Automatic constraints violations in stack view, Swift 2, iOS 9.3, XCode 7

Question

I couldn't find a similar problem anywhere, so here is my first post: I am working on a swift 2 project for iPhone. I have devised a "settings" screen with embedded stack views:

Layout showing bounds boxes and constraints

So, there is a main "vertical" stack view with two subviews, spaced equally: the top one corresponds to color palette settings, with the following elements:

The window's main label ("Color Scheme")
A slider to change hue (tint), with name label ("Hue")
A switch to toggle color/B&W, with name label ("Color")
A switch to toggle monochrome palette vs "rainbow" palette, with name label ("Rainbow")
An horizontal stack view with 9 subviews, one for each palette color

The bottom view is for weather view settings, and doesn't have any problems.

The main stack view is set up to change from "vertical" to "horizontal" when changing from portrait to landscape orientation:

Landscape orientation layout, showing all bounds boxes and constraints

The storyboard editor shows no constraint error whatsoever in either orientation (all show in blue), but at runtime, when running in the simulator, changing the settings screen orientation to landscape generates multiple "unable to satisfy constraints" errors, although the screen shows and behaves perfectly.

The weird thing, is that the constraint error messages repeat several times, each time trying to recover by breaking constraints within the second stack view! (which are supposed to be handled automatically by the stack view itself). In particular, the system is trying to break multiple "equal bottom" constraints setting the bottoms of different pairs of embedded subviews to equal values. That is, the nine subviews within the second stack view are supposed to have their bottom bounds at the same level (this is imposed by the stack view itself, so I can't modify it in my program). Each time the systems tries this, it fails, so a new error is output to the console, and the system tries to break another "bottom == bottom" constraint within the stack view, with a new failure.

This goes on several times (from 5 to 8 times), until it settles by trying to break the main label ("Color Scheme") vertical size constraint. In view of this final result, I changed the compression priority of the main label vertical size, but I am still getting the errors. Here is a console dump:

2016-04-22 10:06:14.061 Just Now[9468:534833] 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. 
(
    "<_UILayoutSupportConstraint:0x7fa918c200e0 V:[_UILayoutGuide:0x7fa91f55acf0(44)]>",
    "<_UILayoutSupportConstraint:0x7fa91f582a20 V:|-(0)-[_UILayoutGuide:0x7fa91f55acf0]   (Names: '|':UIView:0x7fa91f53a530 )>",
    "<_UILayoutSupportConstraint:0x7fa91f5c65a0 V:[_UILayoutGuide:0x7fa91f5d2350(0)]>",
    "<_UILayoutSupportConstraint:0x7fa918c11a20 _UILayoutGuide:0x7fa91f5d2350.bottom == UIView:0x7fa91f53a530.bottom>",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    ""
)

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  may also be helpful.
2016-04-22 10:06:14.063 Just Now[9468:534833] 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. 
(
    "<_UILayoutSupportConstraint:0x7fa918c200e0 V:[_UILayoutGuide:0x7fa91f55acf0(44)]>",
    "<_UILayoutSupportConstraint:0x7fa91f582a20 V:|-(0)-[_UILayoutGuide:0x7fa91f55acf0]   (Names: '|':UIView:0x7fa91f53a530 )>",
    "<_UILayoutSupportConstraint:0x7fa91f5c65a0 V:[_UILayoutGuide:0x7fa91f5d2350(0)]>",
    "<_UILayoutSupportConstraint:0x7fa918c11a20 _UILayoutGuide:0x7fa91f5d2350.bottom == UIView:0x7fa91f53a530.bottom>",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    ""
)

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  may also be helpful.
2016-04-22 10:06:14.064 Just Now[9468:534833] 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. 
(
    "<_UILayoutSupportConstraint:0x7fa918c200e0 V:[_UILayoutGuide:0x7fa91f55acf0(44)]>",
    "<_UILayoutSupportConstraint:0x7fa91f582a20 V:|-(0)-[_UILayoutGuide:0x7fa91f55acf0]   (Names: '|':UIView:0x7fa91f53a530 )>",
    "<_UILayoutSupportConstraint:0x7fa91f5c65a0 V:[_UILayoutGuide:0x7fa91f5d2350(0)]>",
    "<_UILayoutSupportConstraint:0x7fa918c11a20 _UILayoutGuide:0x7fa91f5d2350.bottom == UIView:0x7fa91f53a530.bottom>",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    ""
)

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  may also be helpful.
2016-04-22 10:06:14.065 Just Now[9468:534833] 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. 
(
    "<_UILayoutSupportConstraint:0x7fa918c200e0 V:[_UILayoutGuide:0x7fa91f55acf0(44)]>",
    "<_UILayoutSupportConstraint:0x7fa91f582a20 V:|-(0)-[_UILayoutGuide:0x7fa91f55acf0]   (Names: '|':UIView:0x7fa91f53a530 )>",
    "<_UILayoutSupportConstraint:0x7fa91f5c65a0 V:[_UILayoutGuide:0x7fa91f5d2350(0)]>",
    "<_UILayoutSupportConstraint:0x7fa918c11a20 _UILayoutGuide:0x7fa91f5d2350.bottom == UIView:0x7fa91f53a530.bottom>",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    "",
    ""
)

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  may also be helpful.

I have tried moving things around, but I get no errors in the storyboard editor and the runtime errors continue when rotating the screen. The fact that it's trying to break automatic constraints within a stack view makes me think this may be a bug. Regardless of the errors, the program works and looks as it is supposed to, so I am at a loss.

Any help will be appreciated. Thanks in advance!

Automatic constraints violations in stack view, Swift 2, iOS 9.3, XCode 7

Answers (1)

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