NSDictionary & NSMutableDictionary keys

NSDictionary keys must implement NSCopying. I required a map that used an UIView as a key, which does not implement NSCopying. The solution:

UISwitch* view = [[UISwitch alloc] init];

NSMutableDictionary* controls = [[NSMutableDictionary alloc] init];

[controls setObject:value forKey:[NSValue valueWithNonretainedObject:view]];

value = [controls objectForKey:[NSValue valueWithNonretainedObject: view]];