NSMutableDictionary thread safety

Question

I have a question on thread safety while using NSMutableDictionary.

The main thread is reading data from NSMutableDictionary where:

Answer 1

I have two options to using nsmutabledictionary.

One is:

NSLock* lock = [[NSLock alloc] init];
[lock lock];
[object.dictionary setObject:image forKey:name];
[lock unlock];

Two is:

//Let's assume var image, name are setup properly
dispatch_async(dispatch_get_main_queue(), 
^{ 
        [object.dictionary setObject:image forKey:name];
});

I dont know why some people want to overwrite setting and getting of mutabledictionary.