Background thread confusion

Question

I'm trying to improve the responsiveness of my app but am a complete newbie to threading and have become confused.

On launch an alert is shown:

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
    if (buttonIndex == 0)
    {
        NSString *user = [[alertView textFieldAtIndex:0] text];
        NSString *pass = [[alertView textFieldAtIndex:1] text];
        [self loginToServerWithUsername:user andPassword:pass];
    }
}

within the loginToServerWithUsername: method, the app calls a method:

[self checkFiles:sessionID];

This can take several seconds so I tried to execute it in the background:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                        [self checkFiles:sessionID];
                    });

checkFiles method:

fileList = [[NSMutableString alloc] init];

NSArray *directoryPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [directoryPaths objectAtIndex:0];
NSString *downloadsFolderString = [documentsDirectory stringByAppendingPathComponent:DOWNLOADS_FOLDER];
NSError *error = nil;
NSString* file;
NSDirectoryEnumerator* enumerator = [[NSFileManager defaultManager] enumeratorAtPath:downloadsFolderString];
while (file = [enumerator nextObject])
{
    BOOL isDirectory = NO;        
    [[NSFileManager defaultManager] fileExistsAtPath: [NSString stringWithFormat:@"%@/%@",downloadsFolderString,file]
                                         isDirectory: &isDirectory];

            if (!isDirectory)
            {
                [fileList appendString:[NSString stringWithFormat:@"%@|", file]];
            }
}
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"dd/MM/yyyy HH:mm"];
NSString *timeOpened = [formatter stringFromDate:[NSDate date]];

NSString *post = [NSString stringWithFormat:@"sessionID=%@&fileList=%@&dateTime=%@&userID=%@", sessionID, fileList, timeOpened, userID];
NSData *postData = [post dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES];
NSString *postLength = [NSString stringWithFormat:@"%d", [postData length]];
NSString *comparisonURLString = SERVER_COMPARE_URL_STRING;
NSURL *comparisonURL = [NSURL URLWithString:comparisonURLString];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:comparisonURL];
[request setHTTPMethod:@"POST"];
[request addValue:postLength forHTTPHeaderField:@"Content-Length"];
[request setHTTPBody:postData];
NSHTTPURLResponse *urlResponse = nil;
error = nil;
NSData *responseData = [NSURLConnection sendSynchronousRequest:request returningResponse:&urlResponse error:&error];

if (responseData)
{
    NSString *requiredFilesList = [[NSString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];

    NSArray *lines = [requiredFilesList componentsSeparatedByString: @"\n"];        
    if (lines.count > 2)
    {
        dispatch_async(dispatch_get_main_queue(), ^(void){

            NSRange theRange;
            theRange.location = 2;
            theRange.length = [lines count] -3;
            numberOfFilesToBeDownloaded = theRange.length;

            if (numberOfFilesToBeDownloaded <= 0)
            {
                _jobStatusLabel.text = @"Documents up to date"; // is this the issue?
            }            
            if (numberOfFilesToBeDownloaded > 0)
            {
                NSArray *subArray = [lines subarrayWithRange:theRange];
                [self animateProgressBar];
                if (![eraseDevice isEqualToString:@"true"])
                {
                [self getFiles:subArray];
                }
            }
        });
    }
}
else
{
    NSLog(@"no response data from check files");
}

However, the alertView is not dismissed until the checkFiles method is complete. Can anyone tell me how to have the alert dismiss whilst checkFiles runs in the background?

Answer 1

The UI operations should be done in the main thread.

For instance :

_jobStatusLabel.text = @"Documents up to date"; // is this the issue?

should be

dispatch_async(dispatch_get_main_queue(), ^(void){
    _jobStatusLabel.text = @"Documents up to date"; // is this the issue?
});

change all such possible cases.