ios - iOS8 上的自动布局错误，但 iOS9 上没有

Question

我有一段代码可以在 UITableView 的页眉中设置自动布局。此代码在 iOS9 上运行良好，但在 iOS8 上它引发了一个 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;
}

辅助方法有:

-(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
             };
}

引发的异常是:

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 )>

查看自动布局的文档，我没有发现从 iOS8 到 iOS9 的任何更改可能会影响此代码。我错过了什么吗？

Answer 1

你的问题不是约束本身。至少在您提供给我们的那部分代码中。

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

这种类型的约束由系统添加，一旦 View 已经设置，它们就会被删除。我的猜测是您在代码中的某处强制进行了布局传递(例如使用 layoutIfNeeded)。如果是这种情况，请将其删除。请改用 setNeedsLayout。该方法会将 View 标记为 MUST_BE_LAYOUTED_IN_THE_NEXT_LAYOUT_PASS 但不会在准备就绪之前强制执行。