设置UIImageView的高度时AutoLayout失败

Question

我正在尝试使用约束创建带图像和标签的自动调整大小的UITableViewCell。当我为图像定义高度时，某些单元格的标签高度设置不正确，为什么会这样？

下图显示了标签高度不正确且高度正确的单元格。

带有约束的自定义单元格的代码是：

import UIKit

class NewsItemCell : UITableViewCell {

   var msg: UILabel
   var img: UIImageView!

   required init(coder aDecoder: NSCoder) {

      msg = UILabel()
      msg.numberOfLines = 0
      msg.font = UIFont.preferredFontForTextStyle(UIFontTextStyleBody)
      msg.lineBreakMode = .ByWordWrapping

      img = UIImageView()
      img.contentMode = .ScaleAspectFill
      img.clipsToBounds = true

      super.init(coder: aDecoder)
  }

  override func awakeFromNib() {

      super.awakeFromNib()

      addSubview(msg)
      addSubview(img)

      contentView.setTranslatesAutoresizingMaskIntoConstraints(false)
      msg.setTranslatesAutoresizingMaskIntoConstraints(false)
      img.setTranslatesAutoresizingMaskIntoConstraints(false)

      let c0 = NSLayoutConstraint(item: img, attribute: .Top,  relatedBy: .Equal, toItem: self, attribute: .Top, multiplier: 1, constant: 0);
      let c1 = NSLayoutConstraint(item: img, attribute: .Leading,  relatedBy: .Equal, toItem: self, attribute: .Leading, multiplier: 1, constant: 0)
      let c2 = NSLayoutConstraint(item: img, attribute: .Trailing, relatedBy: .Equal, toItem: self, attribute: .Trailing, multiplier: 1, constant: 0)
      let c3 = NSLayoutConstraint(item: msg, attribute: .Top,  relatedBy: .Equal, toItem: img, attribute: .Bottom, multiplier: 1, constant: 5)
      let c4 = NSLayoutConstraint(item: msg, attribute: .Bottom,  relatedBy: .Equal, toItem: self, attribute: .Bottom, multiplier: 1, constant: -5)
      let c5 = NSLayoutConstraint(item: img, attribute: .Height,  relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 120)
      let c6 = NSLayoutConstraint(item: msg, attribute: .Trailing,  relatedBy: .Equal, toItem: self, attribute: .Trailing, multiplier: 1, constant:-5)
      let c7 = NSLayoutConstraint(item: msg, attribute: .Leading,  relatedBy: .Equal, toItem: self, attribute: .Leading, multiplier: 1, constant:5)

      addConstraint(c0)
      addConstraint(c1)
      addConstraint(c2)
      addConstraint(c3)
      addConstraint(c4)
      addConstraint(c5)
      addConstraint(c6)
      addConstraint(c7)
  }

  override func setSelected(selected: Bool, animated: Bool) {
      super.setSelected(selected, animated: animated)
  }
}

Answer 1

啊！那么你看，问一半解决方案^。^

事实证明我正在更新错误的委托功能中的msg.text值。我在msg.text设置了willDisplayCell，但我必须在cellForRowAtIndexPath中设置它。