swift用animateWithDuration实现简单view切换动画

[code] UIView.animateWithDuration(duration, animations: {      
 }, completion: { finished in
 })

[code]//
//  ViewController.swift
//  ChangeView
//
//  Created by lifubing on 15/3/18.
//  Copyright (c) 2015年 lifubing. All rights reserved.
//

import UIKit

class ViewController: UIViewController {
    @IBAction func but(sender: UIButton) {
        changeOrder()
    }
    var flag = 0//flag作为一个标志，切换view时作为判断
    var viewA = UIView(frame: CGRect(x: 75, y: 150, width: 100, height: 100))//手动创建View，也可以拖控件
    var viewB = UIView(frame: CGRect(x: 75, y: 175, width: 50, height: 50))

    override func viewDidLoad() {
        super.viewDidLoad()
        viewA.backgroundColor = UIColor.blueColor()
        viewA.alpha = 0.8
        self.view.addSubview(viewA)

        viewB.backgroundColor = UIColor.redColor()
        self.view.addSubview(viewB)

        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    func changeOrder() {
        if flag == 0 {//判断是不是第一次切换
            UIView.animateWithDuration(1.5, animations: {
                self.viewB.center = CGPointMake(self.viewB.center.x + 100,self.viewB.center.y)
                }, completion: {
                    (Bool finished) in
                    self.view.bringSubviewToFront(self.viewA)
                    UIView.animateWithDuration(1.5, animations: {
                        self.viewB.center = CGPointMake(self.viewB.center.x - 100,self.viewB.center.y)
                    })
            })
            flag = 1//改变标志的值
        } else {
            UIView.animateWithDuration(1.5, animations: {
                self.viewB.center = CGPointMake(self.viewB.center.x + 100,self.viewB.center.y)
                }, completion: {
                    (Bool finished) in
                    self.view.bringSubviewToFront(self.viewB)
                    UIView.animateWithDuration(1.5, animations: {
                        self.viewB.center = CGPointMake(self.viewB.center.x - 100,self.viewB.center.y)
                    })
            })
            flag = 0//恢复标志的值
        }
    }

}