Adding information returned by threads to an array

Question

I am relatively new to Objective C and I am trying to read information from a URL then add its info to my Name object then add the name objects to an array. I am able to read the info from the URLs fine and furthermore I am successful in adding their info to an array however I cannot seem to successfully add the names to my nameArray. I declare the array at the top and initialize it in the viewDidLoad function then furthermore the names are meant to be added to the array when I call the methods readURLOne, readURLTwo and readURLThree. However, when I check the array, after calling these methods in viewDidLoad, it is empty. Why might this be?

@interface ViewController () {
    Name *n1;
    Name *n2;
    Name *n3;
    NSMutableArray *nameArray; //this is the array I am having trouble with
}
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    nameArray = [[NSMutableArray alloc] init];
    NSOperationQueue *queue = [NSOperationQueue new];
    NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(readURLOne)object:nil];
    [queue addOperation:operation];

    operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(readURLTwo)object:nil];
    [queue addOperation:operation];

    operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(readURLThree) object:nil];
    [queue addOperation:operation];
    NSLog(@"NAME ARRAY: %lu", (unsigned long)[nameArray count]); //This is where I am trying to check if they are added to the array, the are not
} 

-(void) readURLOne {
    NSURL *dataURL = [NSURL URLWithString:@"http://example.org/data/1.txt"];
    NSArray *tmp = [NSArray arrayWithContentsOfURL:dataURL];
    for (NSString *str in tmp){ //add contents of URL successfully to tmp
        NSLog(@"%@", str);
    }
    n1 = [[Name alloc] init];
    [n1 setFirstName:tmp[0]];
    [n1 setLastName:tmp[1]];
    [n1 setNumber:tmp[2]];
    [nameArray addObject:n1];
 }

-(void) readURLTwo {
    NSURL *dataURL = [NSURL URLWithString:@"http://example.org/data/2.txt"];
    NSArray *tmp = [NSArray arrayWithContentsOfURL:dataURL];
    for (NSString *str in tmp){
        NSLog(@"%@", str);
    }
    n2 = [[Name alloc] init];
    [n2 setFirstName:tmp[0]];
    [n2 setLastName:tmp[1]];
    [n2 setNumber:tmp[2]];
    [nameArray addObject:n2];
}

-(void) readURLThree {
    NSURL *dataURL = [NSURL URLWithString:@"http://example.org/data/3.txt"];
    NSArray *tmp = [NSArray arrayWithContentsOfURL:dataURL];
    for (NSString *str in tmp){
        NSLog(@"%@", str);
    }
    n3 = [[Name alloc] init];
    [n3 setFirstName:tmp[0]];
    [n3 setLastName:tmp[1]];
    [n3 setNumber:tmp[2]];
    [nameArray addObject:n3];
}

-(NSInteger) numberOfSectionsInTableView:(UITableView *)tableView {
    return 1;
}

-(NSInteger) tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    return nameArray.count;
}

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    static NSString *cellIdentifier = @"Cell";
    CustomCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
    if (!cell) {
        cell = [[CustomCell alloc]initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];
    }

    cell.firstName.text = [[nameArray objectAtIndex:indexPath.row] getFirstName];
    cell.lastName.text = [[nameArray objectAtIndex:indexPath.row] getLastName];
    return cell;
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

Answer 1

I declare the array at the top and initialize it in the viewDidLoad function then furthermore the names are meant to be added to the array when I call the methods readURLOne, readURLTwo and readURLThree.

As I can see you are NOT calling readURLOne, readURLTwo and readURLThree - you are adding them to NSOperationQueue. In such a case your methods are called asynchronously and nameArray is probably updated after you check it at the end of viewDidLoad.

Change your method to:

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    nameArray = [[NSMutableArray alloc] init];
    NSOperationQueue *queue = [NSOperationQueue new];
    queue.maxConcurrentOperationCount = 1; // It is needed to avoid synchronisation problems with nameArray
    NSInvocationOperation *operation1 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(readURLOne)object:nil];
    [queue addOperation:operation1];

    NSInvocationOperation *operation2 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(readURLTwo)object:nil];
    [queue addOperation:operation2];

    NSInvocationOperation *operation3 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(readURLThree) object:nil];
    [queue addOperation:operation3];


    NSOperation *finalOperation = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"NAME ARRAY: %lu", (unsigned long)[nameArray count]);
    }];
    [finalOperation addDependency:operation1];
    [finalOperation addDependency:operation2];
    [finalOperation addDependency:operation3];
    [queue addOperation:finalOperation];
} 

Answer 2

NSOperationQueue s operate asynchronously, so there's no guarantee that they will be complete by the time you're accessing nameArray .

You could add this line before your NSLog :

[queue waitUntilAllOperationsAreFinished];

But really, I think you should probably just call the functions directly without using an NSOperationQueue .

---

On further inspection of the methods you're calling, it looks like you're performing network operations. This should definitely be done on a different thread like you've already done, so you really just need to add a callback for the completion. You could do this pretty simply by adding a finish operation:

  NSInvocationOperation *completionOperation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(urlDataRead) object:nil];

  // ...
  [completionOperation addDependency:operation];
  // ...
  [completionOperation addDependency:operation];
  // ...
  [completionOperation addDependency:operation];

  [queue addOperation:completionOperation];
}

- (void)urlDataRead {
  NSLog(@"NAME ARRAY: %lu", (unsigned long)[nameArray count]); //This is where I am trying to check if they are added to the array, the are not
}