约束添加到父视图上
意思就是imageView的高度等于self.view的height的0.5倍+0
self.view.addConstraint(NSLayoutConstraint(item: imageView, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem: self.view, attribute: NSLayoutAttribute.Height, multiplier: 0.5, constant: 0))
注意：
When added to a view, the constraint‘s items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. Break on -[UIView(UIConstraintBasedLayout)
这个方法需要在hierarchy 也就是整个的层级关系组合完成之后使用
在viewDidLoad(:_)方法中写的时候会出错
在viewDidAppear(:_)方法正常使用
原文：http://www.cnblogs.com/baaingSheep/p/5299533.html