视图层次结构不准备

Question

我是一位新手快速开发人员，遵循一个好的iOS 9 Swift书中的代码示例。本书刚刚演示了如何在视图层次结构的两个不同分支中的两个对象（标签，按钮）之间添加约束（以代码形式）。

myLabel位于顶部视图控制器中的viewBB容器中。 myButton位于viewBB之外的顶部视图控制器中。

示例代码正常工作。但是对于练习，我想在viewBB中创建第二个label2，并将其位置限制为与myLabel偏移。所以这两个标签（我认为）都在viewBB中。

在代码中创建label2并在代码中添加约束可以正常工作。

但是......如果我在界面生成器中创建了label2，并尝试删除IB约束并将其替换为我自己的，那么构建成功，但应用程序崩溃时出现通常的“视图层次结构不准备”错误。

***有趣的是错误消息提到一个标签和一个按钮，暗示书本示例代码（我称之为下面的BLOCK 0）是罪犯。

我在做什么错？ （我读过的一切，我能找到的错误信息，包括在这个论坛4个员额但我不知所措知道为什么书代码失败。）

错误消息：

2016-02-25 12：52：32.796 TwoViewConstraints [5713：1860768] 未对约束准备视图层次结构：NSLayoutConstraint：0x7c9ce990 UILabel：0x7c9dbec0'Label'.centerX == UIButton：0x7c9deed0'Button'.centerX

当添加到视图中时，约束条目必须是该视图的后代（或视图本身）。如果在组装视图层次结构之前需要解析约束，这将会崩溃。打破 - [UIView（UIConstraintBasedLayout）_viewHierarchyUnpreparedForConstraint：]进行调试。从调试

消息：由于终止信号15

这里是我的代码：

class ViewController: UIViewController { 

@IBOutlet weak var myLabel: UILabel! 
@IBOutlet weak var myButton: UIButton! 
@IBOutlet weak var centerConstraint: NSLayoutConstraint! 
@IBOutlet weak var viewBB: UIView! 


@IBOutlet weak var lab2cxh: NSLayoutConstraint! 
@IBOutlet weak var lab2cxv: NSLayoutConstraint! 
@IBOutlet weak var label2: UILabel! 


override func viewDidLoad() { 
    super.viewDidLoad() 
    // Do any additional setup after loading the view, typically from a nib. 

    // BLOCK 0 - works fine, from the iOS9 book 
    // myLabel and myButton were created with the interface builder 
    // Goal is to remove an IB constraint, and replace it with one 
    // that is built in code. (Because label and buttons are in 
    // different branches of the view hierarchy). 
    // 
    // remove auto center constraint from parent label 
    viewBB.removeConstraint(centerConstraint) 

    // center the label and the button across views 
    let ccx = NSLayoutConstraint(item: myLabel, 
     attribute: NSLayoutAttribute.CenterX, 
     relatedBy: NSLayoutRelation.Equal, 
     toItem: myButton, 
     attribute: NSLayoutAttribute.CenterX, 
     multiplier: 1.0, 
     constant: 0) 
    self.view.addConstraint(ccx) 

//  BLOCK 1 - Create a second label object in code 
//  this code works fine with the two added constraints below 
//  create a second label object and add it to the view 
//  let label2 = UILabel() 
//  label2.translatesAutoresizingMaskIntoConstraints = false 
//  label2.text="Label2" 
//  viewBB.addSubview(label2) 

    // BLOCK 2 - create a second label object in the interface builder 
    // Suggested horiz/vert constraints are set in the IB. 
    // So now I want to remove them, and replace them with my own, 
    // as was done in the Swift book example code in BLOCK 0. 
    // remove original label 2 constraints 
    viewBB.removeConstraint(lab2cxv) 
    viewBB.removeConstraint(lab2cxh) 

    // adding these two constraints works fine when the label2 object 
    // is created in code. But when I create label2 in the interface 
    // builder and try to remove the IB constraints (as was done in the 
    // first block of code), I get the error: 
    // 
    // ... "The view hierarchy is not prepared 
    // for the constraint "Label.CenterX to Button.CenterX", which is 
    // the first block of code. 
    // 
    // NOTE** To me, it looks like the error is coming from the 
    // BLOCK 0 constraint, since it cites a label and a button, not 
    // two labels. 
    // What am I doing wrong? 
    let c2h = NSLayoutConstraint(item: label2, 
     attribute: NSLayoutAttribute.CenterY, 
     relatedBy: NSLayoutRelation.Equal, 
     toItem: myLabel, 
     attribute: NSLayoutAttribute.CenterY, 
     multiplier: 1.0, 
     constant: -50) 
    self.viewBB.addConstraint(c2h) 

    // constrain label 2 to be diagonally left of label one 
    let c2v = NSLayoutConstraint(item: label2, 
     attribute: NSLayoutAttribute.CenterX, 
     relatedBy: NSLayoutRelation.Equal, 
     toItem: myLabel, 
     attribute: NSLayoutAttribute.CenterX, 
     multiplier: 1.0, 
     constant: -100) 
    self.viewBB.addConstraint(c2v) 


} 

Answer 1

我做了整片的代码一遍，从无到有，和它的工作。去搞清楚。这是正常工作的代码。据我所知，它几乎是相同的代码，除了在这里和那里有一个小的名称更改。

我不知道为什么这个版本可以工作，而以前的版本没有。它让我想知道Interface Builder设置中是否有不同的东西。

class ViewController: UIViewController { 

@IBOutlet weak var viewbb: UIView! 
@IBOutlet weak var myLabel: UILabel! 
@IBOutlet weak var centerConstraint: NSLayoutConstraint! 
@IBOutlet weak var myButton: UIButton! 

@IBOutlet weak var label2: UILabel! 
@IBOutlet weak var label2centerConstraint: NSLayoutConstraint! 
@IBOutlet weak var label2Yconstraint: NSLayoutConstraint! 

override func viewDidLoad() { 
    super.viewDidLoad() 
    // Do any additional setup after loading the view, typically from a nib. 

    // remove the IB centering constraint, then add our own between label and button 
    viewbb.removeConstraint(centerConstraint) 

    // make a new constraint to center label over button 
    let newcon = NSLayoutConstraint(item: myLabel, 
     attribute: NSLayoutAttribute.CenterX, 
     relatedBy: NSLayoutRelation.Equal, 
     toItem: myButton, 
     attribute: NSLayoutAttribute.CenterX, 
     multiplier: 1.0, 
     constant: 0) 
    self.view.addConstraint(newcon) 

    // remove the IB centering constraint, then add our own between label and button 
    viewbb.removeConstraint(label2centerConstraint) 
    viewbb.removeConstraint(label2Yconstraint) 

    // make a new constraint to center label over button 
    let newcon2 = NSLayoutConstraint(item: label2, 
     attribute: NSLayoutAttribute.CenterX, 
     relatedBy: NSLayoutRelation.Equal, 
     toItem: myLabel, 
     attribute: NSLayoutAttribute.CenterX, 
     multiplier: 1.0, 
     constant: -100) // left of mylabel 

    // make a new constraint to center label over button 
    let newcon3 = NSLayoutConstraint(item: label2, 
     attribute: NSLayoutAttribute.CenterY, 
     relatedBy: NSLayoutRelation.Equal, 
     toItem: myLabel, 
     attribute: NSLayoutAttribute.CenterY, 
     multiplier: 1.0, 
     constant: 0) // left of mylabel 

    self.view.addConstraint(newcon3) 
    self.view.addConstraint(newcon2) 
}