iOS - Issue When Consuming API - dispatch_async - ios

Question

I was trying to get a basic prototype of an API working. I have the web service setup and now I'm trying to have an iOS app request the data and display it in a table. It's pretty basic, however, random parts of the data are displaying as null in the table and I haven't been able to find a common thread.
But basically, I make a call to this API, I parse the data, i use a StopCollection class to createItem (of type StopItem) and add it to an NSMutableArray in StopCollection. And then in the MainTableViewController, I use that array as a data source.
The data comes in correctly from the API. I'm able to add it the NSMutableArray just fine. However, it seems that in the MainTableViewController, once I get to the following line, I no longer have access to the same data in the array and bunch of the values are null:
dispatch_async(dispatch_get_main_queue(), ^{
[self.tableView reloadData];
});
Here's what I'm working with:
MainTableViewController.m
#import "MainTableViewController.h"
#import "StopCollection.h"
#import "StopItem.h"
#interface MainTableViewController ()
#property (nonatomic, strong) NSURLSession *session;
#end
#implementation MainTableViewController
- (instancetype)init
{
self = [super initWithStyle:UITableViewStylePlain];
if (self) {
NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
_session = [NSURLSession sessionWithConfiguration:config
delegate:nil
delegateQueue:nil];
[self fetchFeed];
}
return self;
}
- (void)fetchFeed
{
NSString *requestString = #"http://nexttrain.dev/station?include=stopTime";
NSURL *url = [NSURL URLWithString:requestString];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSURLSessionDataTask *dataTask =
[self.session dataTaskWithRequest:request
completionHandler:
^(NSData *data, NSURLResponse *response, NSError *error) {
NSDictionary *jsonObject = [NSJSONSerialization JSONObjectWithData:data
options:0
error:nil];
for(NSDictionary *stop in jsonObject[#"data"])
{
for (NSDictionary *stopTime in stop[#"stopTime"][#"data"]) {
[[StopCollection sharedStore] createItem:stop[#"station"]
withRoute:stopTime[#"route"]
withHeadsign:stopTime[#"trip"]
withArrivalTime:stopTime[#"arrival_time"]];
NSLog(#"%#", stop[#"station"]);
}
}
dispatch_async(dispatch_get_main_queue(), ^{
[self.tableView reloadData];
});
}];
[dataTask resume];
}
- (void)viewDidLoad
{
[super viewDidLoad];
[self.tableView registerClass:[UITableViewCell class]
forCellReuseIdentifier:#"UITableViewCell"];
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
}
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
return 1;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
return [[[StopCollection sharedStore] stops] count];
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:#"UITableViewCell"
forIndexPath:indexPath];
NSArray *allStops = [[StopCollection sharedStore] stops];
StopItem *stop = allStops[indexPath.row];
NSString *formattedText = [NSString stringWithFormat:#"%# - %# - %#", stop.station, stop.route, stop.arrivalTime];
cell.textLabel.text = formattedText;
cell.textLabel.font = [UIFont systemFontOfSize:12];
return cell;
}
StopCollection.m
#import "StopCollection.h"
#import "StopItem.h"
#interface StopCollection()
#property (nonatomic) NSMutableArray *privateStops;
#end
#implementation StopCollection
// Override Accessor for Stops, and return from privateStops.
- (NSArray *)stops
{
return [self.privateStops copy];
}
// Singleton Access to Object.
+ (instancetype)sharedStore
{
// Static will be Strong and will remain.
static StopCollection *sharedStore;
// Create initial instance.
if (!sharedStore) {
sharedStore = [[self alloc] initPrivate];
}
// Return instance if exists.
return sharedStore;
}
// Make defualt init inaccessible.
- (instancetype)init
{
// Throw Exception if accesssed.
[NSException raise:#"Singleton"
format:#"Use + (instancetype)sharedStore"];
return nil;
}
// Private initializer will call Super init.
- (instancetype)initPrivate
{
self = [super init];
if (self) {
_privateStops = [[NSMutableArray alloc] init];
}
return self;
}
- (void)createItem:(NSString *)station withRoute:(NSString *)route withHeadsign:(NSString *)headsign withArrivalTime:(NSString *)arrivalTime
{
StopItem *stop = [[StopItem alloc] initWithStation:station
lattitude:22
longitude:56
route:route
tripHeadsign:headsign
arrivalTime];
[self.privateStops addObject:stop];
}
#end
StopCollection.h
#import <Foundation/Foundation.h>
#interface StopCollection : NSObject
+ (instancetype)sharedStore;
#property(nonatomic, readonly, copy) NSArray *stops;
- (void)createItem:(NSString *)station
withRoute:(NSString *)route
withHeadsign:(NSString *)headsign
withArrivalTime:(NSString *)arrivalTime;
#end
StopItem.m
#import "StopItem.h"
#implementation StopItem
- (instancetype)initWithStation:(NSString *)station lattitude:(NSInteger)lattitude longitude:(NSInteger)longitude route:(NSString *)route tripHeadsign:(NSString *)headsign arrivalTime:(NSString *)arrivalTime
{
self = [super init];
if (self) {
_station = station;
_lattitude = lattitude;
_longitude = longitude;
_route = route;
_tripHeadsign = headsign;
_arrivalTime = arrivalTime;
}
return self;
}
- (instancetype)init {
return [self initWithStation:#"Hey" lattitude:0 longitude:0 route:#"" tripHeadsign:#"" arrivalTime:[[NSString alloc] init]];
}
#end
StopItem.h
#interface StopItem : NSObject
#property (nonatomic, weak) NSString *station;
#property (nonatomic) NSInteger lattitude;
#property (nonatomic) NSInteger longitude;
#property (nonatomic, weak) NSString *route;
#property (nonatomic, weak) NSString *tripHeadsign;
#property (nonatomic, weak) NSString *arrivalTime;
- (instancetype)initWithStation:(NSString *)station
lattitude:(NSInteger)lattitude
longitude:(NSInteger)longitude
route:(NSString *)route
tripHeadsign:(NSString *)headsign
arrivalTime:(NSString *)arrivalTime;
#end
And here's the JSON data set I'm working with:
{
"data":[
{
"station":"2 Av",
"lattitude":"40.723402",
"longitude":"-73.989938",
"stopTime":{
"data":[
{
"arrival_time":"02:40:30",
"trip":"JAMAICA - 179 ST",
"route":"some longer word with a -"
}
]
}
},
{
"station":"2 Av",
"lattitude":"40.723402",
"longitude":"-73.989938",
"stopTime":{
"data":[
{
"arrival_time":"00:54:00",
"trip":"CONEY ISLAND - STILLWELL AV",
"route":"some longer word with a -"
}
]
}
},
{
"station":"Delancey St",
"lattitude":"40.718611",
"longitude":"-73.988114",
"stopTime":{
"data":[
{
"arrival_time":"02:39:00",
"trip":"JAMAICA - 179 ST",
"route":"some longer word with a -"
}
]
}
},
{
"station":"Delancey St",
"lattitude":"40.718611",
"longitude":"-73.988114",
"stopTime":{
"data":[
{
"arrival_time":"00:55:30",
"trip":"CONEY ISLAND - STILLWELL AV",
"route":"some longer word with a -"
}
]
}
},
{
"station":"Essex St",
"lattitude":"40.718315",
"longitude":"-73.987437",
"stopTime":{
"data":[
{
"arrival_time":"01:23:30",
"trip":"JAMAICA CENTER - PARSONS/ARCHER",
"route":"some longer word with a -"
}
]
}
},
{
"station":"Essex St",
"lattitude":"40.718315",
"longitude":"-73.987437",
"stopTime":{
"data":[
{
"arrival_time":"00:52:30",
"trip":"BROAD ST",
"route":"some longer word with a -"
}
]
}
},
{
"station":"Bowery",
"lattitude":"40.72028",
"longitude":"-73.993915",
"stopTime":{
"data":[
{
"arrival_time":"01:22:00",
"trip":"JAMAICA CENTER - PARSONS/ARCHER",
"route":"some longer word with a -"
}
]
}
},
{
"station":"Bowery",
"lattitude":"40.72028",
"longitude":"-73.993915",
"stopTime":{
"data":[
{
"arrival_time":"00:54:00",
"trip":"BROAD ST",
"route":"some longer word with a -"
}
]
}
}
]
}

Answer 1

That's not related to dispatch. It's because you have weak properties, which get nilled whenever possible, i.e. when there is no reference holding them. Change weak to strong or copy in your StopItem model, and everything will work.