NSTask Blocking My UI when NSTask encounter large data - multithreading

Question

I want to update my nstextview with the data generated during nstask execution(Ipa Generation). But when i run my code to execute nstask, in the middle my nstask blocks my ui but the task continues to execute. At last when nstask terminates my ui starts working properly.
This is my code where i am running my nstask:
dispatch_queue_t taskQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
NSTask *task=[[NSTask alloc]init];
dispatch_async(taskQueue, ^{
#try {
[task setArguments:arguments];
[task setLaunchPath: launchPath];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(taskCompletion:) name: NSTaskDidTerminateNotification object:task];
// Output Handling
NSPipe *outputPipe = [[NSPipe alloc] init];
outputFileHandle = [[NSFileHandle alloc]init];
[task setStandardOutput:outputPipe];
outputFileHandle=[outputPipe fileHandleForReading];
[outputFileHandle waitForDataInBackgroundAndNotify];
[[NSNotificationCenter defaultCenter] addObserverForName:NSFileHandleDataAvailableNotification object:outputFileHandle queue:nil usingBlock:^(NSNotification *notification){
NSData *output = [outputFileHandle availableData];
NSString *outStr = [[NSString alloc] initWithData:output encoding:NSUTF8StringEncoding];
dispatch_sync(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(taskCompletion:) name: NSTaskDidTerminateNotification object:task];
NSLog(#"%#",outStr);
NSString *textViewData =[self.outputView string];
self.outputView.string = [textViewData stringByAppendingString:[NSString stringWithFormat:#"\n%#", outStr]];
// Scroll to end of outputText field
NSRange range;
range = NSMakeRange([self.outputView.string length], 0);
[self.outputView scrollRangeToVisible:range];
});
[outputFileHandle waitForDataInBackgroundAndNotify];
}];
[task launch];
[task waitUntilExit];
}
#catch (NSException *exception) {
NSLog(#"Problem Running Task: %#", [exception description]);
}
#finally {
NSLog(#"i m in finally xbuild");
}
});
I really stuck in that .Your suggestions will be helpful for me.