Constraints to superview not working - swift

Question

I add a subView to a superview and I want to constrain the subview. But it does not do anything. The subview is added fullscreen over the superview. Please help me, what is going wrong?
This is my code:
override func viewWillAppear(animated: Bool) {
super.viewWillAppear(animated)
setupConstraints()
}
func setupConstraints(){
heightConstraint = NSLayoutConstraint(item: view, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem: nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1, constant: 80)
bottomConstraint = NSLayoutConstraint(item: view, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, toItem: view.superview!, attribute: NSLayoutAttribute.CenterY, multiplier: 1, constant: 30)
widthConstraint = NSLayoutConstraint(item: view, attribute: NSLayoutAttribute.Width, relatedBy: NSLayoutRelation.Equal, toItem: view.superview!, attribute: NSLayoutAttribute.Width, multiplier: 1, constant: 0)
view.setTranslatesAutoresizingMaskIntoConstraints(false)
view.superview!.setTranslatesAutoresizingMaskIntoConstraints(false)
view.addConstraint(heightConstraint!)
view.addConstraint(widthConstraint!)
view.addConstraint(bottomConstraint!)
}

Answer 1

The problem is that your constraints are ambiguous (underdetermined). There is not enough information to know where to put your view.
There are four pieces of information that must be known:
* x position
* y position
* width
* height
Look at your constraints, and consider what each one determines:
heightConstraint = NSLayoutConstraint(item: view, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem: nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1, constant: 80)
That's height.
bottomConstraint = NSLayoutConstraint(item: view, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, toItem: view.superview!, attribute: NSLayoutAttribute.CenterY, multiplier: 1, constant: 30)
That's y position.
widthConstraint = NSLayoutConstraint(item: view, attribute: NSLayoutAttribute.Width, relatedBy: NSLayoutRelation.Equal, toItem: view.superview!, attribute: NSLayoutAttribute.Width, multiplier: 1, constant: 0)
That's width.
Ooops! What about x position?
Another problem with your code could be this line:
view.superview!.setTranslatesAutoresizingMaskIntoConstraints(false)
You remove view.superview's automatically generated constraints by saying that — but you do not replace them by any new constraints. So it may be ambiguously configured too at this point (unless it already has a complete set of constraints we don't know about, but then in that case that line is unnecessary).
In my book, I provide code for some utility methods to help track down this kind of thing:
extension NSLayoutConstraint {
class func reportAmbiguity (var v:UIView?) {
if v == nil {
v = UIApplication.sharedApplication().keyWindow
}
for vv in v!.subviews as! [UIView] {
println("\(vv) \(vv.hasAmbiguousLayout())")
if vv.subviews.count > 0 {
self.reportAmbiguity(vv)
}
}
}
class func listConstraints (var v:UIView?) {
if v == nil {
v = UIApplication.sharedApplication().keyWindow
}
for vv in v!.subviews as! [UIView] {
let arr1 = vv.constraintsAffectingLayoutForAxis(.Horizontal)
let arr2 = vv.constraintsAffectingLayoutForAxis(.Vertical)
NSLog("\n\n%#\nH: %#\nV:%#", vv, arr1, arr2);
if vv.subviews.count > 0 {
self.listConstraints(vv)
}
}
}
}
If you run NSLayoutConstraint.reportAmbiguity(view.superview!), preferably in viewDidLayoutSubviews at a time after your view has been injected into the view hierarchy, you will find that view reports as ambiguous. That's why you are not seeing what you expect.