Question

我对自动版式有疑问。

background =>我想在UITableViewCell中排列UIImageView，使UIImageView异步获取图像，并通过AutoLayout使用图像的纵横比动态更改单元格的高度。其中，UIImageView位于我的自定义UIView之上，而后者又位于Cell的ContentView上。

AutoLayout可以很好地使用此代码，而不会出现任何错误。

private var contentHeightConstraint = NSLayoutConstraint()

// this method called in constructor 
func setupAutolayout() {
  NSLayoutConstraint.activate([
    messageView.topAnchor.constraint(equalTo: contentView.topAnchor, constant: 8),
    messageView.rightAnchor.constraint(equalTo: contentView.rightAnchor, constant: 16),
    messageView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor, constant: -8),
    messageView.widthAnchor.constraint(equalToConstant: 240),


    contentImageView.topAnchor.constraint(equalTo: messageView.topAnchor),
    contentImageView.leftAnchor.constraint(equalTo: messageView.leftAnchor),
    contentImageView.rightAnchor.constraint(equalTo: messageView.rightAnchor),
    contentImageView.bottomAnchor.constraint(equalTo: messageView.bottomAnchor)
  ])
}

// After view loaded, this method called
func something() {
  contentHeightConstraint = contentImageView.heightAnchor.constraint(equalToConstant: 150)
  contentHeightConstraint.isActive = true
}

但是将DispatchQueue.main中的代码用作异步任务（将在Main线程中执行），AutoLayout不能很好地工作。
这会导致[LayoutConstraints] Unable to simultaneously satisfy constraints错误。

private var contentHeightConstraint = NSLayoutConstraint()

// this method called in constructor 
func setupAutolayout() {
  NSLayoutConstraint.activate([
    messageView.topAnchor.constraint(equalTo: contentView.topAnchor, constant: 8),
    messageView.rightAnchor.constraint(equalTo: contentView.rightAnchor, constant: 16),
    messageView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor, constant: -8),
    messageView.widthAnchor.constraint(equalToConstant: 240),


    contentImageView.topAnchor.constraint(equalTo: messageView.topAnchor),
    contentImageView.leftAnchor.constraint(equalTo: messageView.leftAnchor),
    contentImageView.rightAnchor.constraint(equalTo: messageView.rightAnchor),
    contentImageView.bottomAnchor.constraint(equalTo: messageView.bottomAnchor)
  ])
}

// After view loaded, this method called
func something() {
  DispatchQueue.main.async {
    // of course, exec in MainThread
    self.contentHeightConstraint = contentImageView.heightAnchor.constraint(equalToConstant: 150)
    self.contentHeightConstraint.isActive = true
  }
}

这是错误消息（此错误消息仅在异步运行时出现）。

2019-03-31 13:10:06.791758+0900 app[42515:4243718] [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:0x600002a3dbd0 V:|-(8)-[UIView:0x7fe775f3f830]   (active, names: '|':UITableViewCellContentView:0x7fe775f3c210 )>",
    "<NSLayoutConstraint:0x600002a3f1b0 UIView:0x7fe775f3f830.bottom == UITableViewCellContentView:0x7fe775f3c210.bottom - 8   (active)>",
    "<NSLayoutConstraint:0x600002a32a80 V:|-(0)-[UIImageView:0x7fe775f40690]   (active, names: '|':UIView:0x7fe775f3f830 )>",
    "<NSLayoutConstraint:0x600002a32350 UIImageView:0x7fe775f40690.bottom == UIView:0x7fe775f3f830.bottom   (active)>",
    "<NSLayoutConstraint:0x600002a2c190 UIImageView:0x7fe775f40690.height == 150   (active)>",
    "<NSLayoutConstraint:0x600002a2d310 'UIView-Encapsulated-Layout-Height' UITableViewCellContentView:0x7fe775f3c210.height == 116   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x600002a2c190 UIImageView:0x7fe775f40690.height == 150   (active)>

点
需要注意的一点是，如果您在UIImageView的HeihgtAnchor之后正常添加约束，它会按预期工作，但是异步添加约束将无法正常工作。

其他信息
*表格设置

table.estimatedRowHeight = 800
table.rowHeight = UITableView.automaticDimension

Answer 1

您是否同时使用底部约束和高度？如果是这样，那将行不通。

您可以尝试删除UIImageView的高度限制或将UIImageView的底部限制去除为UIView。

注意：不要在后台线程中使用与视图相关的代码。

DispatchQueue.main中的自动布局不起作用

1 个答案: