Centering Modal View with Autolayout

Question

I'm presenting a UIViewController using presentViewController and a custom modalPresentationStyle, in an effort to implement a Facebook POP animated transition.

The modal view itself is completely dynamic, defined using Autolayout constraints in code. There is no xib/storyboard to back the modal.

I can't get the modal view to center on screen! Autolayout isn't sufficient, because there is no superview to add constraints on!

My presenting code looks like this (taken from a FB POP code sample):

- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext
{
    UIView *fromView = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey].view;
    fromView.tintAdjustmentMode = UIViewTintAdjustmentModeDimmed;
    fromView.userInteractionEnabled = NO;

    UIView *dimmingView = [[UIView alloc] initWithFrame:fromView.bounds];
    dimmingView.backgroundColor = [UIColor colorWithRed:(24/255.0) green:(42/255.0) blue:(15/255.0) alpha:1.0];
    dimmingView.layer.opacity = 0.0;

    UIView *toView = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey].view;
    toView.frame = CGRectMake(0,
                              0,
                              CGRectGetWidth(transitionContext.containerView.bounds) - 104.f,
                              CGRectGetHeight(transitionContext.containerView.bounds) - 320.f);
    toView.center = CGPointMake(transitionContext.containerView.center.x, -transitionContext.containerView.center.y);

    [transitionContext.containerView addSubview:dimmingView];
    [transitionContext.containerView addSubview:toView];

    POPSpringAnimation *positionAnimation = [POPSpringAnimation animationWithPropertyNamed:kPOPLayerPositionY];
    positionAnimation.toValue = @(transitionContext.containerView.center.y);
    positionAnimation.springBounciness = 10;
    [positionAnimation setCompletionBlock:^(POPAnimation *anim, BOOL finished) {
        [transitionContext completeTransition:YES];
    }];

    POPSpringAnimation *scaleAnimation = [POPSpringAnimation animationWithPropertyNamed:kPOPLayerScaleXY];
    scaleAnimation.springBounciness = 20;
    scaleAnimation.fromValue = [NSValue valueWithCGPoint:CGPointMake(1.2, 1.4)];

    POPBasicAnimation *opacityAnimation = [POPBasicAnimation animationWithPropertyNamed:kPOPLayerOpacity];
    opacityAnimation.toValue = @(0.2);

    [toView.layer pop_addAnimation:positionAnimation forKey:@"positionAnimation"];
    [toView.layer pop_addAnimation:scaleAnimation forKey:@"scaleAnimation"];
    [dimmingView.layer pop_addAnimation:opacityAnimation forKey:@"opacityAnimation"];
}

This works beautifully, but I need the actual view size to be dynamic (sometimes the modal will have four lines of text and two buttons, etc). To accomplish this, I need to set translatesAutoresizingMaskIntoConstraints=NO in the VC subclass. This obviously negates the frame centering I'm doing in the presentation animator.

The end result is a modal that's stuck to the left edge of the screen; curiously, it centers itself vertically, but not horizontally. Visually, it looks something like this (pardon the black squares, I had to do it for legal purposes):

The obvious solution would be to add a view constraint that centers the view. No problem, right?

But where do I add it? view.superview is nil; there is no superview. I tried creating a custom 'superview' property and setting it, but autolayout doesn't know how to handle a view that's outside of its view hierarchy (the presenting vc). This is what my view hierarchy looks like, annotated:

You're apparently not supposed to access the UITransitionView directly. Constraints on the UIWindow have no effect.

Does anyone have any advice? How do you guys handle this sort of thing?

Answer 1

You can programmatically add a centering constraint from containerView to toView in your animateTranisition method:

(in Swift, but you should be able to get the idea...)

containerView.addSubview(toView)

let centerXLayoutConstraint = NSLayoutConstraint(item: toView, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal, toItem: containerView, attribute: NSLayoutAttribute.CenterX, multiplier: 1, constant: 0)
let centerYLayoutConstraint = NSLayoutConstraint(item: toView, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, toItem: containerView, attribute: NSLayoutAttribute.CenterY, multiplier: 1, constant: 0)

containerView.addConstraint(centerXLayoutConstraint)
containerView.addConstraint(centerYLayoutConstraint)

When I tried this, I also added width and height constraints to toView to size it relative to containerView. It worked -- no problem.

I think it should work with a self-sizing toView as well. You might have to override intrinsicSize in your toView class and/or play around with forcing it to update its constraints.

Answer 2

Could you try this?

Declare your modal window view as subclass of UIView, and implement didMoveToSuperView.

- (void)didMoveToSuperview {
    UIView *superView = self.superview;

    if(superView == nil) {
        return;
    }

    [superView addConstraint:[NSLayoutConstraint constraintWithItem:self
                                                          attribute:NSLayoutAttributeCenterX
                                                          relatedBy:NSLayoutRelationEqual
                                                             toItem:superView
                                                          attribute:NSLayoutAttributeCenterX
                                                         multiplier:1.0
                                                           constant:0]];
    [superView addConstraint:[NSLayoutConstraint constraintWithItem:self
                                                          attribute:NSLayoutAttributeCenterY
                                                          relatedBy:NSLayoutRelationEqual
                                                             toItem:superView
                                                          attribute:NSLayoutAttributeCenterY
                                                         multiplier:1.0
                                                           constant:0]];

    // [superView layoutIfNeeded]; // If this does not work, try uncomment this.
}

This should automatically do centering itself when added to any superview.

Needles to say, you also need translatesAutoresizingMaskIntoConstraints = NO and any width/height constraints.

I haven't tested with UITransitionView, though. Maybe this conflict with your positionAnimation.

EDIT: 2014/09/22

Instead of using Autolayout constraints to modal view itself, I think you should use systemLayoutSizeFittingSize: method without translatesAutoresizingMaskIntoConstraints = NO.

- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext
{
    // ...snip

    UIView *toView = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey].view;

    toView.translatesAutoresizingMaskIntoConstraints = YES; // To clarify. You don't need this line because this is the default.

    CGSize sysSize = [toView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
    toView.bounds = CGRectMake(0, 0, sysSize.width, sysSize.height);
    toView.center = CGPointMake(transitionContext.containerView.center.x, -transitionContext.containerView.center.y);
    toView.autoresizingMask = UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleBottomMargin | UIViewAutoresizingFlexibleRightMargin;

    // ...snip

}

and If you want resize modal view after display (as a side effect of modifying it's content), do like following code in your modal views UIViewController subclass:

- (void)yourAppMethod {
    NSString *message = @""; // <- as u like
    UILabel *label = self.messageLabel;
    label.text = message;
    [self resizeViewIfNeeded]; // <- this will resize self.view
}

- (void)resizeViewIfNeeded {
    CGSize sysSize = [self.view systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
    if(!CGSizeEqualToSize(sysSize, self.view.bounds.size)) {
        self.view.bounds = CGRectMake(0, 0, sysSize.width, sysSize.height);
    }
}