Can't get the constraints to change programmatically

Question

I am trying to get the scrolling parallax effect. The following code is of the CollectionViewCell.

#import "NearbyViewCell.h"

@interface NearbyViewCell ()

@property (nonatomic) CGFloat parallaxOffset;

@end
@implementation NearbyViewCell

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {

        self.clipsToBounds = YES;

        self.locationImage = [[UIImageView alloc] initWithFrame:CGRectMake(0.0, 0.0, frame.size.width, frame.size.height)];
        self.locationImage.frame = self.contentView.bounds;
        self.locationImage.contentMode = UIViewContentModeScaleAspectFill;

        self.locationLabel = [[UILabel alloc] initWithFrame:CGRectMake(0.0, 0.0, frame.size.width, frame.size.height - 50.0)];
        self.locationLabel.numberOfLines = 0;
        self.locationLabel.textAlignment = NSTextAlignmentCenter;
        self.locationLabel.textColor = [UIColor whiteColor];
        self.locationLabel.backgroundColor = [UIColor colorWithRed:0.0/255.0 green:0.0/255.0 blue:0.0 /255.0 alpha:0.5];


        self.otherLabel = [[UILabel alloc] initWithFrame:CGRectMake(0.0, (frame.size.height - 50.0), frame.size.width, 50.0)];
        self.otherLabel.textAlignment = NSTextAlignmentCenter;
        self.otherLabel.textColor = [UIColor whiteColor];
        self.otherLabel.backgroundColor = [UIColor colorWithRed:0.0/255.0 green:0.0/255.0 blue:0.0 /255.0 alpha:0.5];

        [self.contentView addSubview:self.locationImage];
        [self.contentView addSubview:self.locationLabel];
        [self.contentView addSubview:self.otherLabel];
    }
    return self;
}

The following code is called from the scrollViewDidScroll in the collectionViewController. The constraint is an IBOutlet and is declared in the header file.

- (void)updateParallaxOffset:(CGRect)bounds {
    CGPoint center = CGPointMake(CGRectGetMidX(bounds),   CGRectGetMidY(bounds));
    CGPoint offsetFromCenter = CGPointMake(center.x - self.center.x, center.y - self.center.y);
    CGFloat maxVerticalOffset = (CGRectGetHeight(bounds) / 2) + (CGRectGetHeight(self.bounds) / 2);
    CGFloat scaleFactor = 4000 / maxVerticalOffset;

    self.parallaxOffset = -offsetFromCenter.y * scaleFactor;

    self.locationLabelCenterYConstraint.constant = self.parallaxOffset;
    NSLog(@"%f", self.locationLabelCenterYConstraint.constant);
    NSLog(@"%f", self.parallaxOffset);
}

@end

The problem is that the value of 'parallaxOffset' is changing but the value of 'locationLabelCenterYConstraint' is not changing. The result of the NSLog is given below.

2016-01-13 22:55:43.926 Parallax[1597:382314] 0.000000
2016-01-13 22:55:43.926 Parallax[1597:382314] -161.566707
2016-01-13 22:55:43.926 Parallax[1597:382314] 0.000000
2016-01-13 22:55:43.926 Parallax[1597:382314] 1307.221542
2016-01-13 22:55:43.926 Parallax[1597:382314] 0.000000
2016-01-13 22:55:43.927 Parallax[1597:382314] 2776.009792
2016-01-13 22:55:44.108 Parallax[1597:382314] 0.000000
2016-01-13 22:55:44.108 Parallax[1597:382314] -3094.247246
2016-01-13 22:55:44.109 Parallax[1597:382314] 0.000000
2016-01-13 22:55:44.109 Parallax[1597:382314] -1625.458996
2016-01-13 22:55:44.110 Parallax[1597:382314] 0.000000
2016-01-13 22:55:44.110 Parallax[1597:382314] -156.670747
2016-01-13 22:55:44.110 Parallax[1597:382314] 0.000000
2016-01-13 22:55:44.111 Parallax[1597:382314] 1312.117503
2016-01-13 22:55:44.118 Parallax[1597:382314] 0.000000
2016-01-13 22:55:44.118 Parallax[1597:382314] 2780.905753

Thank You in advance!

Answer 1

I have got this to work by adding the constraint programmatically rather than through interface builder. This works fine now.