ios - Unrecognized selector when adding an object to NSMutableArray

Question

I have a singleton class with a NSMutableArray property to which I want to add objects and remove objects. For some reason I am getting:

-[__NSDictionaryI setObject:forKey:]: unrecognized selector sent to instance 0x1edf24c0

exception when trying to add to it. Here is the relevant code for the interface of the singleton:

//outbox item is the type of objects to be held in the dictionary
@interface OutboxItem : NSObject
@property (nonatomic, assign) unsigned long long size;
@end

@interface GlobalData : NSObject
@property (nonatomic, copy) NSMutableDictionary *p_outbox;
+ (GlobalData*)sharedGlobalData;
@end

The implementation of the singleton:

@implementation GlobalData
@synthesize  p_outbox;
static GlobalData *sharedGlobalData = nil;
+ (GlobalData*)sharedGlobalData {
    if (sharedGlobalData == nil) {
        sharedGlobalData = [[super allocWithZone:NULL] init];
        sharedGlobalData.p_outbox = [[NSMutableDictionary alloc] init];
    }
    return sharedGlobalData;
}

+ (id)allocWithZone:(NSZone *)zone {
    @synchronized(self)
    {
        if (sharedGlobalData == nil)
        {
            sharedGlobalData = [super allocWithZone:zone];
            return sharedGlobalData;
        }
    }
    return nil;
}
- (id)copyWithZone:(NSZone *)zone {
    return self;
}
@end

And here is the code that throws the exception:

GlobalData* glblData=[GlobalData sharedGlobalData] ;
OutboxItem* oItem = [OutboxItem alloc];
oItem.size = ...;//some number here
[glblData.p_outbox setObject:oItem forKey:...];//some NSString for a key

Am I missing something very obvious??

Answer 1

The problem is with your property:

@property (nonatomic, copy) NSMutableDictionary *p_outbox;

The copy semantics of the property result in a copy of the dictionary being made when you assign a value to the property. But the copy method for a dictionary always returns an immutable NSDictionary, even when called on an NSMutableDictionary.

To solve this problem you must create your own setter method for the property:

// I'm a little unclear what the actual name of the method will be.
// It's unusual to use underscores in property names. CamelCase is the standard.
- (void)setP_outbox:(NSMutableDictionary *)dictionary {
    p_outbox = [dictionary mutableCopy];
}