silentValueForKey.m

// helper function for silentValueForKey
id silentValueForUndefinedKey(id self, SEL _cmd, NSString *key) {
	return nil;
}

// The purpose of this function is to have the same result as
// [object valueForKey:key], but instead of letting [NSObject valueForUndefinedKey:]
// raise an NSUndefinedKeyException, it silently returns nil instead.
id silentValueForKey(id object, NSString *key) {
	// Does object provide the same implementations of valueForKey: and valueForUndefinedKey: as NSObject?
	// If it does not, don't mess with it.
	Class originalClass = [object class];
	SEL vfukSelector = @selector(valueForUndefinedKey:);
	SEL vfkSelector = @selector(valueForKey:);
	if ((class_getMethodImplementation([NSObject class], vfukSelector) !=
		 class_getMethodImplementation(originalClass, vfukSelector))
		||
		(class_getMethodImplementation([NSObject class], vfkSelector) !=
		 class_getMethodImplementation(originalClass, vfkSelector)))
	{
		return [object valueForKey:key];
	}
	
	// Now the magic: let's temporarily switch object's class with a subclass
	// whose implementation for valueForUndefinedKey: just returns nil.
	const char *silentClassName = [[NSString stringWithFormat:@"GRMustacheSilent%@", originalClass] UTF8String];
	Class silentClass = objc_lookUpClass(silentClassName);
	if (silentClass == nil) {
		silentClass = objc_allocateClassPair(originalClass, silentClassName, 0);
		class_addMethod(silentClass, vfukSelector, (IMP)silentValueForUndefinedKey, "@@:@");
		objc_registerClassPair(silentClass);
	}
	object->isa = silentClass;
	
	// Silently call valueForKey!!!!
	@try {
		id value = [object valueForKey:key];
		object->isa = originalClass;
		return value;
	}
	@catch (NSException *exception) {
		object->isa = originalClass;
		[exception raise];
	}
}