NSMutableDictionary crashes with "mutating message sent to immutable object" - objective-c

Question

I have a class that has a NSMutableDictionary as a property:
#interface Alibi : NSObject <NSCopying>
#property (nonatomic, copy) NSMutableDictionary * alibiDetails;
#end
With the following constructor:
- (Alibi *)init
{
self = [super init];
_alibiDetails = [NSMutableDictionary dictionary];
return self;
}
and copy method:
- (Alibi *)copyWithZone:(NSZone *)zone
{
Alibi *theCopy = [[Alibi alloc] init];
theCopy.alibiDetails = [self.alibiDetails mutableCopy];
return theCopy;
}
When I try to call setObject:ForKey: I get a runtime error mutating method sent to immutable object.
I have the Alibi object declared in the view controller as #property (copy, nonatomic) Alibi * theAlibi; and I initialize it with self.theAlibi = [[Alibi alloc] init]; in viewDidLoad.
The line which crashes is:
NSString * recipient;
recipient = #"Boss";
[self.theAlibi.alibiDetails setObject:recipient forKey:#"Recipient"];
Please let me know what I am doing wrong here. I am coding for iOS 5 on iPhone.

Answer 1

You have a 'copy' property, which means exactly that - your NSMutableDictionary will get the -copy method called and return a regular NSDictionary before being assigned to the synthesized instance variable. This thread provides some information on some of your options as to solving this.

Answer 2

For the sake of completing this thread I will include my revised Alibi class below, this works as I require it to. If anyone notices any memory leaks or other issues, that would be appreciated.
#implementation Alibi
NSMutableDictionary *_details;
- (Alibi *)init
{
self = [super init];
_details = [NSMutableDictionary dictionary];
return self;
}
- (NSMutableDictionary *)copyDetails
{
return [_details mutableCopy];
}
- (NSMutableDictionary *)setDetails:(NSMutableDictionary *)value
{
_details = value;
return value;
}
- (void)addDetail:(id)value forKey:(id)key
{
[_details setObject:value forKey:key];
}
- (id)getDetailForKey:(id)key
{
return [_details objectForKey:key];
}
- (Alibi *)copyWithZone:(NSZone *)zone
{
Alibi *theCopy = [[Alibi alloc] init];
theCopy.serverId = [self.serverId copyWithZone:zone];
theCopy.user = [self.user copyWithZone:zone];
theCopy.startTime = [self.startTime copyWithZone:zone];
theCopy.endTime = [self.endTime copyWithZone:zone];
[theCopy setDetails:[self copyDetails]];
return theCopy;
}
#end