Updating UILabel in the middle of a for() loop

Question

I have a method with a for() loop. In that loop, mylabel.text is updated many times. However, the actual label does not update on the screen until the method is done, updati

Answer 1

From my earlier comment:

Note that this (runMode:beforeDate:) can have all kinds of bizarre side-effects. When you call runMode:beforeDate:, all kinds of things could happen in the middle of your loop. Timers could fire; WebKit can do all kinds of madness; delayed selectors can fire. This is a very dangerous trick. Sometimes useful, occasionally necessary (especially on Mac), but not a general-purpose tool.

The better solution is to schedule your updates on the main dispatch queue:

  for (NSInteger i = 0; i < 10; i++) {
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, i * NSEC_PER_SEC), 
                   dispatch_get_main_queue(), ^{
      [self.label setText:[NSString stringWithFormat:@"%d", i]];
    });
  }

This schedules 10 updates 1 second apart. It can be adapted to all kinds of other requirements without creating a blocking method on the main run loop.

Answer 2

You can make the UI update by telling the run loop to run like this:

for (NSInteger i = 0; i < 10; i++) {
    [label setText:...];
    [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantPast]];
}

Answer 3

try

[yourLabel setNeedsDisplay];