Programmatically using constraints to center a UIIMageView - ios

Question

I'm working on an iOS project in which I need to programmatically use constraints for my views. I'm used to storyboards, but am using xibs for this particular project due to the nature of the project specification.
I have a MainViewController, in which I create the following properties in the .h file:
#property (nonatomic, strong) IBOutlet UIImageView *backgroundImageView;
#property (nonatomic, strong) IBOutlet UIImageView *logoImage;
I added these UIImageView instances to my XIB file, and selected the appropriate images via the Attributes inspector.
In my .m file, I have a addConstraints method, which is called in viewDidLoad. Inside this method, I make sure to turn off converting autoresizingMasks into constraints:
self.backgroundImageView.translatesAutoresizingMaskIntoConstraints = NO;
self.logoImage.translatesAutoresizingMaskIntoConstraints = NO;
Then, I've set up constraints so that the background image takes up the entirety of the superview:
id mainView = #{#"background": self.backgroundImageView};
//Set constraints so that the background takes up the entirety of the superview
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"H:|[background]|" options:0 metrics:nil views:mainView]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"V:|[background]|" options:0 metrics:nil views:mainView]];
Finally, I set constraints so that the logo view is in the center of the view (which is where I am going wrong):
// Width constraint
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.logoImage
attribute:NSLayoutAttributeWidth
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeWidth
multiplier:0.5
constant:0]];
// Height constraint
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.logoImage
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeHeight
multiplier:0.5
constant:0]];
// Center horizontally
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.logoImage
attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeCenterX
multiplier:1.0
constant:0.0]];
// Center vertically
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.logoImage
attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeCenterY
multiplier:1.0
constant:0.0]];
The error I receive is
*** +[NSLayoutConstraint constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]:
Constraint items must each be an instance of UIView or subclass'
, which I don't fully understand, as my constraint items (two UIImageView instances) are either subclasses of UIView, but I may be misunderstanding this. Can anyone help point out where I'm going wrong?

Answer 1

Try that :
// Width constraint
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.logoImage
attribute:NSLayoutAttributeWidth
relatedBy:NSLayoutRelationEqual
toItem:self.backgroundImageView
attribute:NSLayoutAttributeWidth
multiplier:0.5
constant:0]];
// Height constraint
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.logoImage
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:self.backgroundImageView
attribute:NSLayoutAttributeHeight
multiplier:0.5
constant:0]];
// Center horizontally
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.logoImage
attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationEqual
toItem:self.backgroundImageView
attribute:NSLayoutAttributeCenterX
multiplier:1.0
constant:0.0]];
// Center vertically
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.logoImage
attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationEqual
toItem:self.backgroundImageView
attribute:NSLayoutAttributeCenterY
multiplier:1.0
constant:0.0]];
You were trying to define constraints between your UIImageView and your view controller. You must define constraints between views which are subviews of a same view.