自动布局约束抛出异常

Question

我在我加载到表视图单元格的字段上使用自动布局。对于每个单元格，我将清除contentView中的所有子视图，并根据所需字段的类型加载我自己的视图。大多数细胞都很好，但是当这个细胞出现时，我会得到一个异常。

这里是小区的代码：

UILabel *questionText = [[UILabel alloc] init]; 
questionText.text = self.field.questionText; 
questionText.font = font; 
questionText.numberOfLines = 0; 
questionText.adjustsFontSizeToFitWidth = NO; 
questionText.lineBreakMode = NSLineBreakByWordWrapping; 
[questionText setTranslatesAutoresizingMaskIntoConstraints:NO]; 
[self.contentView addSubview:questionText]; 

UIView *listContainer = [[UIView alloc] init]; 
listContainer.backgroundColor = [UIColor whiteColor]; 
listContainer.layer.cornerRadius = 8; 
listContainer.layer.borderWidth = 1.0; 
[listContainer setTranslatesAutoresizingMaskIntoConstraints:NO]; 
[self.contentView addSubview:listContainer]; 

// Get the views dictionary 
NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings(questionText, listContainer); 

//Create the constraints using the visual language format 
NSString *format = @"V:|-10-[questionText]-10-[listContainer]-10-|"; 
NSArray *constraintsArray = [NSLayoutConstraint constraintsWithVisualFormat:format options:NSLayoutFormatAlignAllLeft metrics:nil views:viewsDictionary]; 
[self.contentView addConstraints:constraintsArray]; 

format = @"|-10-[questionText]-10-|"; 
constraintsArray = [NSLayoutConstraint constraintsWithVisualFormat:format options:NSLayoutFormatAlignAllLeft metrics:nil views:viewsDictionary]; 
[self.contentView addConstraints:constraintsArray]; 

format = @"|-10-[listContainer]-10-|"; 
constraintsArray = [NSLayoutConstraint constraintsWithVisualFormat:format options:NSLayoutFormatAlignAllLeft metrics:nil views:viewsDictionary]; 
[self.contentView addConstraints:constraintsArray]; 


ToggleButton *prevCheckbox; 
ToggleButton *checkbox; 

for (AssessmentAnswer *answer in self.field.answers) 
{ 
    checkbox = [[ToggleButton alloc] initWithText:answer.text ofType:ToggleButtonTypeCheckbox]; 
    [checkbox setTranslatesAutoresizingMaskIntoConstraints:NO]; 
    [listContainer addSubview:checkbox]; 


    viewsDictionary = NSDictionaryOfVariableBindings(checkbox); 
    format = @"|-10-[checkbox]-10-|"; 
    constraintsArray = [NSLayoutConstraint constraintsWithVisualFormat:format options:NSLayoutFormatAlignAllCenterY metrics:nil views:viewsDictionary]; 
    [listContainer addConstraints:constraintsArray]; 

    if (prevCheckbox) 
    { 
     // This is not the first item 
     [listContainer addConstraint:[NSLayoutConstraint constraintWithItem:checkbox attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:prevCheckbox attribute:NSLayoutAttributeBottom multiplier:1.0 constant:20.0]]; 
    } 
    else 
    { 
     // This is the first item 
     [listContainer addConstraint:[NSLayoutConstraint constraintWithItem:checkbox attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:listContainer attribute:NSLayoutAttributeTop multiplier:1.0 constant:10.0]]; 
    } 

    prevCheckbox = checkbox; 
} 

if (checkbox) 
{ 
    // This is the last checkbox 
    [listContainer addConstraint:[NSLayoutConstraint constraintWithItem:listContainer attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:checkbox attribute:NSLayoutAttributeBottom multiplier:1.0 constant:10.0]]; 
} 

在这种情况下，存在self.field.answers只有一个项目。字体与下面使用的值相同。

这里是单元格高度的代码：

UIFont *font = [UIFont fontWithName:@"Helvetica" size:14.0]; 

//Find height of question text 
CGFloat questionTextHeight = [field.questionText sizeWithFont:font constrainedToSize:CGSizeMake(width - (2 * PAD_X), CGFLOAT_MAX) lineBreakMode:NSLineBreakByWordWrapping].height; 

//Find height of each row 
CGFloat listHeight = 0.0; 

for (AssessmentAnswer *answer in field.answers) 
{ 
    listHeight += MAX([answer.text sizeWithFont:font constrainedToSize:CGSizeMake(width - 43.0 - [ToggleButton getImageWidth], CGFLOAT_MAX) lineBreakMode:NSLineBreakByWordWrapping].height, [ToggleButton getImageHeight]) + 20.0; 
} 

// Question text height + list height + spacers 
return questionTextHeight + listHeight + 30.0; 

下面是创建一个切换按钮，这是一个UIButton的内部代码：

for (UIView *subview in self.subviews) 
{ 
    [subview removeFromSuperview]; 
} 

// Add the toggle button image 
self.toggleImgView = [[UIImageView alloc] init]; 
[self.toggleImgView setTranslatesAutoresizingMaskIntoConstraints:NO]; 
[self addSubview:self.toggleImgView]; 

// Add the label 
self.toggleLabel = [[UILabel alloc] init]; 
self.toggleLabel.font = [UIFont fontWithName:@"Helvetica" size:14.0f]; 
self.toggleLabel.numberOfLines = 0; 
self.toggleLabel.adjustsFontSizeToFitWidth = NO; 
self.toggleLabel.lineBreakMode = NSLineBreakByWordWrapping; 
self.toggleLabel.backgroundColor = [UIColor clearColor]; 
[self.toggleLabel setTranslatesAutoresizingMaskIntoConstraints:NO]; 
[self addSubview:self.toggleLabel]; 

// Get the views dictionary 
NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings(_toggleImgView, _toggleLabel); 

//Create the constraints using the visual language format 
NSString *format = [NSString stringWithFormat:@"|-0-[_toggleImgView(%f)]-3-[_toggleLabel]-0-|", IMAGE_WIDTH]; 
NSArray *constraintsArray = [NSLayoutConstraint constraintsWithVisualFormat:format options:nil metrics:nil views:viewsDictionary]; 
[self addConstraints:constraintsArray]; 

viewsDictionary = NSDictionaryOfVariableBindings(_toggleLabel); 
format = [NSString stringWithFormat:@"V:|-0-[_toggleLabel(>=%f)]-0-|", IMAGE_HEIGHT]; 
constraintsArray = [NSLayoutConstraint constraintsWithVisualFormat:format options:NSLayoutFormatAlignAllLeft metrics:nil views:viewsDictionary]; 
[self addConstraints:constraintsArray]; 


[self addConstraint:[NSLayoutConstraint constraintWithItem:self.toggleImgView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:self.toggleImgView attribute:NSLayoutAttributeWidth multiplier:1.0 constant:0.0]]; 

[self addConstraint:[NSLayoutConstraint constraintWithItem:self.toggleImgView attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.toggleLabel attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0.0]]; 

这里是例外，读出我得到：

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:0x1f28c4f0 V:[UILabel:0x1f288f10(>=27)]>", 
    "<NSLayoutConstraint:0x1f291460 V:[ToggleButton:0x1f068e50]-(10)-| (Names: '|':UIView:0x1f0182a0)>", 
    "<NSLayoutConstraint:0x1f293830 V:[UILabel:0x1f288f10]-(0)-| (Names: '|':ToggleButton:0x1f068e50)>", 
    "<NSLayoutConstraint:0x1f28b480 V:|-(10)-[UILabel:0x1f062890] (Names: '|':UITableViewCellContentView:0x1f1c23d0)>", 
    "<NSLayoutConstraint:0x1f29b070 V:|-(10)-[ToggleButton:0x1f068e50] (Names: '|':UIView:0x1f0182a0)>", 
    "<NSLayoutConstraint:0x1f289810 V:[UIView:0x1f0182a0]-(10)-| (Names: '|':UITableViewCellContentView:0x1f1c23d0)>", 
    "<NSLayoutConstraint:0x1f292eb0 V:|-(0)-[UILabel:0x1f288f10] (Names: '|':ToggleButton:0x1f068e50)>", 
    "<NSLayoutConstraint:0x1f28dc90 V:[UILabel:0x1f062890]-(10)-[UIView:0x1f0182a0]>", 
    "<NSAutoresizingMaskLayoutConstraint:0x1f1e5ed0 h=--& v=--& V:[UITableViewCellContentView:0x1f1c23d0(60)]>" 
) 

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x1f293830 V:[UILabel:0x1f288f10]-(0)-| (Names: '|':ToggleButton:0x1f068e50)> 

Break on objc_exception_throw to catch this in the debugger. 
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful. 

我不明白为什么我得到例外，因为所有的高度加起来。这种情况发生在其他几种细胞类型中，因此任何见解都会有所帮助。

Answer 1

错误消息非常清楚地告诉你，它不能满足约束，要求在ToggleButton内的UILabel的高度大于或等于27。据推测，在布局过程中的某个时刻，在保持其他垂直间距规则的同时，没有空间做到这一点。

有趣的是，我实际上没有请参阅关于代码中UILabel高度的约束;也许你正在把它放在笔尖上？如果是这样，请尝试降低优先级，或者设置一个插座并在加载笔尖后将其移除。这里有点神秘，因为你没有解释什么是ToggleButton。但它内部似乎确实有一个UILabel，而且UILabel确实似乎有一个高度不平等，这是造成麻烦的原因。