iOS8上的自动布局错误，但iOS9上没有

Question

I have a code that set an Autolayout in a Header of an UITableView . This code is working fine on iOS9 but on iOS8 it raises an UIViewAlertForUnsatisfiableConstraints .

-(NSArray *)layoutConstraints
{
    NSMutableArray * result = [NSMutableArray array];

    NSDictionary * views = [self views];

    NSDictionary * metrics = [self metrics];

    [result addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-20-[section]-20-|"
                                                                        options:0
                                                                        metrics:nil
                                                                          views:views]];

    [result addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[section]|"
                                                                        options:0
                                                                        metrics:nil
                                                                          views:views]];


    [result addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-20-[subtitleLabel]-20-|"
                                                                        options:0
                                                                        metrics:nil
                                                                          views:views]];

    [result addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-20-[titleLabel]-20-|"
                                                                        options:0
                                                                        metrics:metrics
                                                                          views:views]];

    [result addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[sectionContainer]|"
                                                                 options:0
                                                                 metrics:nil
                                                                   views: views]];

    [result addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[sectionContainer(section_heigth)]-[subtitleLabel]-[titleLabel]-20-|"
                                                                              options:0
                                                                              metrics:metrics
                                                                                views:views]];

    return result;
}

The auxiliary methods are:

-(NSDictionary *)views
{
    return   @{   @"sectionContainer": self.sectionContainer,
                  @"section": self.section,
                  @"subtitleLabel": self.subtitleLabel,
                  @"titleLabel": self.titleLabel
                  };
}

-(NSDictionary *)metrics
{
    return  @{@"section_heigth" : @(HEIGHT_FOR_HEADER_IN_SECTION),
              @"margin"         :@20
             };
}

The exception raised is:

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) 
(
    "<NSLayoutConstraint:0x1b954150 H:|-(20)-[UILabel:0x12834d40'Ability to Round']   (Names: '|':_UITableViewHeaderFooterContentView:0x239a5510 )>",
    "<NSLayoutConstraint:0x1b954190 H:[UILabel:0x12834d40'Ability to Round']-(20)-|   (Names: '|':_UITableViewHeaderFooterContentView:0x239a5510 )>",
    "<NSLayoutConstraint:0x1b94f390 'UIView-Encapsulated-Layout-Width' H:[_UITableViewHeaderFooterContentView:0x239a5510(0)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x1b954190 H:[UILabel:0x12834d40'Ability to Round']-(20)-|   (Names: '|':_UITableViewHeaderFooterContentView:0x239a5510 )>

Looking into the documentation for Auto-layout I haven't found anything that changed from iOS8 to iOS9 that might impact this code. Am I missing something?

Answer 1

Your problem here are not the constraints itself. At least in that portion of code you provided us.

    "<NSLayoutConstraint:0x1b94f390 'UIView-Encapsulated-Layout-Width' H:[_UITableViewHeaderFooterContentView:0x239a5510(0)]>"

This type of constriants are added by the system, and they are removed once the view is already set up. My guess is that you are forcing a layout pass somewhere in your code (with layoutIfNeeded for example). If that's the case, remove that. Use setNeedsLayout instead. That method will mark the view as MUST_BE_LAYOUTED_IN_THE_NEXT_LAYOUT_PASS but will not force it before it's ready.