Return NSMutableArray from completionHandler (Objective C) - ios

Question

I did post request to a web service and get response. I convert the response to NSMutableArray. My response in NSURLSessionDataTask and now I want to return NSMutableArray for using outside of NSURLSessionDataTask. Here is my code:
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
[request setURL:[NSURL URLWithString:#"url"]];
[request setHTTPMethod:#"POST"];
[request setValue:#"application/x-www-form-urlencoded" forHTTPHeaderField:#"Content-Type"];
NSString *postString = #"params";
NSString *postLength = [NSString stringWithFormat:#"%lu", ( unsigned long )[postString length]];
[request setValue:postLength forHTTPHeaderField:#"Content-Length" ];
[request setHTTPBody:[postString dataUsingEncoding:NSUTF8StringEncoding]];
NSURLSessionDataTask *task = [[self getURLSession] dataTaskWithRequest:request completionHandler:^( NSData *data, NSURLResponse *response, NSError *error )
{
dispatch_async( dispatch_get_main_queue(),
^{
NSDictionary *dicData = [NSJSONSerialization
JSONObjectWithData:data
options:NSJSONReadingAllowFragments
error:nil];
NSDictionary *values = [dicData valueForKeyPath:#"smth"];
NSArray * dataArr = [dicData objectForKey:#"smth"];
NSArray * closeArr = [values objectForKey:#"0"];
NSUInteger dataCount = [dataArr count] ;
NSUInteger closeCount = [closeArr count] ;
NSMutableArray * newData = [NSMutableArray new] ; //<-- THIS ARRAY
for(int i = 0 ; i<dataCount && i<closeCount ; i++)
{
NSMutableDictionary * temp = [NSMutableDictionary new] ;
NSString * dataString = [dataArr objectAtIndex:i];
NSString * closeString = [closeArr objectAtIndex:i];
[temp setObject:dataString forKey:#"smth"];
[temp setObject:closeString forKey:#"smth"];
[newData addObject:temp];
}
NSLog(#"%#", newData);
} );
}];
[task resume];
I need return NSMutableArray * newData = [NSMutableArray new];
Long story short, I get json data from web service, then transform it to appropriate json format for displaying it in the chart(I use shinobicontrols). Now, I display chart with the help of local json. Here is the code:
_timeSeries = [NSMutableArray new];
NSString* filePath = [[NSBundle mainBundle] pathForResource:#"AppleStockPrices" ofType:#"json"];
NSData* json = [NSData dataWithContentsOfFile:filePath];
NSArray* data = [NSJSONSerialization JSONObjectWithData:json
options:NSJSONReadingAllowFragments
error:nil];
for (NSDictionary* jsonPoint in data) {
SChartDataPoint* datapoint = [self dataPointForDate:jsonPoint[#"smth1"]
andValue:jsonPoint[#"smth2"]];
[_timeSeries addObject:datapoint];
}
When I am trying to implement this code in NSURLSessionDataTask, the chart doesn't appear. So I need return NSMutableArray(where my data in appropriate json format) outside.
How can I do this? Any ideas?
Thank you!

Answer 1

You cannot add a return statement in the completion handler since it may not be called if the session return an error. As a matter of fact, Xcode will give you an "Incompatible pointer type" error if you try to do that.
The best way I found to go around it is to set up your newData array as a property and make it available to the other methods in the class. If a specific method will need to handle this array when the url session task is over, you can call that method from the completion handler or use a notification.
Alternatively, if for some reason you do not want to user a class property, you can use the NSNotificationCenter, and pass the newData to the listener in the notification object.
EDIT: code example using a property
If you need the newData outside the completion block, an easy way is declaring the array as a property. This is not the only approach and probably not the best. But it doesn't add much complexity to the code.
You can declare the newData array in your .m class file:
#interface "whatever class you are using"
#property (nonatomic, strong) NSMutableArray *newData;
#end
Your can initialize the array in your viewDidLoad method:
- (void)viewDidLoad {
_newdata = [[NSMutableArray alloc] init];
}
In Your completion block, you would remove the initialization and add data to the array.
//NSMutableArray * newData = [NSMutableArray new] ; // REMOVE THE INTIALIZATION
for(int i = 0 ; i<dataCount && i<closeCount ; i++) {
NSMutableDictionary * temp = [NSMutableDictionary new] ;
NSString * dataString = [dataArr objectAtIndex:i];
NSString * closeString = [closeArr objectAtIndex:i];
[temp setObject:dataString forKey:#"smth"];
[temp setObject:closeString forKey:#"smth"];
[_newData addObject:temp];
}
Again, this is not the best approach but it is relatively simple. One problem with doing this, is that since you have a strong reference to the array, if you need to perform another URL call, and load new data into the array, you will need to empty it. Otherwise, the new data will be appended to the old one. You can do it by calling [_newData removeAllObjects]; before the URL session is called again.
EDIT 2: changed code based on the user comment:
- (void)loadChartData {
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
[request setURL:[NSURL URLWithString:#"url"]];
[request setHTTPMethod:#"POST"];
[request setValue:#"application/x-www-form-urlencoded" forHTTPHeaderField:#"Content-Type"];
NSString *postString = #"params";
NSString *postLength = [NSString stringWithFormat:#"%lu", ( unsigned long )[postString length]];
[request setValue:postLength forHTTPHeaderField:#"Content-Length" ];
[request setHTTPBody:[postString dataUsingEncoding:NSUTF8StringEncoding]];
NSURLSessionDataTask *task = [[self getURLSession] dataTaskWithRequest:request completionHandler:^( NSData *data, NSURLResponse *response, NSError *error ) {
dispatch_async( dispatch_get_main_queue(), ^{
NSDictionary *dicData = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:nil];
NSDictionary *values = [dicData valueForKeyPath:#"smth"];
NSArray * dataArr = [dicData objectForKey:#"smth"];
NSArray * closeArr = [values objectForKey:#"smth0"];
NSUInteger dataCount = [dataArr count] ;
NSUInteger closeCount = [closeArr count] ;
NSMutableArray * newData = [NSMutableArray new] ;
for(int i = 0 ; i<dataCount && i<closeCount ; i++) {
NSMutableDictionary * temp = [NSMutableDictionary new] ;
NSString * dataString = [dataArr objectAtIndex:i];
NSString * closeString = [closeArr objectAtIndex:i];
[temp setObject:dataString forKey:#"smth"];
[temp setObject:closeString forKey:#"smth"];
[newData addObject:temp];
}
NSLog(#"%#", newData);
_timeSeries = [NSMutableArray new];
NSString* filePath = [[NSBundle mainBundle] pathForResource:#"AppleStockPrices" ofType:#"json"];
NSData* json = [NSData dataWithContentsOfFile:filePath];
NSArray* data = [NSJSONSerialization JSONObjectWithData:json options:NSJSONReadingAllowFragments error:nil];
for (NSDictionary* jsonPoint in data) {
SChartDataPoint* datapoint = [self dataPointForDate:jsonPoint[#"smth"] andValue:jsonPoint[#"smth"]];
[_timeSeries addObject:datapoint];
}
});
}];
[task resume];
// Code here has a good chance of being executed before the completion block is complete
// _newdata = [[NSMutableArray alloc] init];
// NSLog(#"%#", _newdata);
}