DispatchQueue.main中的自動布局不起作用

Question

我對自動版式有疑問。

背景=>我想在UITableViewCell中排列UIImageView，使UIImageView異步獲取圖像，並通過AutoLayout使用圖像的縱橫比動態更改單元格的高度。 其中，UIImageView在我的自定義UIView之上，而后者又在Cell的ContentView上。

AutoLayout與此代碼配合良好，沒有任何錯誤。

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
}

但是將DispatchQueue.main中的代碼用作異步任務（將在Main線程中執行），AutoLayout效果不佳。
這會導致[LayoutConstraints] Unable to simultaneously satisfy constraints錯誤。

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
  }
}

這就是錯誤消息（此錯誤消息僅在異步運行時出現）。

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)>

點
需要注意的一點是，如果您在UIImageView的HeihgtAnchor之后正常添加約束，它會按預期工作，但是異步添加約束將無法正常工作。

附加信息
* 桌面設置

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

Answer 1

您是否同時使用底部約束和高度？ 如果是這樣，那將行不通。

您可以嘗試通過移除UIImageView高度約束或UIImageView到UIView底部約束來進行嘗試。

注意：請勿在后台線程中使用與視圖相關的代碼。