Ios 未准备视图层次结构

我是一个新手swift开发人员，下面是一本很好的iOS9SWIFT书籍中的代码示例。本书刚刚演示了如何在视图层次的两个不同分支中的两个对象（标签、按钮）之间添加约束（在代码中）

myLabel位于viewBB容器中，该容器位于俯视图控制器中。 myButton位于俯视图控制器中，在viewBB之外

示例代码运行良好。但作为练习，我想在viewBB内部创建第二个label2，并将其位置约束为与myLabel偏移。所以这两个标签（我想）都在viewBB中

在代码中创建label2并在代码中添加约束效果良好

但是。。。如果我在interface builder中创建label2，并尝试删除IB约束并用我自己的约束替换它们，则构建成功，但应用程序崩溃，出现通常的“视图层次结构未准备好”错误

***有趣的是，错误消息谈到了一个标签和一个按钮，表明图书示例代码（我在下面称它为BLOCK 0）就是违规者

我做错了什么？（我已经阅读了错误信息中的所有内容，包括本论坛中的4篇帖子。但是我不知道为什么书的代码会失败。）

错误消息：

2016-02-25 12:52:32.796两个视图约束[5713:1860768] 未为约束准备视图层次结构：NSLayoutConstraint:0x7c9ce990 UILabel:0x7c9dbec0'Label'。centerX== UI按钮：0x7c9deed0“按钮”。centerX

添加到视图时，约束项必须是该视图（或视图本身）的后代。如果在装配视图层次之前需要解决约束，则此操作将崩溃。中断-[UIView（UIConstraintBasedLayout）\u viewHierarchyUnpreparedForConstraint:]以进行调试

来自调试器的消息：由于信号15而终止

这是我的密码：

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)


}

---

我从头开始重新编写了整段代码，它成功了。仔细想想，这是正确运行的代码。就我所见，它几乎是相同的代码，只是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有效，而上一个版本没有。这让我想知道Interface Builder的设置是否有所不同

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)
}

---

尝试使用

ccx.active=true

而不是

self.view.addConstraint（ccx）

。并为您的财产使用更好的名称。谢谢您回答我的问题。很抱歉，建议的更改无效。这行代码是直接从书中写出来的，在我将代码添加到下面之前，它工作得很好。（当然，cx=constraint、c=center、cxv=constraint-vertical等对任何人都没有多大帮助。）是否有什么让视图层次结构毫无准备的好的列表，或者我可以研究和遵循的约定？将所有代码移出viewDidLoad并移入VIEW将显示。视图层次结构在viewDidLoad中没有完全建立。我还尝试将两个新的约束添加到self.viewbb而不是self.view，但没有什么不同。代码仍然运行良好。（回想一下，viewbb是父顶级视图中的视图容器。viewbb容器包含两个标签视图，因此新约束可以放置在直接父视图（viewbb）或顶级视图（self.view）中。