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<title>Unowned Properties in Swift</title>
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<h1>Unowned Properties in Swift</h1>
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Published on 02 May 2018
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<p>It's very common to use the <code>weak</code> keyword in order to prevent reference cycles in properties like delegates:</p>
<pre class="language-swift"><code class="language-swift">weak var delegate: HomeViewDelegate?</code></pre>
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<p>Unfortunately, since it protects you from references being lost, the <code>weak</code> keyword forces the usage of <code>var</code> and an optional type, which can be quite jarring if you're building something like an <code>UIView</code> that is not supposed to be used without a delegate - much less change delegates suddenly:</p>
<pre class="language-swift"><code class="language-swift">class HomeView: UIView {
weak var delegate: HomeViewDelegate?
func renderView() {
guard let delegate = delegate else {
//Timmy: this should never happen!
return
}
let category = delegate.currentlySelectedCategory()
categoryView.render(category: category)
delegate.homeViewDidUpdate()
}
}</code></pre>
<p>But <code>weak</code> is not the only way of breaking reference cycles. Just like in capture lists, the <code>unowned</code> keyword can be used in properties to create non-strong references:</p>
<pre class="language-swift"><code class="language-swift">unowned let delegate: HomeViewModelDelegate</code></pre>
<p>Unlike <code>weak</code>, <code>unowned</code> references are expected to always have a value. This allows you to not only use non-optional types when declaring them, but also to do so with <code>let</code>, bringing back immutability and making sure your objects won't act in unexpected ways:</p>
<pre class="language-swift"><code class="language-swift">class HomeView: UIView {
private unowned let delegate: HomeViewDelegate
init(delegate: HomeViewDelegate) {
self.delegate = delegate
super.init(frame: .zero)
}
func renderView() {
let category = delegate.currentlySelectedCategory()
categoryView.render(category: category)
delegate.homeViewDidUpdate()
}
}</code></pre>
<p>With <code>weak</code>, there's nothing stopping <code>HomeView</code> from being used without a delegate, and a <code>guard</code> is required to unwrap values retrieved from the delegate due to the optional requirement of the keyword. On the other hand, <code>unowned</code> allows you to break reference cycles while still using the reference just like if it was strong.</p>
<p>However, be aware that <code>unowned</code> properties will work just like the capture list keyword. If you try to access an unowned reference that has already deallocated, your app will crash.</p>
<p>If your object is guaranteed to never outlive its unowned reference (like a <code>ViewModel</code>, for example), usage of <code>unowned</code> can greatly improve both code quality and performance.</p>
<h2>But <code>unowned</code> can crash your app just like implicitly unwrapped optionals. Shouldn't you use <code>weak</code> for everything?</h2>
<p>Most people rely on <code>weak</code> for everything and frown upon <code>unowned</code> due to its potential crashes, specially on capture lists. This is a safe way to use Swift, but not Apple's intended practice.</p>
<p>Unlike implicitly unwrapped optionals, which in my opinion are just a lazy way of overcoming architectural problems, <code>unowned</code> references have significant advantages over <code>weak</code> ones: They have better performance, allow immutability, and since they can't be manually set to <code>nil</code>, your code will not accidentally follow unexpected paths. <code>unowned</code> references are perfectly safe - problems will only arise if you misuse the keyword.</p>
<p>According to Apple, you should always use <code>unowned</code> references when your object can't outlive its reference:</p>
<blockquote>
<p>Use an unowned reference only when you are sure that the reference always refers to an instance that has not been deallocated.</p>
<p>If you try to access the value of an unowned reference after that instance has been deallocated, you’ll get a runtime error.</p>
<p>If the captured reference will never become nil, it should always be captured as an unowned reference, rather than a weak reference.</p>
</blockquote>
<p>In any other cases, you should keep using <code>weak</code> references.</p>
<h2>What else?</h2>
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<p>The next time you create a delegate or an <code>@escaping</code> closure, think about its context and see if it's possible for it to outlive its reference. If it doesn't, you might find <code>unowned</code> properties an interesting tool to improve your code's quality.</p>
<p>Follow me on my Twitter - <a href="https://twitter.com/rockbruno_">@rockbruno_</a>, and let me know of any suggestions and corrections you want to share.</p>
<h2>References and Good reads</h2>
<a href="https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/AutomaticReferenceCounting.html">Apple Docs: ARC and Keywords</a>
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