NSLayoutConstraints加动画来改变约束

//
//  ViewController.m
//  NSLayoutAnimationDemo
//
//  Created by ebaotong on 15/7/22.
//  Copyright (c) 2015年 com.csst. All rights reserved.
//

#import "ViewController.h"
#define kWeakSelf(weakSelf) __weak typeof(self)weakSelf = self
#define WS(weakSelf)  __weak __typeof(&*self)weakSelf = self
@interface ViewController ()
@property (weak, nonatomic) IBOutlet NSLayoutConstraint *topConstraint;
@property (weak, nonatomic) IBOutlet UIView *topView;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}
/**需要对顶部约束拖线条来实现改变view的顶部约束*/
- (IBAction)btnClick:(UIButton *)sender
{

   kWeakSelf(weakSelf);
  [UIView animateWithDuration:0.8 delay: options:UIViewAnimationOptionCurveEaseInOut animations:^{

      weakSelf.topConstraint.constant = [UIScreen mainScreen].bounds.size.height;
      /**必须要添加不然看起来无明显效果*/
      [weakSelf.view layoutIfNeeded];
  } completion:^(BOOL finished) {

      weakSelf.topConstraint.constant = .f;
      /**必须要添加不然看起来无明显效果*/
      [weakSelf.view layoutIfNeeded];
  }];

}
/**不需要对顶部约束拖线条来实现改变view的顶部约束*/
- (IBAction)btnAction:(UIButton *)sender {
     kWeakSelf(weakSelf);
    [self.view.constraints enumerateObjectsUsingBlock:^(NSLayoutConstraint *constraint, NSUInteger idx, BOOL *stop)
     {
            if ((constraint.firstItem ==weakSelf.topView)&&(constraint.firstAttribute == NSLayoutAttributeTop))
        {
            [UIView animateWithDuration: delay: options:UIViewAnimationOptionCurveEaseInOut animations:^{

                constraint.constant = [UIScreen mainScreen].bounds.size.height;
                /**必须要添加不然看起来无明显效果*/
                [ weakSelf.view layoutIfNeeded];

            } completion:^(BOOL finished) {

                constraint.constant = ;
                /**必须要添加不然看起来无明显效果*/
                [ weakSelf.view layoutIfNeeded];
            }];

        }

    }];

}

@end