黏性tableView 下拉变长 毛玻璃效果渐变

//
//  MeController.m
//  E-flyer
//
//  Created by Jason_Msbaby on 16/2/28.
//  Copyright © 2016年 Jason_Msbaby. All rights reserved.
//

#import "MeController.h"
#import "SearchController.h"
@interface MeController ()<UIScrollViewDelegate,UITableViewDelegate,UITableViewDataSource>
@property (weak, nonatomic) IBOutlet UITableView *tableView;
@property (weak, nonatomic) IBOutlet UIImageView *Header;
@property (weak, nonatomic) IBOutlet UIVisualEffectView *HeaderMask;
@property (weak, nonatomic) IBOutlet NSLayoutConstraint *HeaderH;
@property (weak, nonatomic) IBOutlet NSLayoutConstraint *HeaderMaskH;

@property(assign,nonatomic) CGFloat basicH;
@end

@implementation MeController
-(void)viewDidLoad{
[super viewDidLoad];
self.tableView.contentInset = UIEdgeInsetsMake(130, 0, 0, 0);
_basicH = 150;
}

#pragma mark - ScrollViewDelegate
/*!
*  计算scrollView的滚动距离 改变顶部view的高度实现黏性headerView
*
*  @param scrollView
*/
- (void)scrollViewDidScroll:(UIScrollView *)scrollView{
CGFloat h = -scrollView.contentOffset.y;

//在拉伸的过程中偏移量总是保持在1->0.5之间
//所以变化不明显
//根据h的变化规律  从0开始依次逐大递增至0.5
//然后减掉这个变化的值就能够实现透明度在短距离的拉伸过程中从1->0的变化😄
CGFloat regulation = (h/_basicH-1)*0.5;
CGFloat alpha = 150 / h - regulation;
alpha -= regulation;
NSLog(@"%f",alpha);
if (h <= _basicH) {
h = _basicH;
}

//更改毛玻璃透明度
self.HeaderMask.alpha = alpha;
self.HeaderH.constant = h;
self.HeaderMaskH.constant = h;

}

#pragma mark - tableViewDataSource
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section{
return 40;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
UITableViewCell *cell = [_tableView dequeueReusableCellWithIdentifier:@"meCell"];
//    cell.backgroundColor = kRandomColor;
return  cell;
}

@end