为什么我的spring-label-label-spring自动布局约束不起作用？

Question

我想在它的容器内水平居中标签，使用UIView作为弹簧的双方。

不幸的是，系统告诉它可以满足所有约束，因此错误的结果。但我可以想出一个解决方案。

我的代码如下：

func addSpringConstraints(items: [UIView], mainView: UIView, spacing: CGFloat) { 

    let pad = spacing/2 

    var constraints = [NSLayoutConstraint]() 


    // Add two spacer to serve as springs 
    let lSpring = UIView() 
    lSpring.frame.size.width = 50 
    //lSpring.hidden = true 
    lSpring.backgroundColor = UIColor.redColor().colorWithAlphaComponent(0.5) 
    lSpring.translatesAutoresizingMaskIntoConstraints = false 
    addSubview(lSpring) 

    let rSpring = UIView() 
    rSpring.frame.size.width = lSpring.frame.size.width 
    //rSpring.hidden = true 
    rSpring.backgroundColor = UIColor.blueColor().colorWithAlphaComponent(0.5) 
    rSpring.translatesAutoresizingMaskIntoConstraints = false 
    addSubview(rSpring) 


    // Constraint left spring 
    constraints.append(NSLayoutConstraint(item: lSpring, attribute: .Top, relatedBy: .Equal, toItem: mainView, attribute: .Top, multiplier: 1, constant: 0)) 
    constraints.append(NSLayoutConstraint(item: lSpring, attribute: .Bottom, relatedBy: .Equal, toItem: mainView, attribute: .Bottom, multiplier: 1, constant: 0)) 
    constraints.append(NSLayoutConstraint(item: lSpring, attribute: .Left, relatedBy: .Equal, toItem: mainView, attribute: .Left, multiplier: 1, constant: 0)) 


    // Constraint right spring 
    constraints.append(NSLayoutConstraint(item: rSpring, attribute: .Top, relatedBy: .Equal, toItem: mainView, attribute: .Top, multiplier: 1, constant: 0)) 
    constraints.append(NSLayoutConstraint(item: rSpring, attribute: .Bottom, relatedBy: .Equal, toItem: mainView, attribute: .Bottom, multiplier: 1, constant: 0)) 
    constraints.append(NSLayoutConstraint(item: rSpring, attribute: .Right, relatedBy: .Equal, toItem: mainView, attribute: .Right, multiplier: 1, constant: 0)) 


    for (index, segment) in items.enumerate() { 

     // Add top and bottom constraints 

     constraints.append(NSLayoutConstraint(item: segment, attribute: .Top, relatedBy: .Equal, toItem: mainView, attribute: .Top, multiplier: 1, constant: 0)) 
     constraints.append(NSLayoutConstraint(item: segment, attribute: .Bottom, relatedBy: .Equal, toItem: mainView, attribute: .Bottom, multiplier: 1, constant: 0)) 


     // Add left constraint if any and width constraint 

     if index == 0 { 
      let prev = lSpring   // first label is attached to the left spring 
      constraints.append(NSLayoutConstraint(item: segment, attribute: .Left, relatedBy: .Equal, toItem: prev, attribute: .Right, multiplier: 1, constant: 0)) 
     } 
     else { 
      let prev = items[index-1] // label attached to the previous label by its left side 
      constraints.append(NSLayoutConstraint(item: segment, attribute: .Left, relatedBy: .Equal, toItem: prev, attribute: .Right, multiplier: 1, constant: pad)) 

      // Same width than first segment 
      constraints.append(NSLayoutConstraint(item: segment, attribute: .Width, relatedBy: .Equal, toItem: items.first, attribute: .Width, multiplier: 1, constant: 0)) 
     } 


     // Add right constraints if any 

     if index == items.count - 1 { 
      let next = rSpring   // last label is attached to the right spring 
      constraints.append(NSLayoutConstraint(item: segment, attribute: .Right, relatedBy: .Equal, toItem: next, attribute: .Left, multiplier: 1, constant: 0)) 
     } 
     else { 
      let next = items[index+1] // label attached to the next label by its right side 
      constraints.append(NSLayoutConstraint(item: segment, attribute: .Right, relatedBy: .Equal, toItem: next, attribute: .Left, multiplier: 1, constant: -pad)) 
     } 
    } 

    // Add all constraints to the view 
    mainView.addConstraints(constraints) 
} 

这里是春天的问题 - “PHOTO” - “实况视频” - 春：

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. 
    (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSAutoresizingMaskLayoutConstraint:0x14ddf0d60 h=--& v=--& H:[Visage.ZEENSUISegmentedControl:0x14df01300(0)]>", 
    "<NSLayoutConstraint:0x14dec1490 H:|-(0)-[UIView:0x14debe390](LTR) (Names: '|':Visage.ZEENSUISegmentedControl:0x14df01300)>", 
    "<NSLayoutConstraint:0x14deaabb0 UIView:0x14deaaa40.right == Visage.ZEENSUISegmentedControl:0x14df01300.right>", 
    "<NSLayoutConstraint:0x14deb7780 H:[UIView:0x14debe390]-(0)-[UILabel:0x14df3af70'PHOTO'](LTR)>", 
    "<NSLayoutConstraint:0x14deaada0 UILabel:0x14df3af70'PHOTO'.right == UILabel:0x14debe9e0'LIVE VIDEO'.left - 22.5>", 
    "<NSLayoutConstraint:0x14dea5930 UILabel:0x14debe9e0'LIVE VIDEO'.width == UILabel:0x14df3af70'PHOTO'.width>", 
    "<NSLayoutConstraint:0x14dea5980 UILabel:0x14debe9e0'LIVE VIDEO'.right == UIView:0x14deaaa40.left>" 
) 

Answer 1

OK，我发现这个问题，通过进一步试验。

的事情是，间隔必须为“春天”的行为相同的尺寸（实际宽度），从而，所以我增加了额外的约束如下：

// Constraint right spring 
    constraints.append(NSLayoutConstraint(item: lSpring, attribute: .Width, relatedBy: .Equal, toItem: rSpring, attribute: .Width, multiplier: 1, constant: 0)) 

事实证明，这必须设置额外的限制，以限制只有一个自由度（左右均匀空间）。如果我们不添加这个约束，负责解决约束的底层代码有多种解决方案：其中包括left = 0或right = 0。