How to trap on UIViewAlertForUnsatisfiableConstraints?

Question

I'm seeing an error appear in my debugger log:

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x191f0920 H:[MPKnockoutButton:0x17a876b0]-(34)-[MPDetailSlider:0x17a8bc50](LTR)>

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.

How do I trap on that call? It does not appear anywhere in my code.

Tangerine

In 9 out of 10 cases: this is just caused by: for some view or item on yoru storyboard, you uncheck "Installed". (For example, just a development button or something you didn't need any more.) In general, it handles "not-Installed" badly: it often leaves constraints there which become meaningless without the not-Installed item. Often the solution is simply to delete items you have forgotten about, that are sitting around "not-Installed" - just delete 'em. – Minelayer

Answer 1

This post helped me A LOT!

I added UIViewAlertForUnsatisfiableConstraints symbolic breakpoint with suggested action:

Obj-C project

po [[UIWindow keyWindow] _autolayoutTrace]

Swift project

expr -l objc++ -O -- [[UIWindow keyWindow] _autolayoutTrace]

With this hint, the log became more detailed, and It was easier for me identify which view had the constraint broken.

UIWindow:0x7f88a8e4a4a0
|   UILayoutContainerView:0x7f88a8f23b70
|   |   UINavigationTransitionView:0x7f88a8ca1970
|   |   |   UIViewControllerWrapperView:0x7f88a8f2aab0
|   |   |   |   •UIView:0x7f88a8ca2880
|   |   |   |   |   *UIView:0x7f88a8ca2a10
|   |   |   |   |   |   *UIButton:0x7f88a8c98820'Archived'
|   |   |   |   |   |   |   UIButtonLabel:0x7f88a8cb0e30'Archived'
|   |   |   |   |   |   *UIButton:0x7f88a8ca22d0'Download'
|   |   |   |   |   |   |   UIButtonLabel:0x7f88a8cb04e0'Download'
|   |   |   |   |   |   *UIButton:0x7f88a8ca1580'Deleted'
|   |   |   |   |   |   |   UIButtonLabel:0x7f88a8caf100'Deleted'
|   |   |   |   |   *UIView:0x7f88a8ca33e0
|   |   |   |   |   *_UILayoutGuide:0x7f88a8ca35b0
|   |   |   |   |   *_UILayoutGuide:0x7f88a8ca4090
|   |   |   |   |   _UIPageViewControllerContentView:0x7f88a8f1a390
|   |   |   |   |   |   _UIQueuingScrollView:0x7f88aa031c00
|   |   |   |   |   |   |   UIView:0x7f88a8f38070
|   |   |   |   |   |   |   UIView:0x7f88a8f381e0
|   |   |   |   |   |   |   |   •UIView:0x7f88a8f39fa0, MISSING HOST CONSTRAINTS
|   |   |   |   |   |   |   |   |   *UIButton:0x7f88a8cb9bf0'Retrieve data'- AMBIGUOUS LAYOUT for UIButton:0x7f88a8cb9bf0'Retrieve data'.minX{id: 170}, UIButton:0x7f88a8cb9bf0'Retrieve data'.minY{id: 171}
|   |   |   |   |   |   |   |   |   *UIImageView:0x7f88a8f3ad80- AMBIGUOUS LAYOUT for UIImageView:0x7f88a8f3ad80.minX{id: 172}, UIImageView:0x7f88a8f3ad80.minY{id: 173}
|   |   |   |   |   |   |   |   |   *App.RecordInfoView:0x7f88a8cbe530- AMBIGUOUS LAYOUT for App.RecordInfoView:0x7f88a8cbe530.minX{id: 174}, App.RecordInfoView:0x7f88a8cbe530.minY{id: 175}, App.RecordInfoView:0x7f88a8cbe530.Width{id: 176}, App.RecordInfoView:0x7f88a8cbe530.Height{id: 177}
|   |   |   |   |   |   |   |   |   |   +UIView:0x7f88a8cc1d30- AMBIGUOUS LAYOUT for UIView:0x7f88a8cc1d30.minX{id: 178}, UIView:0x7f88a8cc1d30.minY{id: 179}, UIView:0x7f88a8cc1d30.Width{id: 180}, UIView:0x7f88a8cc1d30.Height{id: 181}
|   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8cc1ec0- AMBIGUOUS LAYOUT for UIView:0x7f88a8cc1ec0.minX{id: 153}, UIView:0x7f88a8cc1ec0.minY{id: 151}, UIView:0x7f88a8cc1ec0.Width{id: 154}, UIView:0x7f88a8cc1ec0.Height{id: 165}
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8e68e10- AMBIGUOUS LAYOUT for UIView:0x7f88a8e68e10.minX{id: 155}, UIView:0x7f88a8e68e10.minY{id: 150}, UIView:0x7f88a8e68e10.Width{id: 156}
|   |   |   |   |   |   |   |   |   |   |   |   *UIImageView:0x7f88a8e65de0- AMBIGUOUS LAYOUT for UIImageView:0x7f88a8e65de0.minX{id: 159}, UIImageView:0x7f88a8e65de0.minY{id: 182}
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8e69080'8-6-2015'- AMBIGUOUS LAYOUT for UILabel:0x7f88a8e69080'8-6-2015'.minX{id: 183}, UILabel:0x7f88a8e69080'8-6-2015'.minY{id: 184}, UILabel:0x7f88a8e69080'8-6-2015'.Width{id: 185}
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8cc0690'16:34'- AMBIGUOUS LAYOUT for UILabel:0x7f88a8cc0690'16:34'.minX{id: 186}, UILabel:0x7f88a8cc0690'16:34'.minY{id: 187}, UILabel:0x7f88a8cc0690'16:34'.Width{id: 188}, UILabel:0x7f88a8cc0690'16:34'.Height{id: 189}
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8cc2050- AMBIGUOUS LAYOUT for UIView:0x7f88a8cc2050.minX{id: 161}, UIView:0x7f88a8cc2050.minY{id: 166}, UIView:0x7f88a8cc2050.Width{id: 163}
|   |   |   |   |   |   |   |   |   |   |   |   *UIImageView:0x7f88a8e69d90- AMBIGUOUS LAYOUT for UIImageView:0x7f88a8e69d90.minX{id: 190}, UIImageView:0x7f88a8e69d90.minY{id: 191}, UIImageView:0x7f88a8e69d90.Width{id: 192}, UIImageView:0x7f88a8e69d90.Height{id: 193}
|   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3cc00
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8e618d0
|   |   |   |   |   |   |   |   |   |   |   |   *UIImageView:0x7f88a8e5ba10
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3cd70
|   |   |   |   |   |   |   |   |   |   |   |   *UIImageView:0x7f88a8e58e10
|   |   |   |   |   |   |   |   |   |   |   |   *UIImageView:0x7f88a8e5e7a0
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3cee0
|   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3dc70
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8e64dd0
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8e65290'Average flow rate'
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8e712d0'177.0 ml/s'
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8c97150'1299.4'
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3dde0
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8f3df50'Maximum flow rate'
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8cbfdb0'371.6 ml/s'
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8cc0230'873.5'
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3e2a0
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8f3e410'Total volume'
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8cc0f20'371.6 ml'
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3e870
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8f3ea00'Time do max. flow'
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8cc0ac0'3.6 s'
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3ee10
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8f3efa0'Flow time'
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8cbf980'2.1 s'
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3f3e0
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8f3f570'Voiding time'
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8cc17e0'3.5 s'
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3f9a0
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8f3fb30'Voiding delay'
|   |   |   |   |   |   |   |   |   |   |   |   *UILabel:0x7f88a8cc1380'1.0 s'
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8e65000
|   |   |   |   |   |   |   |   |   |   |   |   *UIButton:0x7f88a8e52f20'Show'
|   |   |   |   |   |   |   |   |   |   |   |   *UIImageView:0x7f88a8e6e1d0
|   |   |   |   |   |   |   |   |   |   |   |   *UIButton:0x7f88a8e52c90'Send'
|   |   |   |   |   |   |   |   |   |   |   |   *UIImageView:0x7f88a8e61bb0
|   |   |   |   |   |   |   |   |   |   |   |   *UIButton:0x7f88a8e528e0'Delete'
|   |   |   |   |   |   |   |   |   |   |   |   *UIImageView:0x7f88a8e6b3f0
|   |   |   |   |   |   |   |   |   |   |   |   *UIView:0x7f88a8f3ff60
|   |   |   |   |   |   |   |   |   *UIActivityIndicatorView:0x7f88a8cba080
|   |   |   |   |   |   |   |   |   |   UIImageView:0x7f88a8cba700
|   |   |   |   |   |   |   |   |   *_UILayoutGuide:0x7f88a8cc3150
|   |   |   |   |   |   |   |   |   *_UILayoutGuide:0x7f88a8cc3b10
|   |   |   |   |   |   |   UIView:0x7f88a8f339c0
|   |   UINavigationBar:0x7f88a8c96810
|   |   |   _UINavigationBarBackground:0x7f88a8e45c00
|   |   |   |   UIImageView:0x7f88a8e46410
|   |   |   UINavigationItemView:0x7f88a8c97520'App'
|   |   |   |   UILabel:0x7f88a8c97cc0'App'
|   |   |   UINavigationButton:0x7f88a8e3e850
|   |   |   |   UIImageView:0x7f88a8e445b0
|   |   |   _UINavigationBarBackIndicatorView:0x7f88a8f2b530

Legend:
    * - is laid out with auto layout
    + - is laid out manually, but is represented in the layout engine because translatesAutoresizingMaskIntoConstraints = YES
    • - layout engine host

Then I paused execution and I changed problematic view's background color with the command (replacing 0x7f88a8cc2050 with the memory address of your object of course)...

Obj-C

expr ((UIView *)0x7f88a8cc2050).backgroundColor = [UIColor redColor]

Swift 3.0

expr -l Swift -- import UIKit
expr -l Swift -- unsafeBitCast(0x7f88a8cc2050, to: UIView.self).backgroundColor = UIColor.red

... and the result It was awesome!

Simply amazing!

Hairpiece

@TomCalmon Where did u fire this command to change background color of your view?? – Crist

@Crist You can run commands in console area, when execution is paused. – Phraseograph

@TomCalmon I did the same...but it shows following error rror: Execution was interrupted, reason: EXC_BAD_ACCESS (code=1, address=0x7f88a8cc2050). The process has been returned to the state before expression evaluation. – Crist

It's happening to me the same :/ After all the steps, I write in console: expr -l Swift -- import UIKit expr -l Swift -- unsafeBitCast(0x102083e00, to: UITableView.self).backgroundColor = UIColor.red and I get the same error: error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=1, address=0x5214c1438). The process has been returned to the state before expression evaluation. @Crist could you find a solution? – Hortative

I get error: use of undeclared identifier 'expr' & error: use of undeclared identifier 'l' – Epigeal

@Crist this could happen if the view with address=0x7f88a8cc2050 has been already deallocated and not in memory anymore. – Gemeinschaft

expr -l objc++ -O -- [[UIWindow keyWindow] _autolayoutTrace] returns nil for me – Georgie

Make sure you replace 0x7f88a8cc2050 with the memory address of your object and run the command in the console when execution pauses. – Walkup

Still working in Swift 4 and Xcode 9.4, thanks a lot! – Arturoartus

Unfreakingbelievable. Great tip here, totally helped me go straight to the issue. Once the item is changed to red, continue the execution if possible and you'll see the highlight. – Suter

expr (void)[CATransaction flush] after changing color – Jill

I also had to check "Automatically continue after evaluating actions" otherwise nothing was dumped into the console. – Jaggers

@TomHoward any ideas why I don't see any clues in the output: AMBIGUOUS LAYOUT, MISSING HOST CONSTRAINTS, etc? – Teethe

NSURLConnection finished with error - code -1001 Stopped due to an error evaluating condition of breakpoint 1.1: "expr -l objc++ -O -- [[UIWindow keyWindow] _autolayoutTrace]" Couldn't parse conditional expression: error: use of undeclared identifier 'expr' error: use of undeclared identifier 'l' – Devil

How to find the problematic view? I have 30 views... and where to add this line -> Swift -- unsafeBitCast(0x7f88a8cc2050, to: UIView.self).backgroundColor = UIColor.red Is there a YouTube video for this ? – Barbuto

When trying this I'm getting error: Execution was interrupted, reason: internal ObjC exception breakpoint(-5).. – Orji

Sometimes I get this error when hitting the breakpoint: 2021-03-24 15:41:42.141413+0100 NB[29965:5238269] *** Assertion failure in -[NSISEngine _optimizeWithoutRebuilding], NSISEngine.m:1639 error: Execution was interrupted, reason: internal ObjC exception breakpoint(-5).. The process has been returned to the state before expression evaluation. – Eulau

For those who get the error ` Error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=1, address=0x7f88a8cc2050). The process has been returned to the state before expression evaluation ` or smth similar, that is because you are using the example memory address provided in the answer instead of the one found in your project which is causing the issue. You should be replacing the "0x7f88a8cc2050" with the object's memory address which is causing the issue, to find it, look for a "AMBIGUOUS LAYOUT" or a "MISSING HOST CONSTRAINTS" etc in the tree log you receive by using the first command. – Chromatolysis

For those who are working in swift 5.0 can execute below command as I was getting error: <user expression 5>:1:7: use of undeclared identifier 'l' expr -l Swift -- unsafeBitCast(0x107e6e7f0, to: UIView.self).backgroundColor = UIColor.red So this command worked expr -l objc -- (void)[(UIView *)0x107e6e7f0 setBackgroundColor:[UIColor redColor]] – Stemware

Answer 2

You'll want to add a Symbolic Breakpoint. Apple provides an excellent guide on how to do this.

1. Open the Breakpoint Navigator cmd+7 (cmd+8 in Xcode 9)
2. Click the Add button in the lower left
3. Select Add Symbolic Breakpoint...
4. Where it says Symbol just type in UIViewAlertForUnsatisfiableConstraints

You can also treat it like any other breakpoint, turning it on and off, adding actions, or log messages.

Younglove

I just don't understand how I can debug the problem better with this hint. I added a symbolic breakpoint but it still doesn't give me enough information whats the problem. The only way is trying to read line by line and understand what causes the problem....otherwise clearing the constraints and again adding them together with the Preview in the asisstent view should help most! – Spindling

This might help to get more info after stopping at the breakpoint: staxmanade.com/2015/06/debugging-ios-autolayout-issues – Oakum

Just adding that you can now give identifiers to the constraints directly in IB so when you're debugging them, that's the name you'll see. – Mincey

(follow-up on @MarqueIV ) NSLayoutConstraint has had an identifier property since iOS 7 — Xcode 7 and above, which can be set both from IB Storyboards as well from code. By setting the identifier you can easier distinguish between system-generated and user-generated constraints in the debug log, e.g. myConstraint.identifier = "centered image" (source & examples: useyourloaf.com/blog/using-identifiers-to-debug-autolayout) – Kei

@AlexCio How it helps? The least it does is it pauses at the moment that it happens. It gives a stack trace where you can backtrack and find the origin... – Epigeal

@Honey the debug message printed in 2015 was not helpful at all. They've gotten better over the years, and there are better ways of debugging now that allow you to label identifiers with constraints – Younglove

It will help you if you have one main view compose from 2-3 child views, you will know where you need to fix the constrains. – Pendant

find the memory address in the log when breakpoint stopped and print like: po 0x7f9141cdXXX – Fire

Answer 3

Followed Stephen's advice and tried to debug the code and whoa! it worked. The answer lies in the debug message itself.

Will attempt to recover by breaking constraint NSLayoutConstraint:0x191f0920 H:[MPKnockoutButton:0x17a876b0]-(34)-[MPDetailSlider:0x17a8bc50](LTR)>

The line above tells you that the runtime worked by removing this constraint. May be you don't need Horizontal Spacing on your button (MPKnockoutButton). Once you clear this constraint, it won't complain at runtime & you would get the desired behaviour.

Serpasil

The compiler? You mean the runtime? The compiler didn't remove the constraint. The compiler left it there for the runtime to deal with, hence "recover by breaking constraint" during runtime. – Rental

Answer 4

Whenever I attempt to remove the constraints that the system had to break, my constraints are no longer enough to satisfy the IB (ie "missing constraints" shows in the IB, which means they're incomplete and won't be used). I actually got around this by setting the constraint it wants to break to low priority, which (and this is an assumption) allows the system to break the constraint gracefully. It's probably not the best solution, but it solved my problem and the resulting constraints worked perfectly.

Bracy

Typically that is a situation where you want to use a placeholder constraint which is removed at runtime. To make a constraint a placeholder constraint, go to the constraint inspector and click "Remove at build time". Notice how the constraint I-beam symbol in the IB drawing area turns from blue to grey to indicate this. – Carlenecarleton

I had the same issue. when i remove the broken constraint my design broke. So i set the priority to medium. – Ignatz

Answer 5

This usually appears when you want to use UIActivityViewController in iPad.

Add below, before you present the controller to mark the arrow.

activityViewController.popoverPresentationController?.sourceRect = senderView.frame // senderView can be your button/view you tapped to call this VC

I assume you already have below, if not, add together:

activityViewController.popoverPresentationController?.sourceView = self.view

Corporal

Answer 6

I think the problem I have is when I see something like this:

(
    "<NSLayoutConstraint:0x600002faf7a0 'UISV-alignment' UIStackView:0x7fcdda716770.top == UIStackView:0x7fcdda7291d0.top   (active)>",
    "<NSLayoutConstraint:0x600002f84dc0 'UISV-canvas-connection' UILayoutGuide:0x6000035a8620'UIViewLayoutMarginsGuide'.top == UILabel:0x7fcdda716900.top   (active)>",
    "<NSLayoutConstraint:0x600002f8dfe0 'UISV-spacing' V:[UIImageView:0x7fcdda7170c0]-(2)-[UIStackView:0x7fcdda7291d0]   (active)>",
    "<NSLayoutConstraint:0x600002f84e60 'UISV-spacing' V:[UILabel:0x7fcdda716900]-(2)-[UIImageView:0x7fcdda7170c0]   (active)>",
    "<NSLayoutConstraint:0x600002f84c80 'UIView-topMargin-guide-constraint' V:|-(20)-[UILayoutGuide:0x6000035a8620'UIViewLayoutMarginsGuide']   (active, names: '|':UIStackView:0x7fcdda716770 )>"
)

I assume, ew. They don't really expect me to deal with this garbage... But maybe that is all we get and maybe it isn't so bad.

Lets break it somewhat down, the codes refer to ids of constraints. You can actually find out which constraint they correspond to by simply printing out your constraints like so:

print("my Constraint", ibOutletConstraint)

You'll get:

my Constraint Optional(<NSLayoutConstraint:0x600001625a90 UIStackView:0x7fdca0c0d1f0.width == 2*UIStackView: 0x7fcdda716770.width   (inactive)>)

Now I can grab: 0x7fcdda716770 and search my output to match it to the constraint in question. Its tedious and certainly not glamorous, but it is a way to step by step break it down. I think Apple should really make it clearer which constraint is having an issue.

Renaldo