ios - UITableViewCell 的 NSLayoutConstraint : Constraint must contain a first layout item

Question

我正在尝试在我的每个自定义单元格(继承自 UITableViewCell)中设置 NSLayoutConstraint，而不是在 UIViewController 中指定它们以通过公开每种单元格类型的约束来避免困惑。

问题是我找不到单元格生命周期的适当部分来创建布局约束(我尝试了 layoutSubviews() 和 awakeFromNib())，但是它们都在单元格的生命周期中过早地被调用，当它的 superview 还没有被定义时——它会导致这个错误:

Constraint must contain a first layout item

因为 UITableViewCell 没有像 UIViewController 那样的 viewDidLoad() 方法，如何在单元格内定义布局约束？

自定义 UITableViewCell 的

layoutSubviews():

- (void) layoutSubviews {
    self.translatesAutoresizingMaskIntoConstraints = false;
    [[NSLayoutConstraint constraintWithItem: self.attributeNameLabel
                                  attribute: NSLayoutAttributeLeading
                                  relatedBy: NSLayoutRelationEqual
                                     toItem: self.contentView
                                  attribute: NSLayoutAttributeLeading
                                 multiplier: 1
                                   constant: 3] setActive:true];
    [[NSLayoutConstraint constraintWithItem: self.attributeNameLabel
                                  attribute: NSLayoutAttributeTop
                                  relatedBy: NSLayoutRelationEqual
                                     toItem: self.contentView
                                  attribute: NSLayoutAttributeTop
                                 multiplier: 1
                                   constant: 3] setActive:true];
    [[NSLayoutConstraint constraintWithItem: self.attributeNameLabel
                                  attribute: NSLayoutAttributeRight
                                  relatedBy: NSLayoutRelationEqual
                                     toItem: self.contentView
                                  attribute: NSLayoutAttributeRight
                                 multiplier: 1
                                   constant: 3] setActive:true];
    [[NSLayoutConstraint constraintWithItem: self.attributeNameLabel
                                  attribute: NSLayoutAttributeBottom
                                  relatedBy: NSLayoutRelationEqual
                                     toItem: self.contentView
                                  attribute: NSLayoutAttributeBottom
                                 multiplier: 1
                                   constant: 3] setActive:true];   
}

UIViewController 中的初始化示例:

- (UITableViewCell *)tableView:(UITableView *)view cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    UITableViewCell *cell;

    if (indexPath.row % 2 == 0) {
        cell = [[CustomCell1 alloc] init];
    } else if (indexPath.row % 2 == 1) {
        cell = [[CustomCell2 alloc] init];
    } else if (...) {
        ...
    }
    return cell;
}

Answer 1

尝试制作

 self.attributeNameLabel.translatesAutoresizingMaskIntoConstraints = NO;

对于每个标签而不是 contentView

像对我有用的那样转换它们

这样做

  - (void)awakeFromNib {
// Initialization code



[super awakeFromNib];


UILabel*ddd = [UILabel new];

ddd.text = @"adsbcujklasdcbaksci";

ddd.translatesAutoresizingMaskIntoConstraints = NO ;

[self.contentView addSubview:ddd];

 NSLayoutConstraint*ss1 =  [NSLayoutConstraint constraintWithItem: ddd
                              attribute: NSLayoutAttributeCenterX
                              relatedBy: NSLayoutRelationEqual
                                 toItem: self.contentView
                              attribute: NSLayoutAttributeCenterX
                             multiplier: 1 constant: 3] ;
NSLayoutConstraint*ss2 =  [NSLayoutConstraint constraintWithItem: ddd
                              attribute: NSLayoutAttributeCenterY
                              relatedBy: NSLayoutRelationEqual
                                 toItem: self.contentView
                              attribute: NSLayoutAttributeCenterY
                             multiplier: 1   constant: 3]  ;

 [self.contentView addConstraints:@[ss1,ss2]];



 [self layoutSubviews];

 [self layoutIfNeeded];



}