更新 UICollectionViewCell 中的高度约束

Question

我有一个类似 Facebook 的博客 post 带有标题、可选图像和文本的视图控制器。

我正在尝试更新图像的高度限制，以便标题和文本之间没有 space，如下所示：

 func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "blogPostCell", for: indexPath) as? BlogPostCell else{
        fatalError("Not an instance of BlogPostCell")
    }
    cell.headlineLabel.text = blogPosts[indexPath.row].headline
    cell.postTextLabel.text = blogPosts[indexPath.row].content

    cell.postImage.translatesAutoresizingMaskIntoConstraints = false
    let cellHeigtConstraint = cell.postImage.heightAnchor.constraint(equalToConstant: 0)

    if let img = blogPosts[indexPath.row].image{
        cell.postImage.contentMode = .scaleAspectFit
        cellHeigtConstraint.isActive = false
        cellHeigtConstraint.constant = self.view.frame.width*0.75
        cellHeigtConstraint.isActive = true
        cell.postImage.kf.indicatorType = .activity
        let url = URL(string: App.rootdir + "/assets/uploads/" + img)
        cell.postImage.kf.setImage(with: url)
    } else{
        cellHeigtConstraint.isActive = false
        cellHeigtConstraint.constant = 0
        cellHeigtConstraint.isActive = true
    }
    return cell
}

所以我试图停用约束，然后更改它，然后再次激活它。但是我似乎有两个约束而不是更新的约束：

2018-01-15 14:20:44.317167+0100 docprice[86466:2062563] [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. 
(
    "<NSLayoutConstraint:0x60800009cc00 UIImageView:0x7f80dd53e280.height == 0   (active)>",
    "<NSLayoutConstraint:0x60c000289970 UIImageView:0x7f80dd53e280.height == 310.5   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x60c000289970 UIImageView:0x7f80dd53e280.height == 310.5   (active)>

我猜是因为单元格出列，我无法像这样解决约束。但我看不到另一种实现方式。

Answer 1

首先，你应该这样做：

if let img = blogPosts[indexPath.row].image{
        cell.postImage.contentMode = .scaleAspectFit
        cellHeigtConstraint.constant = self.view.frame.width*0.75
        cell.postImage.kf.indicatorType = .activity
        let url = URL(string: App.rootdir + "/assets/uploads/" + img)
        cell.postImage.kf.setImage(with: url)
    } else{
        cellHeigtConstraint.constant = 0
    }

cell.layoutIfNeeded()

那么我建议每个单元格都有自己的 cellHeigtConstraint 并且它不是所有单元格的一个。它应该是属性 inside you BlogPostCell class.

所以最后你应该cell.cellHeigtConstraint.constant = 0

更新 UICollectionViewCell 中的高度约束

Update Height Constraint in a UICollectionViewCell

nslayoutconstraint

uicollectionviewcell

swift