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swiftconstraintsnslayoutconstraint

Programatic Constraints Not Obeyed

I run the below code on a button press, however the bottom constraint does not seem to be obeyed. The subview (bandCardView) overflows the bottom of the parent view (formVw). How can I make these constraints obeyed?

 @objc private func cardBtnTouch(){
        self.bandAccountView?.bankBtn.setSelected(selected: false)
        self.bandAccountView?.cardBtn.setSelected(selected: true)

        self.bandAccountView?.selectLbl.isHidden = true
        for subview in self.bandAccountView!.formVw.subviews{
            subview.removeFromSuperview()
        }
        self.bandAccountView!.formVw.addSubview(bandCardView!)
        self.bandAccountView!.formVw.addConstraint(NSLayoutConstraint(item: self.bandAccountView!.formVw, attribute: NSLayoutAttribute.top, relatedBy: NSLayoutRelation.equal, toItem: bandCardView!, attribute: NSLayoutAttribute.top, multiplier: 1.0, constant: 0.0))
        self.bandAccountView!.formVw.addConstraint(NSLayoutConstraint(item: bandCardView!, attribute: NSLayoutAttribute.bottom, relatedBy: NSLayoutRelation.equal, toItem: self.bandAccountView!.formVw, attribute: NSLayoutAttribute.bottom, multiplier: 1.0, constant: 0.0))
        self.bandAccountView!.formVw.addConstraint(NSLayoutConstraint(item: bandCardView!, attribute: NSLayoutAttribute.leading, relatedBy: NSLayoutRelation.equal, toItem: self.bandAccountView!.formVw, attribute: NSLayoutAttribute.leading, multiplier: 1.0, constant: 0.0))
        self.bandAccountView!.formVw.addConstraint(NSLayoutConstraint(item: self.bandAccountView!.formVw, attribute: NSLayoutAttribute.trailing, relatedBy: NSLayoutRelation.equal, toItem: bandCardView!, attribute: NSLayoutAttribute.trailing, multiplier: 1.0, constant: 0.0))
    }
Solution

  • make sure you have translatesAutoresizingMaskIntoConstraints set to false.

    Also after iOS 9 there is an easier way of writing your constraints.

    Add a single constraint without any constant: 

    self.bandAccountView!.formVw.addConstraint(formVw.topAnchor.constraint(equalTo:
 bandCardView.topAnchor)

    Add a single constraint with a constant: 

    self.bandAccountView!.formVw.addConstraint(formVw.topAnchor.constraint(equalTo: 
bandCardView.topAnchor, constant: 10)

    Add multiple constraints:

    self.bandAccountView!.formVw.addConstraints([formVw.topAnchor.constraint(equalTo:
 bandCardView.topAnchor),formVw.bottomAnchor.constraint(equalTo:
 bandCardView.bottomAnchor),formVw.leadingAnchor.constraint(equalTo: 
 bandCardView.leadingAnchor),formVw.trailingAnchor.constraint(equalTo:
 bandCardView.trailingAnchor)]

    NOTE:

    if you ever wrote:

    self.bandAccountView!.formVw.leadingAnchor.constraint(equalTo:
formVw.leadingAnchor, constant: 0)

    then you have actually forgot to "add/activate the constraint". To fix it you either have to do:

    self.bandAccountView!.formVw.leadingAnchor.constraint(equalTo:
formVw.leadingAnchor, constant: 0).isActive = true

    OR

    let leadingConstraint = self.bandAccountView!.formVw.leadingAnchor.constraint(equalTo:
formVw.leadingAnchor, constant: 0)
leadingConstraint.isActive = true // do this whenever you need
leadingConstraint.isActive = false // if you don't need it...

    or simply do like the first snippet

    Additionally the relation between bandAccountView & formVw are instance--instance variable and they way you're doing it isn't good. It's much better to do the constraints in it's own class or create a custom init that would only adjust the constants for you.