How do I animate constraint changes?

I'm updating an old app with an AdBannerView and when there is no ad, it slides off screen. When there is an ad it slides on the screen. Basic stuff.

Old style, I set the frame in an animation block.New style, I have a IBOutlet to the auto-layout constraint which determines the Y position, in this case it's distance from the bottom of the superview, and modify the constant:

- (void)moveBannerOffScreen {    [UIView animateWithDuration:5 animations:^{        _addBannerDistanceFromBottomConstraint.constant = -32;    }];    bannerIsVisible = FALSE;}- (void)moveBannerOnScreen {    [UIView animateWithDuration:5 animations:^{        _addBannerDistanceFromBottomConstraint.constant = 0;    }];    bannerIsVisible = TRUE;}

And the banner moves, exactly as expected, but no animation.

UPDATE: I re-watched WWDC 12 talk Best Practices for Mastering Auto Layout which covers animation. It discusses how to update constraints using CoreAnimation:

I've tried with the following code, but get the exact same results:

- (void)moveBannerOffScreen {    _addBannerDistanceFromBottomConstraint.constant = -32;    [UIView animateWithDuration:2 animations:^{        [self.view setNeedsLayout];    }];    bannerIsVisible = FALSE;}- (void)moveBannerOnScreen {    _addBannerDistanceFromBottomConstraint.constant = 0;    [UIView animateWithDuration:2 animations:^{        [self.view setNeedsLayout];    }];    bannerIsVisible = TRUE;}

On a side note, I have checked numerous times and this is being executed on the main thread.