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POHandling missing NSUserDefault keys after app updates
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  1. POHandling missing NSUserDefault keys after app updates
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    copied!<p>I've just got burned because an update to my iOS app went live on the App Store and unfortunately it crashes when using a new feature because of a bug in the way my code uses <code>NSUserDefaults</code>. My app uses <code>registerDefaults</code>, but the new feature tries to write to a key that doesn't exist after upgrading. It works fine if installing from scratch because the key gets created by <code>registerDefaults</code>.</p> <p>To be specific, my app uses nested dictionaries, so before the update the preferences file has this structure:</p> <pre><code>&lt;dict&gt; &lt;key&gt;Alpha&lt;/key&gt; &lt;dict&gt; &lt;key&gt;Key&lt;/key&gt; &lt;string&gt;Value&lt;/string&gt; &lt;/dict&gt; &lt;key&gt;Beta&lt;/key&gt; &lt;dict&gt; &lt;key&gt;Key&lt;/key&gt; &lt;string&gt;Value&lt;/string&gt; &lt;/dict&gt; &lt;key&gt;Charlie&lt;/key&gt; &lt;dict&gt; &lt;key&gt;Key&lt;/key&gt; &lt;string&gt;Value&lt;/string&gt; &lt;/dict&gt; &lt;/dict&gt; </code></pre> <p>&mdash;and after the update, key <em>Delta</em> is required:</p> <pre><code>&lt;dict&gt; &lt;key&gt;Alpha&lt;/key&gt; &lt;dict&gt; &lt;key&gt;Key&lt;/key&gt; &lt;string&gt;Value&lt;/string&gt; &lt;/dict&gt; &lt;key&gt;Beta&lt;/key&gt; &lt;dict&gt; &lt;key&gt;Key&lt;/key&gt; &lt;string&gt;Value&lt;/string&gt; &lt;/dict&gt; &lt;key&gt;Charlie&lt;/key&gt; &lt;dict&gt; &lt;key&gt;Key&lt;/key&gt; &lt;string&gt;Value&lt;/string&gt; &lt;/dict&gt; &lt;key&gt;Delta&lt;/key&gt; &lt;dict&gt; &lt;key&gt;Key&lt;/key&gt; &lt;string&gt;Value&lt;/string&gt; &lt;/dict&gt; &lt;/dict&gt; </code></pre> <p>My code is falling over when it tries to retrieve the dictionary for <em>Delta</em> from the user defaults in order to write some keys and values to it. For this scenario the dictionary is <code>nil</code> because after an update it's not in the preferences file.</p> <p>What's the best practice way to handle this situation? Should I simply check to see if the dictionary for <em>Delta</em> is <code>nil</code> before attempting to write to it and create it if required, or is there a less belt and braces approach?</p>
 

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