iOS 6 Programming Cookbook

6.17. Invoking Background Methods

Problem

You want to know an easy way to create threads without having to deal with threads directly.

Solution

Use the performSelectorInBackground:withObject: instance method of NSObject:

- (BOOL)            application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{
  
  [self performSelectorInBackground:@selector(firstCounter)
                         withObject:nil];
  
  [self performSelectorInBackground:@selector(secondCounter)
                         withObject:nil];
  
  [self performSelectorInBackground:@selector(thirdCounter)
                         withObject:nil];
  
  self.window = [[UIWindow alloc] initWithFrame:
                 [[UIScreen mainScreen] bounds]];
  self.window.backgroundColor = [UIColor whiteColor];
  [self.window makeKeyAndVisible];
  return YES;
}

The counter methods are implemented in this way:

- (void) firstCounter{
  
  @autoreleasepool {
    NSUInteger counter = 0;
    for (counter = 0;
         counter < 1000;
         counter++){
      NSLog(@"First Counter = %lu", (unsigned long)counter);
    }
  }
  
}

- (void) secondCounter{
  
  @autoreleasepool {
    NSUInteger counter = 0;
    for (counter = 0;
         counter < 1000;
         counter++){
      NSLog(@"Second Counter = %lu", (unsigned long)counter);
    }
  }
  
}

- (void) thirdCounter{
  
  @autoreleasepool {
    NSUInteger counter = 0;
    for (counter = 0;
         counter < 1000;
         counter++){
      NSLog(@"Third Counter = %lu", (unsigned long)counter);
    }
  }
  
}

Discussion

The performSelectorInBackground:withObject: method creates a new thread in the background for us. This is equivalent to the creating a new thread for the selectors. The most important thing we ...