前言：autolayout定义了view的位置，也就是说，在auto layout的工程里，如果不修改约束本身，在视图重新绘制的时候，还会回到最开始的位置。autolayout中的动画与视图的位置和大小有关。
在Storyboard上拖拽一个UIImageview。设置约束为：水平垂直正中心，大小很定100＊100
拖拽Imageview以及Constraint为Outlet
注意拖拽Y相关的约束，也就是这个
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对应代码
@IBOutlet weak var imageview: UIImageView! @IBOutlet weak var yConstraints: NSLayoutConstraint!
在viewDidload中设置imageview的初始状态
yConstraints.constant = yConstraints.constant - CGRectGetHeight(UIScreen.mainScreen().bounds)/2 self.imageview.alpha = 0.0; self.imageview.transform = CGAffineTransformMakeScale(0.1, 0.1) self.view.layoutIfNeeded();
ViewWillAppear中创建动画
yConstraints.constant = yConstraints.constant + CGRectGetHeight(UIScreen.mainScreen().bounds)/2 UIView.animateWithDuration(1.0, animations: { () -> Void in self.imageview.alpha = 1.0 self.imageview.transform = CGAffineTransformIdentity self.view.layoutIfNeeded() })原理比较简单，就是利用修改约束NSLayoutConstraint中的属性constant，然后调用layoutIfNeeded来实现动画。注意，属性multiplier目前（iOS 8.4）还是只读的,不能修改。但是可以通过关系view1.property = view2.property * multiplier + constant进行转换。
上述动画用纯代码实现
class ViewController: UIViewController { var imageview:UIImageView? weak var yConstraint:NSLayoutConstraint? override func viewDidLoad() { super.viewDidLoad() //添加Imageview let image = UIImage(named: "1_hello_hwc.jpg") imageview = UIImageView(image: image) self.imageview?.setTranslatesAutoresizingMaskIntoConstraints(false) self.view.addSubview(self.imageview!) //创建约束，定义最开始的位置 let hC = NSLayoutConstraint(item:self.view, attribute:NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal, toItem: self.imageview, attribute: NSLayoutAttribute.CenterX, multiplier: 1.0, constant: 0.0) let vC = NSLayoutConstraint(item:self.view, attribute:NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, toItem: self.imageview, attribute: NSLayoutAttribute.CenterY, multiplier: 1.0, constant: 0.0) yConstraint = vC; yConstraint!.constant = yConstraint!.constant - CGRectGetHeight(UIScreen.mainScreen().bounds)/2 let widthC = NSLayoutConstraint(item:self.imageview!, attribute: NSLayoutAttribute.Width, relatedBy: NSLayoutRelation.Equal, toItem:nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1.0, constant: 100) let widthH = NSLayoutConstraint(item:self.imageview!, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem:nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1.0, constant: 100) self.view.addConstraints([hC,vC,widthC,widthH]) //定义最开始的状态 self.imageview?.alpha = 0.0; self.imageview?.transform = CGAffineTransformMakeScale(0.1, 0.1); self.view.layoutIfNeeded() } override func viewWillAppear(animated: Bool) { yConstraint!.constant = yConstraint!.constant + CGRectGetHeight(UIScreen.mainScreen().bounds)/2 UIView.animateWithDuration(1.0, animations: { () -> Void in self.imageview!.alpha = 1.0 self.imageview!.transform = CGAffineTransformIdentity self.view.layoutIfNeeded() }) }}设置属性identifier
yConstraint.identifier ＝ "identifier"
然后在过滤，定义到这个Constraints,
let constraint = filter(self.view.constraints() as! [NSLayoutConstraint], { (constraint:NSLayoutConstraint) -> Bool in return constraint.identifier == "identifier" }).first
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