Use GCD dispatch_once function to execute some code only once per instance.

gistfile1.m

/*
 * This may be obvious to some, but the most common use of dispatch_once is for the initialisation of
 * a class singleton, so some people may not know how to do it per instance.
 *
 */

@implementation MyViewController {
  dispatch_once_t _oncePerInstance;
}

- (void)viewDidLayoutSubviews
{
  [super viewDidLayoutSubview];
  
  dispatch_once(&_oncePerInstance, ^{
    // do something after laying out subviews for the first time.
  });
}

@end