CODEWITHSUNDEEP
iosswiftautolayoutnslayoutconstraintuistackview
Renato Stauffer
Renato Stauffer

Reputation: 813

UIStackView spacing breaks constraints when not in set updateConstraints

I have a simple ViewController with a custom subview called MyView:

import UIKit

class ViewController: UIViewController {

    let myView = MyView()

    override func viewDidLoad() {
       super.viewDidLoad()
       myView.translatesAutoresizingMaskIntoConstraints = false
       view.addSubview(myView)
       myView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -8.0).isActive = true
       myView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 8.0).isActive = true
       myView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -8.0).isActive = true
   }

}

class MyView: UIView {

    let stackView: UIStackView
    let label = UILabel()
    let label2 = UILabel()

    init() {
      stackView = UIStackView(arrangedSubviews: [label, label2])
      super.init(frame: .zero)

      stackView.axis = .vertical

      label.translatesAutoresizingMaskIntoConstraints = false
      label.text = "Label 1"
      label2.translatesAutoresizingMaskIntoConstraints = false
      label2.text = "Label 2"
      stackView.spacing = 8.0        
      addSubview(stackView)
   }

   required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
   }

   override func updateConstraints() {
        super.updateConstraints()
        stackView.fillSuperview()
   }

}

When I run this, I get the following breaking constraints in the console:

2018-03-23 16:11:58.960493+0100 ViewWithStackView[75612:10756278] [LayoutConstraints] Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want. 
Try this: 
    (1) look at each constraint and try to figure out which you don't expect; 
    (2) find the code that added the unwanted constraint or constraints and fix it. 
(
"<NSLayoutConstraint:0x608000280f00 'UISV-canvas-connection' UIStackView:0x7f8438e07510.top == UILabel:0x7f8438d06740'Label 1'.top   (active)>",
"<NSLayoutConstraint:0x608000280fa0 'UISV-canvas-connection' V:[UILabel:0x7f8438e07790'Label 2']-(0)-|   (active, names: '|':UIStackView:0x7f8438e07510 )>",
"<NSLayoutConstraint:0x608000280ff0 'UISV-spacing' V:[UILabel:0x7f8438d06740'Label 1']-(8)-[UILabel:0x7f8438e07790'Label 2']   (active)>",
"<NSLayoutConstraint:0x608000280550 'UIView-Encapsulated-Layout-Height' UIStackView:0x7f8438e07510.height == 0   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x608000280ff0 'UISV-spacing' V:[UILabel:0x7f8438d06740'Label 1']-(8)-[UILabel:0x7f8438e07790'Label 2']   (active)>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.
2018-03-23 16:11:58.961737+0100 ViewWithStackView[75612:10756278] 
    [LayoutConstraints] Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want. 
Try this: 
    (1) look at each constraint and try to figure out which you don't expect; 
    (2) find the code that added the unwanted constraint or constraints and fix it. 
(Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
     "<NSAutoresizingMaskLayoutConstraint:0x6080002812c0 h=--& v=--& UIStackView:0x7f8438e07510.height == 0   (active)>",
"<NSLayoutConstraint:0x608000280f00 'UISV-canvas-connection' UIStackView:0x7f8438e07510.top == UILabel:0x7f8438d06740'Label 1'.top   (active)>",
"<NSLayoutConstraint:0x608000280fa0 'UISV-canvas-connection' V:[UILabel:0x7f8438e07790'Label 2']-(0)-|   (active, names: '|':UIStackView:0x7f8438e07510 )>",
"<NSLayoutConstraint:0x608000280ff0 'UISV-spacing' V:[UILabel:0x7f8438d06740'Label 1']-(8)-[UILabel:0x7f8438e07790'Label 2']   (active)>"
)     
Will attempt to recover by breaking constraint 


Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

Now if I move the following line stackView.spacing = 8.0 to the updateConstraints method, the constraints won't break. So the following code works fine without warning / error in the console:

import UIKit

class ViewController: UIViewController {

    let myView = MyView()

    override func viewDidLoad() {
       super.viewDidLoad()
       myView.translatesAutoresizingMaskIntoConstraints = false
       view.addSubview(myView)
       myView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -8.0).isActive = true
       myView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 8.0).isActive = true
       myView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -8.0).isActive = true
   }

}

class MyView: UIView {

    let stackView: UIStackView
    let label = UILabel()
    let label2 = UILabel()

    init() {
      stackView = UIStackView(arrangedSubviews: [label, label2])
      super.init(frame: .zero)

      stackView.axis = .vertical

      label.translatesAutoresizingMaskIntoConstraints = false
      label.text = "Label 1"
      label2.translatesAutoresizingMaskIntoConstraints = false
      label2.text = "Label 2"        
      addSubview(stackView)
   }

   required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
   }

   override func updateConstraints() {
        super.updateConstraints()
        stackView.fillSuperview()
        stackView.spacing = 8.0
   }

}

Why does moving stackView.spacing = 8.0 to updateConstraints solve this issue / remove the warnings / errors?

Thank you for an explanation :)

Upvotes: 1

Views: 2917

Answers (2)

Craig Siemens
Craig Siemens

Reputation: 13276

It looks like this is the constraint you don't want

"<NSLayoutConstraint:0x608000280550 'UIView-Encapsulated-Layout-Height' UIStackView:0x7f8438e07510.height == 0   (active)>"

The first thing I'd try is

stackView.translatesAutoresizingMaskIntoConstraints = false

Also, I don't know what stackView.fillSuperview() is doing but you'd probably be better off adding constraints to it so fills the view.

Upvotes: 4

Shehata Gamal
Shehata Gamal

Reputation: 100503

In the first case the stackView has a zero height because you haven't set a frame for it ( in init before fillSuperview()), so setting a spacing of 8 between the 2 labels will cause a conflict , but in second the frame is zero and spacing is zero and the 2 labels will have a zero height according to (intrinsic content size) so, in this case no conflict will happen , then you set spacing in updateConstraints but at that moment stackView has a frame equal to it's superView according to this line fillSuperview() so that goes smoothly also

Upvotes: 0

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