NSLayoutConstraints在ios7上崩潰而在ios8上崩潰

Question

我在UIViewController中設置了一些可以在ios8上正常工作的佈局約束。但只要我在ios7我已經得到了以下錯誤運行：

*** Assertion failure in -[UIView layoutSublayersOfLayer:], /SourceCache/UIKit_Sim/UIKit-2935.137/UIView.m:8803 

這裏是我的代碼：

class DatacenterIndicatorViewController: UIViewController { 

let sideMargins:Float = 12.0 
var dataCenterPollingLabel:UILabel = UILabel() 
var dataCenterAlarmLabel:UILabel = UILabel() 

//MARK: - Life cycle 
override func viewDidLoad() { 
    super.viewDidLoad() 
    self.view.addSubview(dataCenterPollingLabel) 
    self.view.addSubview(dataCenterAlarmLabel) 
} 

override func viewDidLayoutSubviews() { 
    super.viewDidLayoutSubviews() 
    self.reloadData() 
} 

func reloadData() { 
    self.setupAlarmLabel() 
    self.setupPollingLabel() 
    self.generateConstraints() 
} 

func setupPollingLabel() { 
    // some graphic setup 
} 

func setupAlarmLabel() { 
    // some graphic setup 
} 

func generateConstraints() { 
    self.dataCenterPollingLabel.setTranslatesAutoresizingMaskIntoConstraints(false) 
    self.dataCenterAlarmLabel.setTranslatesAutoresizingMaskIntoConstraints(false) 

    self.view.addConstraint(NSLayoutConstraint(item: dataCenterPollingLabel, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, toItem: self.view, attribute: NSLayoutAttribute.CenterY, multiplier: 1.0, constant: 0.0)) 
    self.view.addConstraint(NSLayoutConstraint(item: dataCenterAlarmLabel, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, toItem: self.view, attribute: NSLayoutAttribute.CenterY, multiplier: 1.0, constant: 0.0)) 
    self.view.addConstraint(NSLayoutConstraint(item: dataCenterAlarmLabel, attribute: NSLayoutAttribute.Width, relatedBy: NSLayoutRelation.Equal, toItem: dataCenterPollingLabel, attribute: NSLayoutAttribute.Width, multiplier: 1.0, constant: 0.0)) 
    self.view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat(NSString(format:"H:|-==%f-[dataCenterPollingLabel]-==%f-[dataCenterAlarmLabel]-==%f-|", sideMargins, sideMargins, sideMargins), options: NSLayoutFormatOptions.allZeros, metrics: nil, views: ["dataCenterPollingLabel": dataCenterPollingLabel, "dataCenterAlarmLabel": dataCenterAlarmLabel])) 

} 
} 

什麼是錯在我的代碼？我甚至可以知道在哪裏尋找一些錯誤，對我來說一切都很好。

Answer 1

我在iOS 7版本中遇到了同樣的問題。在viewDidLayoutSubviews方法的末尾調用self.view.layoutIfNeeded（）函數而不是super.viewDidLayoutSubviews（）函數解決了該問題。

代碼片斷

override func viewDidLayoutSubviews() { 

    // Your statements 
    self.reloadData() 

    //Write the below statement at the end of the function 
    self.view.layoutIfNeeded() 

}