly mirror.
</div>
<footer>
By <a href="http://zeveisenberg.com">Zev Eisenberg</a>, with apologies to <a href="http://lazerwalker.com/">Em Lazer-Walker</a>, who has a very bad memory for this sort of thing.
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