使用Autolayout以编程方式居中UIView使视图从superview中消失

Question

我有以下代码,只需AutoLayout在我ViewController的编程中使用约束来简单地将红色正方形居中view:

class ViewController: UIViewController {

    let square: UIView

    required init?(coder aDecoder: NSCoder) {
        let squareFrame = CGRectMake(0.0, 0.0, 500.0, 500.0)
        self.square = UIView(frame: squareFrame)

        super.init(coder: aDecoder)
    }

    override func viewDidLoad() {
        self.view.addSubview(self.square)
        self.square.backgroundColor = UIColor.redColor()
        self.view.backgroundColor = UIColor.blueColor()
        print(self.square)
        setupConstraints()
        print(self.square)
    }


    func setupConstraints() {
        self.square.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint(item: self.view, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal,
            toItem: self.square, attribute: NSLayoutAttribute.CenterX, multiplier: 1, constant:0).active = true
        NSLayoutConstraint(item: self.view, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal,
            toItem: self.square, attribute: NSLayoutAttribute.CenterY, multiplier: 1, constant:0).active = true
    }
}

然而,生成的屏幕只显示蓝色背景,没有红色方块的标志......即使在Xcode中使用视图调试功能也无法看到.

如果我注释掉它setupConstraints(),它就像我在初始化期间给广场的原始帧一样"预期".

顺便说一句,两个print语句具有完全相同的输出: <UIView: 0x7ff1c8d3f3e0; frame = (0 0; 500 500); layer = <CALayer: 0x7ff1c8d04c00>>

当广场无处可见时,怎么会这样呢？

更新:当我按照@HaydenHolligan建议添加width和height约束时,问题仍然存在setupConstraints():

func setupConstraints() {
    self.square.translatesAutoresizingMaskIntoConstraints = false

    NSLayoutConstraint(item: self.view, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal,
        toItem: self.square, attribute: NSLayoutAttribute.CenterX, multiplier: 1, constant:0).active = true
    NSLayoutConstraint(item: self.view, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal,
        toItem: self.square, attribute: NSLayoutAttribute.CenterY, multiplier: 1, constant:0).active = true

    // the following lines have no effect with respect to the issue mentioned aboove
    let sizeFormat = "[square(100@100)]"
    let size = NSLayoutConstraint.constraintsWithVisualFormat(sizeFormat, options: NSLayoutFormatOptions.AlignAllCenterX, metrics: nil, views: ["square": self.square])
    self.view.addConstraints(size)
}

Answer 1

尝试将setupConstraints func更改为:

func setupConstraints() {

    self.square.translatesAutoresizingMaskIntoConstraints = false

    let centerX = NSLayoutConstraint(item: self.view, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal,toItem: self.square, attribute: NSLayoutAttribute.CenterX, multiplier: 1, constant:0)
    let centerY = NSLayoutConstraint(item: self.view, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal,toItem: self.square, attribute: NSLayoutAttribute.CenterY, multiplier: 1, constant:0)
    let squareWidth = NSLayoutConstraint(item: self.square, attribute: NSLayoutAttribute.Width, relatedBy: NSLayoutRelation.Equal, toItem: nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1, constant:500)
    let squareHeight = NSLayoutConstraint(item: self.square, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem: nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1, constant:500)

    self.view.addConstraints([centerX , centerY ,squareWidth , squareHeight])

}