I have inherited code that parses from a web service and fills in a NSDictionary. Under some circumstances, which I am going to explore, the parser returns a NSDictionary with count of 5, but the objects are invalid. Any message sent to them fails with unrecognized selector, even isKindOfClass: fails.
When I po the object in console, I see this:
error: Execution was interrupted, reason: Attempted to dereference an invalid
ObjC Object or send it an unrecognized selector.
The process has been returned to the state before expression evaluation.
How can I check that the object is invalid, if isKindOfClass: does not work here?
Here is the output from the console during the crash. You see dictionary with 5 empty objects, po on object 9 returns the invalid message.
Here is the po of the big dictionary:
I hope I will be able to check the issue in the parser, but I am also interested in how can I check for that invalid object to prevent the crash.
The problem is that you are trying to access an element of dictionary using an integer, if the object is a dictionary, the key should be any obj-c object that comforms to copy protocol. You are using 0 as an integer index, primitive types aren't obj-c objects, you should wrap it into an NSNumber.
The other issue is that the NSJSONSerializer returns an id type object, basing on the structure of the parsed JSON this could be a dictionary or an array, you should always inspect the returned object to check the type. I usually always expect an arrays of dictionaries, if the system returns just a dictionary, I create an array on the fly with just that object.
your po wants an array - but it is a dictionary.
this wont work
a dictionary can not be indexed with integers as it doesnt have objectForSubscriptIndex
Related
I'm trying to fetch object by object ID in a simple core data implementation. Using this api
let targetObj = CoreDataManager.sharedInstance.privateQueueContext.object(with: self.objectID) as! MyObj
but it seems object id changes. Here is the console log
(lldb) po self.objectID
0xd000000000280000 <x-coredata://551DDB76-537D-41FA-B923-F772E5EE5D29/MyObj/p10>
(lldb) po CoreDataManager.sharedInstance.privateQueueContext.object(with: self.objectID).objectID
0xd000000000280006 <x-coredata://551DDB76-537D-41FA-B923-F772E5EE5D29/MyObj/p10>
Please let me know if I'm doing anything wrong. Actually I'm book keeping these objectIds in an queue and need to dequeue the object based on these objectIds.
Thanks
Ankit
The only difference you're seeing when you print each object ID is the address in memory where the object ID is stored.
So, if you take those two objects and compare them with == they will be different, because that is testing referential equality.
If you compare those two objects with isEqual:, it will return true, because isEqual: is testing whether the values they each represent are equal.
You can't reliably compare objects by reference, you need to use isEqual:.
I am a little confused to how containsObject works. Does it check to see if it contains an instance of an object type or does it compare the inside of the objects variables etc to see if they match?
This is an implementation detail, you can work on the basis that it calls isEqual: on each items and works on the result of that.
Under the hood it's probably calling hash on each item and comparing that, then, if the hash matches it will call isEqual: to make sure it's a real match.
So long story short, there's a discrepancy between the output of a NSMutableDictionary's contents and the result of calling allValues on the same object. Below is some debugger output after inspecting the object which demonstrates my problem: (made generic of course)
(lldb) po self.someDict.allKeys
<__NSArrayI 0xa5a2e00>(
<SomeObject: 0xa5a2dc0>,
<SomeObject: 0xa5a2de0>
)
(lldb) po self.someDict.allValues
<__NSArrayI 0xa895ca0>(
0.5,
0.5
)
(lldb) po self.someDict
{
"<SomeObject: 0xa5a2dc0>" = (null);
"<SomeObject: 0xa5a2de0>" = (null);
}
So as we can see, the actual output of the NSMutableDictionary contains null values for both its entries, but the contents of .allValues contains the proper data. These three outputs were taken at the same time in execution.
I'm not sure why this is happening, but I think it may have something to do with the fact that I'm encoding/decoding the object which this dictionary is a property of using CoreData. I believe I'm doing this properly:
[aCoder encodeObject:self.someDict forKey:#"someDict"];
and to decode
self.someDict = [aDecoder decodeObjectForKey:#"someDict"];
The weird thing is that if I inspect the dictionary before it ever gets encoded, it is still in the state described at the beginning of the post, so this is why I'm doubtful the CoreData actions are screwing with the contents.
As always, please don't hesitate to request additional information.
EDIT: The problem was as answered below. I was using a custom class which didn't cooperate with isEqual, so my solution was to change the storage and structure of my logic, which made using a Dictionary unnecessary.
I have not been able to duplicate the problem using NSString as keys and NSNumber as values. I suspect that your custom class does not properly implement hash and/or isEqual. Specifically, the results from those two methods must be consistent, meaning that if isEqual returns true, then the hash values for the two objects must be identical.
You also need to ensure that your class implements NSCopying properly and that a copy is equal to the original.
As a general rule, don't use custom objects for dictionary keys. Just use strings and be done with it.
As user3386109 points out, custom objects must properly implement the -hash and -isEqual methods in order to be used as dictionary keys, and even then, custom objects don't work correctly for dictionary keys for things like key/value coding.
In my project I have to load a number of json files. I parse them with JSONKit and after every single parsing with
NSMutableDictionary *json = [myJSON objectFromJSONString];
I add them to an array like:
[self.themeArray addObject:json];
This works fine so far. Now I need to pass the dictionaries arround between views. Works so far as well, but I need to add few more objects to the dictionary object-> json. Even it I declared json as NSMutableDictionary it does not allow me to add objects as it seems the JSONKit parser creates non-mutable dictionaries.
I was thinking about creating an object which contains the json dictionary and my additional data side by side so I wouldn´t have to change the json dictionary. I could even change it to NSDictionary because there is no need to change it. But that seems somehow not-elegant to me.
Do you have any idea how I can solve this issue without changing the JSONKit lib?
Thanks in advance!
EDIT
i just tried after changing my code to
NSMutableDictionary *json = [[myJSON objectFromJSONString] mutableCopy];
something like this
[[self.theme objectForKey:#"theme"] setObject:sender forKey:#"sender"];
[[self.theme objectForKey:#"theme"] setValue:sender forKey:#"sender"];
Xcode throws an exception:
* Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '* -[JKDictionary setObject:forKey:]: mutating method sent to immutable object'
I assume that´s due to the fact there are still nested dictionaries in the superior dictionary. Then i would have to interate through my json object to copy all dictionaries to mutable dictionaries, right?
Perhaps it's better to switch to NSJSONSerialization as suggested by Guillaume.
EDIT
I just tried something like this
[self.theme setValue:sender forKey:#"sender"];
And it works now! It was as i assumend. Only the json object was copied to a mutable object. Probably obvious to you, it was not to me.
Thank you all for your help!
EDIT
Finally i changed my code again after i could not manage to change all objects deep inside my dictionary data to mutable objects. I threw out JSONKit and use now NSJSONDeserialization as recommendet here with the option NSJSONReadingMutableContainers. My code looks now like this and all containers (arrays and dictionaries) are mutable deep inside too. That makes me happy! ;-)
NSMutableDictionary *json = [NSJSONSerialization JSONObjectWithData:myJSON options:NSJSONReadingMutableContainers error:&jsonParsingError];
You can always create mutable versions of objects from their non-mutable counterparts by copying them.
NSMutableDictionary* json = [[myJSON objectFromJSONString] mutableCopy];
It is not optimal, but copying smaller dictionaries does is usually not noticable from a performance point of view.
Even it I declared json as NSMutableDictionary it does not allow me to add objects as it seems the JSONKit parser creates non-mutable dictionaries.
What type the variable is declared at means nothing. You could have declared json as NSNumber and that wouldn't make it an NSNumber.
You need to make a mutable copy of the dictionary (with mutableCopy) to get an NSMutableDictionary.
I have three ideas for your.
Create real data model objects and store them in your array. Use the JSON dictionary to init your object.
Store NSMutableDictionary objects in your array. Pass the JSON dictionary to +[NSMutableDictionary dictionaryWithDictionary:] to init the NSMutableDictionary. Others have suggested calling -[NSDictionary mutableCopy] on the JSON dictionary to do the same thing.
Create a category based on NSDictionary that stores the additional data.
NOTES:
Generally creating classes to represent your data is considered the best option, but it is also the most amount of up front work. Basically you are trading more up front work against more maintenance work as you try to keep up maintaining the dictionaries.
Storing mutable dictionary is exactly what you seem to be asking for, but it may be lots of works to find all the places where JSON dictionaries are added to the array and replacing them with the new call.
Creating a category for NSDictionary means you shouldn't need to change any of your current code, but it requires maintainers to understand how you have enhanced NSDictionary. In addition, it will help separate your changes from the original parsed JSON. You can use associated objects to store the data.
Recently I had a problem with such scenario:
In My NSManagedObject I stored longName attribute, called save on NSManagedObjectContext and after a while I close the application. Save ended without an error.
I restarted the application, I fetched the object and I tried to get [object longName]. The value which was returned was nil. When I stopped there with breakpoint and when I tried to display the value by po [object longName] proper value was returned and printed. ALl next calles to [object longName] in sources where correct. Whenever I don't use breakpoints there the value was always nil.
I think I hit the same problem recently in other application. I fetched the user object which contains NSSet of flights objects. When I tried to use [[flight validFrom] intValue] it returns number 0, but when I tried to print it with #"%#" format I've got (null).
The strange thing is that problem occurred not on every object.