ios - UITableViewAutomaticDimension 的约束错误

Question

我很难处理动态高度自定义 tableView 单元格。

所以我得到了“无法同时满足约束条件。”警告(但只是有时，就像千分之一)

---

我认为 UITableViewAutomaticDimension 给出了问题。在示例中，它期望高度为 88.3333，但它可能不正确。 (但大部分时间都有效，我不明白发生了什么)

我做错了什么，有人可以帮忙吗？什么都试过了..

代码:

var cellHeights: [IndexPath : CGFloat] = [:]
    func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
        cellHeights[indexPath] = cell.frame.size.height

        if indexPath.row == UserWorldMessagesStore.shared.worldMessages.count - 1 && userWorldMessagesCanLoadMore == true {
            loadOlderOwnWorldMessages()
        }
    }
    func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
        return UITableViewAutomaticDimension
    }
    func tableView(_ tableView: UITableView, estimatedHeightForRowAt indexPath: IndexPath) -> CGFloat {
        if cellHeights[indexPath] != nil {
            return CGFloat(Float(cellHeights[indexPath] ?? 0.0))
        }
        else {
            return UITableViewAutomaticDimension
        }
    }

警告:

2018-05-19 19:58:54.879876+0200 PipeTest[3378:1282243] [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:0x1c0297fc0 V:|-(10)-[UILabel:0x125d5db40'Heyy']   (active, names: '|':UIView:0x125d03400 )>",
    "<NSLayoutConstraint:0x1c0298010 V:[UILabel:0x125d5db40'Heyy']-(10)-|   (active, names: '|':UIView:0x125d03400 )>",
    "<NSLayoutConstraint:0x1c0298100 PipeTest.profilePictureImageView:0x125d56fc0.height == 22   (active)>",
    "<NSLayoutConstraint:0x1c0298240 UIView:0x125e58150.height == 27   (active)>",
    "<NSLayoutConstraint:0x1c0298600 PipeTest.profilePictureImageView:0x125d56fc0.top == UITableViewCellContentView:0x125d50850.topMargin   (active)>",
    "<NSLayoutConstraint:0x1c02986a0 V:[PipeTest.profilePictureImageView:0x125d56fc0]-(3)-[UIView:0x125d03400]   (active)>",
    "<NSLayoutConstraint:0x1c02988d0 UIView:0x125e58150.bottom == UITableViewCellContentView:0x125d50850.bottomMargin   (active)>",
    "<NSLayoutConstraint:0x1c0298970 V:[UIView:0x125d03400]-(7)-[UIView:0x125e58150]   (active)>",
    "<NSLayoutConstraint:0x1c0299230 'UIView-Encapsulated-Layout-Height' UITableViewCellContentView:0x125d50850.height == 88.3333   (active)>"
)

Answer 1

完整的警告文本应该包括类似

的内容

Will attempt to recover by breaking constraint  
<NSLayoutConstraint:somePointer V:some constraint>

将该约束的优先级降低到 999 应该会使此警告消失。

原因就在那一行:

<NSLayoutConstraint:0x1c0299230 'UIView-Encapsulated-Layout-Height' UITableViewCellContentView:0x125d50850.height == 88.3333   (active)>

这是一个系统提供的约束，它告诉表格 View 单元格的高度应该是多少，它的值基于您从 estimatedHeightForRowAt 返回的值。在计算正确的像元大小(最终会正确计算)之前会抛出警告，因此只需降低一些其他约束的优先级是安全的，因此系统提供的约束优先。