使用 Autolayout 以编程方式居中 UIView 会使视图从超级视图中消失

Question

我有以下代码在我的ViewController 的view 中以编程方式使用AutoLayout 约束简单地居中一个红色方块：

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，没有红色方块的迹象...即使在 Xcode 中使用 查看调试 功能时也看不到它。

如果我注释掉setupConstraints()，它与我在初始化期间给正方形的原始帧一样“预期”。

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

广场不见了怎么办？

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

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

尝试将您的 setupConstraints 函数更改为：

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])

}