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前言：AutoLayout定义了View的位置，也就是说，在Auto Layout的project里，假设不改动约束本身，在视图又一次绘制的时候。还会回到最開始的位置。AutoLayout中的动画与视图的位置和大小有关。

先看看效果

实现过程

在Storyboard上拖拽一个UIImageview。设置约束为：水平垂直正中心，大小非常定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

相应代码

 @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

原理比較简单。就是利用改动约束NSLayoutConstraint中的属性constant。然后调用layoutIfNeeded来实现动画。注意。属性multiplier眼下（iOS 8.4）还是仅仅读的,不能改动。可是能够通过关系view1.property = view2.property * multiplier + constant进行转换。

纯代码的AutoLayout动画

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()
        //加入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，定义最開始的位置

        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])

        //定义最開始的状态
        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()
        })
    }

}

定位Constraints

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