Using nsurlsessiontask causing data in table view to be showed incorrectly and change will scrolling

Question

I am using the google places api to get details about stores near the user. The response I get from the call, I display in a table view. I make two api calls. One call to find nearby stores, and another call to get information about each returned store such as url, phone number, picture etc. So I first made both these calls on the main thread and it slowed down my UI. The table view lagged a lot while scrolling (but all the cells had correct data and when scrolling up or down the data did not change). Then I decided to use nsurlsessiontask to perform both the api calls so it would not slow down the UI. Here is code:

First api call:

s1 = @"Http request";
NSURL *jsonURL = [NSURL URLWithString:[self urlEncodeValue:s1]];
NSString *jsonDataString  = [[NSString alloc]initWithContentsOfURL:jsonURL];

NSURLSessionTask *task = [[NSURLSession sharedSession] dataTaskWithURL:jsonURL completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
    if (data == nil || error != nil) {
        NSLog(@"error: %@", error);
        return;
    }

    NSError *parseError;
    self.jsonDict = [NSJSONSerialization JSONObjectWithData:data options:0 error:&parseError];
    if (self.jsonDict) 
   {
      //perform taks
   }

So basically I created and initialized the jsonDict which the the dictionary that holds the data. Now in my class that shows the tableview, I take the data from jsonDict and display it in the cell. For example:

cell.addresslabel = [[[appDelegate.jsonDict objectForKey@"results"] objectAtIndex:indexPath.row] objectForKey:@"address"];

In order to get the phone number and website of the store I need to make another http request. So I make a a method in the same class that displays the tableView. Here is the code:

s2 = @"Second http request"
NSURL *jsonURL = [NSURL URLWithString:s2];

NSURLSessionTask *task = [[NSURLSession sharedSession] dataTaskWithURL:jsonURL completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
    if (data == nil || error != nil) {
        NSLog(@"error: %@", error);
        return;
    }

    NSError *parseError;
    phoneDic = [NSJSONSerialization JSONObjectWithData:data options:0 error:&parseError];
    if (phoneDic) {
       //perform tasks
    } else {
        NSLog(@"parseError: %@", parseError);
        NSLog(@"responseString: %@", [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding]);
    }
}];
[task resume];

So when I do this the table view does not "lag" however the data shown is incorrect(cells show data of other cells) and when I scroll up or down the data changes? So when I perform task on main thread there is lag but correct data is displayed, when I use nsurlsessiontask there is no lag but the data shown is incorrect. I am doing anything wrong with the nsurlsessiontask?

Answer 1

The reason for your problem is that once the NSURLSessionTask has completed, the cell might be reused at a totally different position. Once you dequeue a cell in cellForRowAtIndexPath, you could stop a previously running task and restart a new one. Ideally the data requesting task is done separately from the presentation in the UITableView, including caching of received data.