Object Passed by Reference Not Working? - ios

Question

New to iOS development. So I passed a method with an object address, thinking that I would be altering the same object by doing so:
_posts = [[NSMutableArray alloc]init];
FetchPosts *dataControl = [[FetchPosts alloc]init];
[dataControl accessPosts: _posts];
And the below code receives the passed in object.
-(void)accessPosts: (NSMutableArray *)transition {
//access the posts here.
_readablePosts = transition;
NSURL *url = [NSURL URLWithString: #"private"];
NSURLRequest *request = [[NSURLRequest alloc]initWithURL:url];
[NSURLConnection connectionWithRequest:request delegate:self];
}
-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
_jsonPosts = [[NSMutableData alloc]init];
[_jsonPosts appendData: data];
}
-(void)connectionDidFinishLoading:(NSURLConnection *)connection {
NSError *error;
NSArray *jsonArray = [NSJSONSerialization JSONObjectWithData:_jsonPosts options:NSJSONReadingAllowFragments error:&error];
for(int i = 0; i<jsonArray.count; i++) {
NSDictionary *element = jsonArray[i];
Posts *newPost = [[Posts alloc]init];
newPost.title = element[#"Title"];
newPost.discovery = element[#"Discovery"];
newPost.summary = element[#"Description"];
newPost.contact = element[#"Contact"];
[_readablePosts addObject:newPost];
}
}
From my perspective, everything seems to be in place properly. However, when I return to my original method, the _posts does not hold the correct object as it should. Am I doing something wrong with my pointers here?

Answer 1

You call this method -
[dataControl accessPosts: _posts];
Which invokes
[NSURLConnection connectionWithRequest:request delegate:self];
This is a non-blocking method, so it returns immediately and calls the delegate methods once it has retrieved data to be processed.
If you access _posts as soon accessPosts returns you will be doing so before the data has been retrieved from the network.
Only after connectionDidFinishLoading: has been called can you access the data in _posts
One solution is to use a delegation pattern. Your outer code would set itself as a delegate to the FetchPosts object, with the delegate method being called from connectionDidFinishLoading. Not only will this address your asynchronous issue it avoid potentially unsafe updating of the NSMutableArray and avoids the use of side-effects.
You will need to create an appropriate protocol in FetchPosts.h but then you can use something like -
-(void) requestPosts {
FetchPosts *dataControl = [[FetchPosts alloc]init];
dataControl.delegate=self;
[dataControl accessPosts];
}
-(void) didReceiveNewPosts:(NSArray *)posts {
// Do something with posts
}
FetchPosts.m
-(void)connectionDidFinishLoading:(NSURLConnection *)connection {
NSError *error;
NSArray *jsonArray = [NSJSONSerialization JSONObjectWithData:_jsonPosts options:NSJSONReadingAllowFragments error:&error];
for(int i = 0; i<jsonArray.count; i++) {
NSDictionary *element = jsonArray[i];
Posts *newPost = [[Posts alloc]init];
newPost.title = element[#"Title"];
newPost.discovery = element[#"Discovery"];
newPost.summary = element[#"Description"];
newPost.contact = element[#"Contact"];
[_readablePosts addObject:newPost];
}
if ([self.delegate respondsToSelector:#selector(didReceiveNewPosts:)]) {
[self.delegate didReceiveNewPosts:_readablePosts];
}
}

Answer 2

I think that the json is empty, the method didReceiveData is called each time that the device obtain a chunk of data, this method can be called more times during a downloading. If you create a new mutable data and append data is like you are resetting the NSDAta object.
You should create the Json post object in didReceiveresponse.