NSMutable Array Property becomes null after function is exited - ios

Question

I am trying take the values of an array of CLLocation's made from a JSON request and assign them to a property that is a NSMutable array.
Here the relevant controller code:
- (void)viewDidLoad
{
[super viewDidLoad];
//using a background thread to receive the json call so that the UI doesn't stall
dispatch_async(directionQueue, ^{
NSData* data = [NSData dataWithContentsOfURL:openMapURL];
[self performSelectorOnMainThread:#selector(fetchedData:)
withObject:data waitUntilDone:YES];
});
NSLog(#"%#", self.coordinates);
}
- (void)fetchedData:(NSData *)responseData
{
//parse out the json data
NSError* error;
NSDictionary* json = [NSJSONSerialization
JSONObjectWithData:responseData //turn the data from the json request into a gigantic dictionary
options:0
error:&error];
NSDictionary* route = [json objectForKey:#"route"]; //created a dictionary out of the contents of route
NSDictionary* shape = [route objectForKey:#"shape"]; //create a sub-dictionary with the contents of shape
NSArray* shapePoints = [shape objectForKey:#"shapePoints"]; //turn shapePoints object into an NSArray
//Loop to turn the array of coordinate strings into CLLocation array
NSMutableArray* locations = [[NSMutableArray alloc] init];
NSUInteger i;
for (i = 0; i < ([shapePoints count])/2 ; i = i+2)
{
[locations addObject: [[CLLocation alloc]
initWithLatitude:[[shapePoints objectAtIndex:i] floatValue]
longitude:[[shapePoints objectAtIndex:i+1] floatValue]
]];
}
//When I NSLog within the function, the array has the correct data. But in viewDidLoad
//the array is null
[self.coordinates initWithArray:locations copyItems:YES];
}
How come this array become's null in viewDidLoad?

Answer 1

You are using GCD to make asynchronous request to fetch the data from viewDidLoad method. As it is asynchronous, it will not block the viewDidLoad method. The array gets populated once the async request downloads and gets parsed. That is the reason your array is nil in viewDidLoad.
If your UI looks blank until the data downloads and populates the array, you can choose to show an activity indicator. This will give the app user an idea that some activity is going on.
Hope that helps!

Answer 2

Probably what you need to do is wait for the process to get completed and the block literally doesn't know what your doing inside it or i would say the inside function is not been captured by the block.so use the array(call the function which is using your array) only after block gets completed.
- (void)fetchedData:(NSData *)responseData
{
//parse out the json data
NSError* error;
NSDictionary* json = [NSJSONSerialization
JSONObjectWithData:responseData //turn the data from the json request into a gigantic dictionary
options:0
error:&error];
NSDictionary* route = [json objectForKey:#"route"]; //created a dictionary out of the contents of route
NSDictionary* shape = [route objectForKey:#"shape"]; //create a sub-dictionary with the contents of shape
NSArray* shapePoints = [shape objectForKey:#"shapePoints"]; //turn shapePoints object into an NSArray
//Loop to turn the array of coordinate strings into CLLocation array
NSMutableArray* locations = [[NSMutableArray alloc] init];
NSUInteger i;
for (i = 0; i < ([shapePoints count])/2 ; i = i+2)
{
[locations addObject: [[CLLocation alloc]
initWithLatitude:[[shapePoints objectAtIndex:i] floatValue]
longitude:[[shapePoints objectAtIndex:i+1] floatValue]
]];
}
//When I NSLog within the function, the array has the correct data. But in viewDidLoad
//the array is null
[self.coordinates initWithArray:locations copyItems:YES];
//Here you call the methods which is accessing your self.coordinates array.
NSLog(#"%#", self.coordinates);
}