Block variable gets null outside of block - xcode - xcode

Question

Im trying to carry over a block variable and put it in my convenience initializer but the variable returns null, can someone show me the right way of doing it, Thanks, also please check if I have done my NSComparison Result correctly, here's the code,
- (void) retrieveData
{
NSURL *url = [NSURL URLWithString:jsonFile];
NSData *data = [NSData dataWithContentsOfURL:url];
_jsonArray = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:nil];
_salesArray = [[NSMutableArray alloc]init];
for (int i = 0; i < _jsonArray.count; i++) {
NSString *sID = [[_jsonArray objectAtIndex:i] objectForKey:#"id"];
NSString *sName = [[_jsonArray objectAtIndex:i] objectForKey:#"name"];
NSString *sAddress = [[_jsonArray objectAtIndex:i] objectForKey:#"address"];
NSString *sPostcode = [[_jsonArray objectAtIndex:i] objectForKey:#"postcode"];
__block NSString *distance;
CLGeocoder *geocoder = [[CLGeocoder alloc]init];
[geocoder geocodeAddressString:sPostcode completionHandler:^(NSArray *placemarks, NSError *error) {
if (error == nil && placemarks.count > 0) {
CLPlacemark *placemark = [placemarks objectAtIndex:0];
CLLocation *location = placemark.location;
CLLocation *myLocation = self.manager.location;
CLLocationDistance miles = [location distanceFromLocation:myLocation];
//this is the variable i want in my convenience init.
distance = [NSString stringWithFormat:#"%.1f m", (miles/1609.344)];
}
}];
[_salesArray addObject:[[sales alloc] initWithSales:sID andName:sName andAddress:sAddress andPostcode:distance]];
}
[_salesArray sortUsingComparator:
^NSComparisonResult(id obj1, id obj2){
sales *p1 = (sales *)obj1;
sales *p2 = (sales *)obj2;
if (p1.postcode > p2.postcode) {
return (NSComparisonResult)NSOrderedDescending;
}
if (p1.postcode < p2.postcode) {
return (NSComparisonResult)NSOrderedAscending;
}
return (NSComparisonResult)NSOrderedSame;
}
];
[self.tableView reloadData];
}
Thanks in advance to anyone that can help, this happens to be the last part of my app until completion :)

Answer 1

-[CLGeocoder geocodeAddressString:completionHandler:] runs the completion handler asynchronously and it won't run the block before the rest of that function finishes. distance doesn't "get null"; rather, it hasn't been set yet.

Answer 2

your block is run AFTER the function completes ... doesn't matter if the 'line of code' is after or before the block.
the solution is to have to block call a 'continuation' function:
e.g. instead of - (void) retrieveData
have - (void) beginRetrieveData, which calls the block and then have a - (void) finishRetrieveData, which is called by the block and doesn't run before the block ;)
here is how your code could look:
- (void) beginRetrieveData
{
NSURL *url = [NSURL URLWithString:jsonFile];
NSData *data = [NSData dataWithContentsOfURL:url];
_jsonArray = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:nil];
_salesArray = [[NSMutableArray alloc]init];
for (int i = 0; i < _jsonArray.count; i++) {
NSString *sID = [[_jsonArray objectAtIndex:i] objectForKey:#"id"];
NSString *sName = [[_jsonArray objectAtIndex:i] objectForKey:#"name"];
NSString *sAddress = [[_jsonArray objectAtIndex:i] objectForKey:#"address"];
NSString *sPostcode = [[_jsonArray objectAtIndex:i] objectForKey:#"postcode"];
CLGeocoder *geocoder = [[CLGeocoder alloc]init];
[geocoder geocodeAddressString:sPostcode completionHandler:^(NSArray *placemarks, NSError *error) {
if (error == nil && placemarks.count > 0) {
CLPlacemark *placemark = [placemarks objectAtIndex:0];
CLLocation *location = placemark.location;
CLLocation *myLocation = self.manager.location;
CLLocationDistance miles = [location distanceFromLocation:myLocation];
//this is the variable i want in my convenience init.
NSString *distance = [NSString stringWithFormat:#"%.1f m", (miles/1609.344)];
[self finishRetrieveData:distance]; //continue
}
}];
}
}
- (void) finishRetrieveData:(NSString*)distance
{
[_salesArray addObject:[[sales alloc] initWithSales:sID andName:sName andAddress:sAddress andPostcode:distance]];
}