ios - 布局约束不起作用和/或失败

Question

一段时间以来，我一直在尝试让这个刷新控件居中，但我似乎无法让约束起作用。我在这里完全错过了什么吗？我阅读了一些教程，并阅读了另一个问题，通常您会向父对象添加位置约束。当我尝试将约束添加到刷新控件时，它告诉我它无法满足约束。

// Set up the refresh control and add it to the tableView
self.refreshControl.attributedTitle = NSAttributedString(string: "Pull to refresh")
self.refreshControl.addTarget(self, action: "refresh:", forControlEvents: UIControlEvents.ValueChanged)
self.tableView.addSubview(refreshControl)

// Center the refreshControl
var centerControl: NSLayoutConstraint = NSLayoutConstraint(
 item: refreshControl,
 attribute: NSLayoutAttribute.CenterX, 
 relatedBy: NSLayoutRelation.Equal, 
 toItem: tableView, 
 attribute: NSLayoutAttribute.CenterX, multiplier: 1.0, constant: 1.0)
tableView.addConstraint(centerControl)

编辑:我试图按照建议设置 refreshControl 的大小，它仍然显示在右侧，就像它像 Any Height Any Width View Controller 一样宽 600。所以显然尝试设置大小没有任何改变

更新代码:

override func viewDidLoad() {
super.viewDidLoad()

// Set tableView delegate and dataSource
tableView.delegate = self
tableView.dataSource = self

// Set up the refresh control and add it to the tableView
self.refreshControl = UIRefreshControl(frame: CGRect(x: 0, y: 0, width: view.bounds.width, height: 40))
self.refreshControl.attributedTitle = NSAttributedString(string: "Pull to refresh")
self.refreshControl.addTarget(self, action: "refresh:", forControlEvents: UIControlEvents.ValueChanged)
self.tableView.addSubview(refreshControl)

// Center the refreshControl
let centerControl: NSLayoutConstraint = NSLayoutConstraint(
  item: refreshControl, 
  attribute: NSLayoutAttribute.CenterX, 
  relatedBy: NSLayoutRelation.Equal, 
  toItem: refreshControl.superview, 
  attribute: NSLayoutAttribute.CenterX, multiplier: 1.0, constant: 0)
refreshControl.superview?.addConstraint(centerControl)

代码更新#2:我已经试过了，但也没有用。我应该有一个 tableViewController 来管理 refreshControl 吗？当我添加任何这些约束(甚至是单独添加)时，刷新控件会随着表格一起被拖下。当我不添加约束时，它会保持不变。我只想得到它，这样图像和文字就不会一直偏到右边!否则它会起作用。

override func viewDidLoad() {
super.viewDidLoad()

// Set tableView delegate and dataSource
tableView.delegate = self
tableView.dataSource = self

// Set up the refresh control and add it to the tableView
refreshControl = UIRefreshControl()
refreshControl.attributedTitle = NSAttributedString(string: "Pull to refresh")
refreshControl.addTarget(self, action: "refresh:", forControlEvents: UIControlEvents.ValueChanged)
tableView.addSubview(refreshControl)
refreshControl.setTranslatesAutoresizingMaskIntoConstraints(false)

// Create the constraints and add them
var constX: NSLayoutConstraint =
  NSLayoutConstraint(item: refreshControl,
                attribute: NSLayoutAttribute.CenterX,
                relatedBy: NSLayoutRelation.Equal,
                   toItem: tableView,
                attribute: NSLayoutAttribute.CenterX,
               multiplier: 1.0, constant: 0)
tableView.addConstraint(constX)

var constW: NSLayoutConstraint =
  NSLayoutConstraint(item: refreshControl,
                attribute: NSLayoutAttribute.Width,
                relatedBy: NSLayoutRelation.Equal,
                   toItem: tableView,
                attribute: NSLayoutAttribute.Width,
               multiplier: 1.0, constant: 0)
tableView.addConstraint(constW)

var constH: NSLayoutConstraint =
  NSLayoutConstraint(item: refreshControl,
                attribute: NSLayoutAttribute.Height,
                relatedBy: NSLayoutRelation.Equal,
                   toItem: nil,
                attribute: NSLayoutAttribute.NotAnAttribute,
               multiplier: 1.0, constant: 50)
tableView.addConstraint(constH)

}

Answer 1

尝试禁用自动调整掩码约束:

self.refreshControl.setTranslatesAutoresizingMaskIntoConstraints(false)