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POUnrecognised selector crash within [NSDictionary valueForKey:]
 

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  1. POUnrecognised selector crash within [NSDictionary valueForKey:]
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    <p>I'm occasionally getting this crash, which is very hard to reproduce:</p> <pre><code>0 CoreFoundation 0x39ff73e2 __exceptionPreprocess + 158 1 libobjc.A.dylib 0x3905095e objc_exception_throw + 26 2 CoreFoundation 0x39ffaf2c -[NSObject(NSObject) doesNotRecognizeSelector:] + 180 3 CoreFoundation 0x39ff9648 ___forwarding___ + 388 4 CoreFoundation 0x39f51204 _CF_forwarding_prep_0 + 20 5 Foundation 0x32914bec -[NSDictionary(NSKeyValueCoding) valueForKey:] + 28 6 MyAppName 0x00032112 -[myViewController stopPlayersWithVolume:] (myViewController.m:151) </code></pre> <p>From this code:</p> <pre><code>- (void)stopPlayersWithVolume:(double)volume { for (NSString *file in players) { AVAudioPlayer *player = [players valueForKey:file]; if (player.volume == volume || volume == 0) [player stop]; } } </code></pre> <p><code>players</code> is an <code>NSMutableDictionary</code> property, accessed without <code>self.</code> because I don't believe it's needed with ARC. The keys are filenames (e.g. "mysound.mp3") and the values are <code>AVAudioPlayer</code> objects.</p> <p>Is the stack trace saying that the parameter <code>file</code> I'm passing to <code>[NSMutableDictionary valueForKey]</code> is not actually an <code>NSString</code>, and hence the unrecognised selector? It is always an <code>NSString</code> in the debugger, as you'd expect as it comes from a fast enumeration through the dictionary, but the crash never occurs in the debugger.</p> <p>My only thought is that there's a threading issue corrupting the dictionary. The code above is being fired by an NSTimer but that's just a message via the run loop, not a separate thread, so I believe there should be no need to worry about cross-thread access?</p> <p>Any suggestions appreciated. Sorry to just dump this out but I'm stuck.</p> <p><strong>Edit:</strong></p> <p>The players dictionary is allocated in <code>viewDidLoad</code> thus:</p> <pre><code>players = [[NSMutableDictionary alloc] init]; </code></pre> <p>And declared as follows:</p> <pre><code>@property (retain, nonatomic) NSMutableDictionary *players; </code></pre> <p>And synthesised as follows:</p> <pre><code>@synthesize players; </code></pre>
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    1. COWhat is the key? `valueForKey` does some m@gic that `objectForKey` does not.
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    2. COIt looks to me that although the players object is defined as an NSMutableDictionary it is pointing to an object of another class (maybe it is as an NSArray?).
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    3. COCould you copy and paste the literal error message? There might be some clue in the details. (Break on Objective-C exceptions if it's not currently logging an exception message.)
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      1. USPhillip Mills
 

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