是什么导致两个具有相同自动布局约束的 UITableViewCell 表现不同？

Question

我正在使用表格中的AFCollectionView，但它没有像我预期的那样呈现。为了调试问题，我创建了另一个具有所需布局的自定义表格单元格，并将其放入 AFCollectionView 单元格中。集合视图单元格仍未正确呈现。

这两个单元都是在没有 xib 或情节提要的情况下构建的，因此我正在努力尝试调试自动布局控制台问题。我知道 UICollectionView 比 UIView 更复杂，但鉴于显示的问题，我什至不确定从哪里开始尝试进行更改。

两个单元格都有以下从 - (id)initWithStyle:(UITableViewCellStyle)style reuseIdentifier:(NSString *)reuseIdentifier 方法内部调用的 updateConstraints 方法。

    [self updateConstraints];
    return self;
}

- (void)updateConstraints{
    [super updateConstraints];

    //Create local versions of the ivars
    UILabel *titleLabelP = self.titleLabel;
    UIView *bottomViewP = self.collectionView;
    NSNumber *bottomViewHeightP = [NSNumber numberWithInteger:self.bottomViewHeight];


    //Build the visual constraints
    NSDictionary *views = NSDictionaryOfVariableBindings(titleLabelP, bottomViewP);
    NSDictionary *metrics = @{ @"padding" : @8.0, @"viewHeight": bottomViewHeightP };

    // title and bottom view fill the width of the superview (cell content view)
    [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-padding-[titleLabelP]-padding-|" options:0 metrics:metrics views:views]];
    [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-padding-[bottomViewP]-padding-|" options:0 metrics:metrics views:views]];
    // title and bottom view are setup vertically with 8px of padding between.  The cell should expand to fit the full size of the bottom view.
    [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-padding-[titleLabelP]-padding-[bottomViewP(viewHeight)]-padding-|" options:0 metrics:metrics views:views]];

}

自动布局问题的控制台日志：

2016-11-21 14:19:06.423003 AutoLayoutTableCellTest[71645:4731377] [Warning] Warning once only: Detected a case where constraints ambiguously suggest a height of zero for a tableview cell's content view. We're considering the collapse unintentional and using standard height instead.
2016-11-21 14:19:06.425136 AutoLayoutTableCellTest[71645:4731377] [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. 
    (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSAutoresizingMaskLayoutConstraint:0x608000096710 h=--& v=--& UITableViewCellContentView:0x7fb87ac1a770.midY == 22.25   (active)>",
    "<NSAutoresizingMaskLayoutConstraint:0x6080000966c0 h=--& v=--& UITableViewCellContentView:0x7fb87ac1a770.height == 44.5   (active)>",
    "<NSLayoutConstraint:0x608000095860 V:|-(8)-[UILabel:0x7fb87ac1b670'CURRENT INDEXPATH ROW: 1']   (active, names: '|':AFTableViewCell:0x7fb87b050400'CellIdentifier' )>",
    "<NSLayoutConstraint:0x608000095900 AFIndexedCollectionView:0x7fb87b051000.height == 150   (active)>",
    "<NSLayoutConstraint:0x608000095950 V:[AFIndexedCollectionView:0x7fb87b051000]-(8)-|   (active, names: '|':UITableViewCellContentView:0x7fb87ac1a770 )>",
    "<NSLayoutConstraint:0x608000095fe0 V:[UILabel:0x7fb87ac1b670'CURRENT INDEXPATH ROW: 1']-(8)-[AFIndexedCollectionView:0x7fb87b051000]   (active)>",
    "<NSAutoresizingMaskLayoutConstraint:0x608000095180 h=-&- v=--& 'UIView-Encapsulated-Layout-Top' AFTableViewCell:0x7fb87b050400'CellIdentifier'.minY == 0   (active, names: '|':UITableViewWrapperView:0x7fb87b03b800 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x608000095fe0 V:[UILabel:0x7fb87ac1b670'CURRENT INDEXPATH ROW: 1']-(8)-[AFIndexedCollectionView:0x7fb87b051000]   (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.
2016-11-21 14:19:06.425934 AutoLayoutTableCellTest[71645:4731377] [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. 
    (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSAutoresizingMaskLayoutConstraint:0x608000096710 h=--& v=--& UITableViewCellContentView:0x7fb87ac1a770.midY == 22.25   (active)>",
    "<NSAutoresizingMaskLayoutConstraint:0x6080000966c0 h=--& v=--& UITableViewCellContentView:0x7fb87ac1a770.height == 44.5   (active)>",
    "<NSLayoutConstraint:0x608000095860 V:|-(8)-[UILabel:0x7fb87ac1b670'CURRENT INDEXPATH ROW: 1']   (active, names: '|':AFTableViewCell:0x7fb87b050400'CellIdentifier' )>",
    "<NSLayoutConstraint:0x6080000958b0 V:[UILabel:0x7fb87ac1b670'CURRENT INDEXPATH ROW: 1']-(8)-[AFIndexedCollectionView:0x7fb87b051000]   (active)>",
    "<NSLayoutConstraint:0x608000095900 AFIndexedCollectionView:0x7fb87b051000.height == 150   (active)>",
    "<NSLayoutConstraint:0x608000095950 V:[AFIndexedCollectionView:0x7fb87b051000]-(8)-|   (active, names: '|':UITableViewCellContentView:0x7fb87ac1a770 )>",
    "<NSAutoresizingMaskLayoutConstraint:0x608000095180 h=-&- v=--& 'UIView-Encapsulated-Layout-Top' AFTableViewCell:0x7fb87b050400'CellIdentifier'.minY == 0   (active, names: '|':UITableViewWrapperView:0x7fb87b03b800 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x6080000958b0 V:[UILabel:0x7fb87ac1b670'CURRENT INDEXPATH ROW: 1']-(8)-[AFIndexedCollectionView:0x7fb87b051000]   (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.

我已经把示例项目放到了github上@https://github.com/propstm/AutoLayoutTableCellTest

使用 COLLECTIONVIEW 编辑以显示单元格的初始化：

- (id)initWithStyle:(UITableViewCellStyle)style reuseIdentifier:(NSString *)reuseIdentifier
{
    if (!(self = [super initWithStyle:style reuseIdentifier:reuseIdentifier])) return nil;

    UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
    layout.sectionInset = UIEdgeInsetsMake(10, 10, 9, 10);
    layout.itemSize = CGSizeMake(100, 100);

    layout.scrollDirection = UICollectionViewScrollDirectionVertical;
    self.collectionView = [[AFIndexedCollectionView alloc] initWithFrame:CGRectZero collectionViewLayout:layout];
    [self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:CollectionViewCellIdentifier];
    [self.collectionView setTranslatesAutoresizingMaskIntoConstraints:FALSE];
    self.collectionView.backgroundColor = [UIColor whiteColor];
    self.collectionView.showsHorizontalScrollIndicator = NO;
    [self.contentView addSubview:self.collectionView];
    if(!self.titleLabel){
        self.titleLabel = [[UILabel alloc] initWithFrame:CGRectMake(16, 8, self.frame.size.width-8, 20)];
        [self.titleLabel setTranslatesAutoresizingMaskIntoConstraints:FALSE];
        UIFont *boldFont = [UIFont fontWithName:@".SFUIText-Bold" size:16.0f];
        [self.titleLabel setFont:boldFont];
        [self addSubview:self.titleLabel];
    }

    self.bottomViewHeight = 150;

    [self updateConstraints];
    return self;
}

Answer 1

通过以下更改，我能够消除这些自动布局警告：

• 在您的AFTableViewCell 的-initWithStyle:reuseIdentifier: 方法中，在返回之前，将单元格的contentView 的translatesAutoresizingMaskIntoConstraints 属性设置为NO。
• 将标题标签添加到单元格的内容视图，而不是单元格本身。

这会导致出现新的警告：

[Warning] Warning once only: Detected a case where constraints ambiguously suggest a height of zero for a tableview cell's content view. We're considering the collapse unintentional and using standard height instead.

但是，布局似乎更接近您的预期。

Answer 2

每当您以编程方式创建视图并设置自动布局约束时，您必须在添加约束的视图上设置translatesAutoresizingMaskIntoConstraints = false。如果没有，自动调整掩码将转换为自动布局约束。

在这种情况下，您需要添加类似：

self.titleLabel.translateAutoresizingMaskIntoConstraints = false
self.collectionView.translateAutoresizingMaskIntoConstraints = false

已添加

根据您添加的日志，collectionView 的高度为 150，但 contentView 的高度小于该高度。您必须返回正确的高度UITableViewCell。