Treat NSLayoutConstraint constant as multiplier? - swift

Question

I have three horizontally aligned UIViews within a container UIView. These three views should span the entire width of the above UIImageView. The problem, however, is that sometimes only one or two of the child views should be shown.
I set up my view hierarchy like so:
Since the child views are set to be equal to the width of the first child (which will always be shown), I simply set the width of the first child to be a fraction of the UIImageView width. So if three child views should be shown, the first child view would have a multiplier of 1/3 the width of the UIImageView. If two child views should be shown, the multiplier would be 1/2. If just one, the multiplier would be 1.
This seemed like a perfect solution, however the multiplier property is read only. My first attempt to solve this was by creating three different NSLayoutConstraints attached to the first child view, all with a different multiplier with 2/3 of them turned off. Then, on runtime, I would enable the appropriate constant with the appropriate multiplier based off of the number of views I wanted to show.
This resulted in a lot of ugly errors, and so did my second solution:
var constraint = NSLayoutConstraint(item: color1, attribute:NSLayoutAttribute.Width, relatedBy: NSLayoutRelation.Equal, toItem: imageView, attribute: NSLayoutAttribute.Width, multiplier: 1/2, constant: 0)
view.addConstraint(constraint)
Where I would add a new constraint to the view based on the multiplier I wanted. This, of course, resulted in an error:
When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled.
My question, therefore, is if I can treat the constant property like the multiplier property. My fear with doing this, however, is that if I set the constant for the width of the first child view, it would not update its width when the phone rotates.
Is there a better solution for all of this?

Answer 1

Firstly, In your question your were using IB but you seemed to suggest there may be a different number of views each time you moved to the view controller which is why I decided to create the NSLayoutConstraints programatically.
Secondly, my solution is fine provided you didn't intend to change the number of views whilst you were on the view controller. If you did, then this needs a bit more work.
In your view controller:
var viewWidthConstraints : [NSLayoutConstraint] = []
override func viewDidLoad() {
super.viewDidLoad()
let numberOfViews = 3
var previousView: UIView = self.view
for i in 0..<numberOfImages {
let myView = UIView()
myView.setTranslatesAutoresizingMaskIntoConstraints(false)
myView.backgroundColor = UIColor.grayColor().colorWithAlphaComponent(CGFloat(i) * (1/CGFloat(numberOfImages)) + 0.1)
self.view.addSubview(myView)
let heightConstraint = NSLayoutConstraint.constraintsWithVisualFormat("V:|[view]|",
options: .DirectionLeadingToTrailing,
metrics: nil,
views: ["view" : myView])
let widthConstraint = NSLayoutConstraint(item: myView,
attribute: .Width,
relatedBy: .Equal,
toItem: nil,
attribute: .NotAnAttribute,
multiplier: 1.0,
constant: self.view.frame.width / CGFloat(numberOfImages))
let attribute: NSLayoutAttribute = (i == 0) ? .Left : .Right
let leftConstraint = NSLayoutConstraint(item: myView,
attribute: .Left,
relatedBy: .Equal,
toItem: previousView,
attribute: attribute,
multiplier: 1.0,
constant: 0.0)
self.view.addConstraints(heightConstraint)
self.view.addConstraint(leftConstraint)
myView.addConstraint(widthConstraint)
viewWidthConstraints.append(widthConstraint)
previousView = myView
}
}
func updateWidthConstraints() {
if viewWidthConstraints.count > 0 {
let width = self.view.frame.width / CGFloat(viewWidthConstraints.count)
for constraint in viewWidthConstraints {
constraint.constant = width
}
}
}
override func viewWillLayoutSubviews() {
updateWidthConstraints()
}
In viewDidLoad you add the UIViews to the view and set up their constraints. The vertical constraint you could change to make the UIViews appear underneath your UIImageView. And change numberOfViews to increase or decrease the number of views.
Then in viewWillLayoutSubviews you update the width of each view using their width constraint. This will make sure, if the device is rotated, each view takes up the correct proportion of the screen.
This is what is looked like with horizontal orientation.
And vertical orientation.