FlexibleContainerView.m

@interface FlexibleContainerView ()
@property (nonatomic, weak) NSLayoutConstraint *heightConstraint;
@property (nonatomic, unsafe_unretained) CGFloat defaultHeight;
@end

@implementation FlexibleContainerView

- (void)awakeFromNib
{
    [super awakeFromNib];
    
    // heightConstraint from nib
    [self.constraints enumerateObjectsWithOptions:NSEnumerationConcurrent
                                       usingBlock:^(NSLayoutConstraint *constraint, NSUInteger index, BOOL *stop) {
                                           if(NSLayoutAttributeHeight == constraint.firstAttribute) {
                                               self.heightConstraint = constraint;
                                               self.defaultHeight = constraint.constant;
                                               *stop = YES;
                                           }
                                       }];
    
    // init heightConstraint if undefined
    if(!self.heightConstraint) {
        NSLayoutConstraint *constraint = [NSLayoutConstraint constraintWithItem:self
                                                                      attribute:NSLayoutAttributeHeight
                                                                      relatedBy:NSLayoutRelationEqual
                                                                         toItem:nil
                                                                      attribute:NSLayoutAttributeNotAnAttribute
                                                                     multiplier:1.0
                                                                       constant:0.0];
        constraint.priority = UILayoutPriorityDefaultLow;
        self.heightConstraint = constraint;
        [self addConstraint:constraint];
    }
}

- (void)setHidden:(BOOL)hidden
{
    [super setHidden:hidden];

    if(self.hidden) {
        self.heightConstraint.priority = UILayoutPriorityRequired - 1;
        self.heightConstraint.constant = 0;
    }
    else {
        self.heightConstraint.priority = UILayoutPriorityDefaultLow;
        if(0 < self.defaultHeight) {
            self.heightConstraint.constant = self.defaultHeight;
        }
        else {
            CGSize fittingSize = [self systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
            self.heightConstraint.constant = ceilf(fittingSize.height);
        }
    }
}

@end