When Is a Constraint Error Breakpoint Hit?

You can see what these ‘magic’ breakpoints do under the covers by using LLDB’s

breakpoint list

(lldb) br list
Current breakpoints:
1: name = 'LAYOUT_CONSTRAINTS_NOT_SATISFIABLE', locations = 0 (pending)

Note that the

locations

The above was on iOS (11, in the simulator). When I run the same test on macOS (10.13) I see this:

(lldb) br list 
Current breakpoints:
1: name = 'LAYOUT_CONSTRAINTS_NOT_SATISFIABLE', locations = 1, resolved = 1, hit count = 0

  1.1: where = AppKit`LAYOUT_CONSTRAINTS_NOT_SATISFIABLE, address = 0x00007fff28d130b1, resolved, hit count = 0

It seems like this is an AppKit-specific feature. Are you testing this on iOS? If so, that explains why you’re not getting any hits.

You wrote:

The outpout in Xcode's console clearly indicates the user interface has conficting constraints and the debugger suggests to add a symbolic breakpoint …

What exactly does it say?

Share and Enjoy
—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

Thank you for your response, Quinn. Your answers are always lifesavers. This clears everything up. I have to admit that it's confusing and I'm wondering if this is a bug. Your answer once again shows how useful it can be to learn more about the LLDB debugger.

To answer your question, this is the output I'm seeing in Xcode's console. I can probably accomplish the same by adding a symbolic breakpoint as Xcode suggests.

2018-03-16 07:54:00.064979+0100 Breakpoints[73295:2788883] [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:0x6180000990f0 UILabel:0x7f9486403a70'Label'.width == 100   (active)>",
    "<NSLayoutConstraint:0x618000099190 UILabel:0x7f9486403a70'Label'.width == 200   (active)>"
)


Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x618000099190 UILabel:0x7f9486403a70'Label'.width == 200   (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.
2018-03-16 07:54:00.065366+0100 Breakpoints[73295:2788883] [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:0x6180000990f0 UILabel:0x7f9486403a70'Label'.width == 100   (active)>",
    "<NSLayoutConstraint:0x61c000097b10 UILabel:0x7f9486403a70'Label'.width == 200   (active)>"
)


Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x61c000097b10 UILabel:0x7f9486403a70'Label'.width == 200   (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.

Thanks again for your time, Quinn. I appreciate it.

Hi. it's great to inspect the stats of a breakpoint using: "br list", however this does not answer the original question of: Why our constraint breakpoints are not firing in the first place? I use "br list" and it shows a "hit count" of 0 on all the constraint breakpoints. I still don't know why they don't get hit. I have constraint errors such as:

2019-09-11 13:22:10.218401-0400 My App [111:222] [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:0x7fbf1f6ac620 UITableView:0x7fbf1a27f200.height == 171 (active)>",

"<NSLayoutConstraint:0x7fbf1f5fa4e0 UIView:0x7fbf1f5f8090.height == 214 (active)>",

"<NSLayoutConstraint:0x7fbf1f5fa620 V:|-(>=40)-[UITableView:0x7fbf1a27f200] (active, names: '|':UIView:0x7fbf1f5f8090 )>",

"<NSLayoutConstraint:0x7fbf1f5faaa0 V:[UITableView:0x7fbf1a27f200]-(25)-| (active, names: '|':UIView:0x7fbf1f5f8090 )>"

)

The "Constraint Error" type breakpoint does not fire.

Here are my breakpoints:

Current breakpoints, as output by "br list":

1: name = 'LAYOUT_CONSTRAINTS_NOT_SATISFIABLE', locations = 0 (pending)

2: name = 'UICollectionViewFlowLayoutBreakForInvalidSizes', locations = 1, resolved = 1, hit count = 0

2.1: where = UIKitCore`UICollectionViewFlowLayoutBreakForInvalidSizes, address = 0x0000000113bacec4, resolved, hit count = 0

3: names = {'-[UIView(UIConstraintBasedLayout) _viewHierarchyUnpreparedForConstraint:]', '-[UIView(UIConstraintBasedLayout) _viewHierarchyUnpreparedForConstraint:]'}, locations = 1, resolved = 1, hit count = 0

3.1: where = UIKitCore`-[UIView(UIConstraintBasedLayout) _viewHierarchyUnpreparedForConstraint:], address = 0x00000001146dfea2, resolved, hit count = 0

Any suggestions?

Thanks in advance.

Alex

OK, adding a Symbolic Breakpoint on UIViewAlertForUnsatisfiableConstraints works great and pauses on the stack trace of the constraint causing the problem. I just didn't have any luck using the standard "Constraint Breakpoint" type in XCode. I'm on version 10.3.

Thanks,

Alex