StackOverflow2013

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copied!<p>While I can't think of a way to find out what keys will an object support you could use the fact that when you set the value for a nonexistent key the default behaviour of your object is to <a href="http://developer.apple.com/mac/library/documentation/cocoa/Conceptual/KeyValueCoding/Concepts/BasicPrinciples.html#//apple_ref/doc/uid/20002170" rel="nofollow noreferrer">throw an exception</a>. You could enclose the <code>setValue:forKey:</code> method invocation in a <code>@try</code> / <code>@catch</code> block to handle these errors.</p> <p>Consider the following code for an object:</p> <pre><code>@interface KVCClass : NSObject { NSString *stuff; } @property (nonatomic, retain) NSString *stuff; @end @implementation KVCClass @synthesize stuff; - (void) dealloc { [stuff release], stuff = nil; [super dealloc]; } @end </code></pre> <p>This should be KVC-compliant for the key <code>stuff</code> but nothing else.</p> <p>If you access this class from the following program:</p> <pre><code>int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; KVCClass *testClass = [[KVCClass alloc] init]; [testClass setValue:@"this is the value" forKey:@"stuff"]; NSLog(@"%@", testClass.stuff); // Error handled nicely @try { [testClass setValue:@"this should fail but we will catch the exception" forKey:@"nonexistentKey"]; } @catch (NSException * e) { NSLog(@"handle error here"); } // This will throw an exception [testClass setValue:@"this will fail" forKey:@"nonexistentKey"]; [testClass release]; [pool drain]; return 0; } </code></pre> <p>You will get a console output similar to the following:</p> <pre><code>2010-01-08 18:06:57.981 KVCTest[42960:903] this is the value 2010-01-08 18:06:57.984 KVCTest[42960:903] handle error here 2010-01-08 18:06:57.984 KVCTest[42960:903] *** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<KVCClass 0x10010c680> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key nonexistentKey.' *** Call stack at first throw: ( 0 CoreFoundation 0x00007fff851dc444 __exceptionPreprocess + 180 1 libobjc.A.dylib 0x00007fff866fa0f3 objc_exception_throw + 45 2 CoreFoundation 0x00007fff85233a19 -[NSException raise] + 9 3 Foundation 0x00007fff85659429 -[NSObject(NSKeyValueCoding) setValue:forKey:] + 434 4 KVCTest 0x0000000100001b78 main + 328 5 KVCTest 0x0000000100001a28 start + 52 6 ??? 0x0000000000000001 0x0 + 1 ) terminate called after throwing an instance of 'NSException' Abort trap </code></pre> <p>This shows that the first attempt to access the key <code>nonexistentKey</code> was nicely caught by the program, the second one generated an exception similar to yours.</p>

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