Crashed if using NSMutableArray to accept json - ios

Question

I create a NSMutableArray and initial it in viewDidLoad
#property (nonatomic, copy) NSMutableArray *arrayDATA;
- (void)viewDidLoad {
[super viewDidLoad];
_arrayDATA = [[NSMutableArray alloc] init];
}
And I request a json by using AFNetworking from the server, the json format like this
{
"code": 0,
"statu": "success",
"datalist": [
{
"id": "1",
"name": "ADPL"
},
{
"id": "2",
"name": "GOOG"
},
{
"id": "3",
"name": "LIKE"
}
]
}
And I give the response data to arrayDATA like this
AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];
manager.responseSerializer = [AFJSONResponseSerializer serializer];
manager.responseSerializer.acceptableContentTypes = [manager.responseSerializer.acceptableContentTypes setByAddingObject:#"application/json"];
NSDictionary *params = #{#"username":username, #"password":password};
[manager POST:URL parameters:params success:^(NSURLSessionDataTask * _Nonnull task, id _Nonnull responseObject) {
[self setArrayDATA:[responseObject objectForKey:#"datalist"]]; // works fine
[_arrayDATA addObject:[responseObject objectForKey:#"datalist"]]; // will crash
[self.tableview reloadData];
} failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
}];
As the code comment say
If I use [self setArrayDB:[responseObject objectForKey:#"datalist"]]; the function works well, but if I use [_arrayDB addObject:[responseObject objectForKey:#"datalist"]];, the function will crash, what's the correct way to use addObject here?
The crash info
[__NSSingleObjectArrayI objectForKey:]: unrecognized selector sent to instance 0x60000001a940
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSSingleObjectArrayI objectForKey:]: unrecognized selector sent to instance 0x60000001a940'

Answer 1

I suppose there is an error and arrayDATA and arrayDB are the same.
But the crash is because you are setting the element "datalist" of the responseObject to arrayDB. Then arrayDB is a NSArray, not an NSMutableArray. So you cannot add more elements.
On the other hand, you don't need to init the mutable array in viewDidLoad if latter you are going to set a new pointer to a new object.
Try to change the setArrayDB: line with this:
self.arrayDB = [NSMutableArray arrayWithArray:[responseObject objectForKey:#"datalist"]];

Answer 2

First of all return type of [responseObject objectForKey:#"datalist"] is Array and not NSDictionary.
Point 1
[self setArrayDATA:[responseObject objectForKey:#"datalist"]]; // works fine
This is fine because you are setting array for an array.
Point 2
This will crash because you are adding array as addObject.
[_arrayDATA addObject:[responseObject objectForKey:#"datalist"]];
You should use as below to make it working
[_arrayDATA addObjectsFromArray:[responseObject objectForKey:#"datalist"]];
^^^^^^^^^^^^^^^^^^^