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POObj-C introspection: how to avoid casting all the time?
 

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  1. POObj-C introspection: how to avoid casting all the time?
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    <p> Hello all, I've been working on some enumeration routines in Objective-C that perform object introspection. In particular, I'm fast enumerating an <code>NSSet</code> and making sure that the objects therein belong to class <code>BBBallView</code> (an fairly unfortunate name, I agree) before tweaking their properties and/or calling their methods.</p> <p>In order to make the parser and compiler happy, however, I end up casting the object to its class on every single line; moreover, in order to access its properties, the object's cast has to be in parentheses, otherwise dot notation won't work. This leads to somewhat messy code:</p> <pre class="lang-obj-c prettyprint-override"><code>for (id otherBall in self.gameField.subviews) { if ([otherBall isKindOfClass:[BBBallView class]]) { if ( !((BBBallView *)otherBall).isEnlarged ) { CGRect otherFrame = ((BBBallView *)otherBall).frame; /* ... */ } } } </code></pre> <p>Is there any way to tell the compiler something like "at this point I know that <code>otherBall</code> is a <code>BBBallView</code>, so stop telling me it doesn't respond to these selectors and properties"? That way, one could just write:</p> <pre class="lang-obj-c prettyprint-override"><code>for (id otherBall in self.gameField.subviews) { if ([otherBall isKindOfClass:[BBBallView class]]) { if ( !otherBall.isEnlarged ) { CGRect otherFrame = otherBall.frame; /* ... */ } } } </code></pre> <p>and so on.</p> <p>I tried <code>otherBall = (BBBallView *)otherBall</code> but "fast enumeration variables can't be modified in ARC by default". Changing the enumeration variable to <code>__strong id</code> fixes it, but doesn't help the fact that any subsequent line gives out errors such as "property <code>isEnlarged</code> not found on object of type 'const __strong id'", so I'm back to square one.</p> <p>I'm not even sure why exactly this happens: shouldn't the compiler stay out of the way when an variable is of type <code>id</code>? In any case, the whole ordeal particularly messy in methods that need to perform several calculations on objects' properties, as it quickly becomes unreadable with all those parentheses.</p> <p>Is there any way around this?</p> <p>Thanks in advance!</p>
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    1. COIn general, introspection heavy patterns are a *code smell*. Collections generally should not hold heterogenous types. Also, having an Abstract class that provides basic services with specific subclasses that refine the behavior oft avoids lots of introspection.
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