Open Radar

 
Area:
UIKit

Summary:
Calling animateAlongsideTransition within presentationTransitionWillBegin creates an animation that seems to be the reverse of what is expected. I used a UIVisualEffectView with a UIBlurEffect that I wanted to fade is as I present a view controller. However, the view comes in with full opacity and then fades out, and at the end of the presentation, appears on the screen abruptly again.

-(UIVisualEffectView *)blurEffectView{
    if(!_blurEffectView){
        UIVisualEffect * blurEffect     = [UIBlurEffect effectWithStyle:UIBlurEffectStyleDark];
        UIVisualEffectView * effectView = [[UIVisualEffectView alloc] initWithEffect:blurEffect];
        _blurEffectView                 = effectView;
    }
    return _blurEffectView;
}

-(void)presentationTransitionWillBegin{
    UIView * containerView        = self.containerView;
    UIVisualEffectView * blurView = self.blurEffectView;
    blurView.frame                = containerView.bounds;
    [containerView addSubview:blurView];
    blurView.alpha = 0;
    
    id<UIViewControllerTransitionCoordinator> transitionCoordinate = [[self presentingViewController] transitionCoordinator];
    [transitionCoordinate animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext>  _Nonnull context) {
        blurView.alpha = 1;
    } completion:nil];
}

-(void)presentationTransitionDidEnd:(BOOL)completed{
    if(!completed){
        [self.blurEffectView removeFromSuperview];
    }    
}

Steps to Reproduce:
1. Present a view controller (animated) and present another view that fades in alongside with transition

Expected Results:
The view transitions along side the view controller being presented

Actual Results:
The view comes on the screen as soon as animation begins, fades out, and then abruptly appears on the screen again

Version:
iOS 10.0

Notes:
Only appears to be on iOS 10 and iOS 10.1 beta

Configuration:
iPhone 5s, 6, 6Plus