現(xiàn)在越來越多的APP中存在下拉放大圖片的效果，今天貢獻(xiàn)一下我的實(shí)現(xiàn)這種方法的原理造成，和我遇到的坑显熏。??效果圖

效果圖

-(void)viewWillAppear:(BOOL)animated
{
    self.navigationController.navigationBar.hidden = YES;
}
-(void)viewWillDisappear:(BOOL)animated
{
    self.navigationController.navigationBar.hidden = NO;
}

使用懶加載的方法把假的導(dǎo)航欄，圖片和表格創(chuàng)建出來

這里有幾點(diǎn)注意事項(xiàng)：

1.圖片不能添加到UITabview的HeaderView中鼓鲁，因?yàn)槿绻砑拥紿eaderView中蕴轨，那圖片就是表格的一部分了，會跟隨表格的移動和移動骇吭。
2.表格要設(shè)置contentInset來顯示出圖片橙弱，不然會有覆蓋問題
3.圖片的填充模式很重要，一定要是UIViewContentModeScaleAspectFill這樣可以節(jié)省很多代碼
4.圖片和假導(dǎo)航欄要設(shè)置clipsToBounds為YES，不然會有Bug

//lazy
-(UITableView *)tableview
{
    if (!_tableview) {
        _tableview = [[UITableView alloc]initWithFrame:CGRectMake(0, 0, kScreenW, kScreenH) style:UITableViewStylePlain];
        CGFloat headImvH = kScreenW / 320 * 300;
        //這句很重要
        _tableview.contentInset = UIEdgeInsetsMake(headImvH-20, 0, 0, 0);
        
        _tableview.dataSource = self;
        _tableview.delegate = self;
    }
    return _tableview;
}

-(UIImageView *)headImv
{
    if (!_headImv) {
        _headImv = [[UIImageView alloc]init];
        CGFloat headImvH = kScreenW / 320 * 300;
        _headImv.frame = CGRectMake(0, 0, kScreenW, headImvH);
        _headImv.image = [UIImage imageNamed:@"head"];
        //一定記住模式
        _headImv.contentMode = UIViewContentModeScaleAspectFill;
        _headImv.clipsToBounds = YES;
    }
    return _headImv;
}

-(UIView *)navView
{
    if (!_navView) {
        _navView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, kScreenW, 64)];
        _navView.backgroundColor = [[UIColor redColor] colorWithAlphaComponent:0.8];
        _navView.clipsToBounds = YES;
        
        _titLab = [[UILabel alloc]init];
        _titLab.centerX = kScreenW * 0.5;
        _titLab.bounds = CGRectMake(0, 0, 150, 44);
        _titLab.textAlignment = NSTextAlignmentCenter;
        _titLab.font = [UIFont systemFontOfSize:12];
        _titLab.textColor = [UIColor blueColor];
        _titLab.numberOfLines = 0;
        _titLab.text = @"CF\n明天休息了";
        [_navView addSubview:_titLab];
    }
    return _navView;
}

??這些代碼很好理解棘脐，不做解釋

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    
    //添加順序不能改變
    [self.view addSubview:self.tableview];
    [self.view addSubview:self.headImv];
    [self.view addSubview:self.navView];
    [self setupNavView];
}

#pragma mark - 設(shè)置導(dǎo)航欄
-(void)setupNavView
{
    UIButton *leftNavBtn = [UIButton buttonWithType:UIButtonTypeCustom];
    leftNavBtn.frame = CGRectMake(0, 20, 64, 44);
    [leftNavBtn setTitle:@"返回" forState:UIControlStateNormal];
    [leftNavBtn addTarget:self action:@selector(leftNavClike) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:leftNavBtn];
    
    UIButton *rightNavBtn = [UIButton buttonWithType:UIButtonTypeCustom];
    rightNavBtn.frame = CGRectMake(kScreenW-64, 20, 64, 44);
    [rightNavBtn setTitle:@"更多" forState:UIControlStateNormal];
    [rightNavBtn addTarget:self action:@selector(rightNavClike) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:rightNavBtn];
}

-(void)leftNavClike
{
    [self.navigationController popViewControllerAnimated:YES];
}

-(void)rightNavClike
{
    
}


#pragma mark - UITableViewDataSource
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return 20;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *cellID = @"cellID";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellID];
    if (cell == nil) {
        cell = [[UITableViewCell alloc]initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellID];
    }
    cell.textLabel.text = [NSString stringWithFormat:@"測試數(shù)據(jù)——%td",indexPath.row];
    cell.detailTextLabel.text = [NSString stringWithFormat:@"測試數(shù)據(jù)——%td",indexPath.row];
    
    return cell;
}

下面為核心代碼

通過表格的代理方法來改變整體的Frame
這里我發(fā)現(xiàn)一個問題斜筐，如果表格滑動的特別快，那么這個代理方法會存在來不及執(zhí)行的效果蛀缝，或者說偏移量不是線性改變的顷链，從而導(dǎo)致根據(jù)偏移量來計(jì)算的Frame出現(xiàn)問題，所以有些控件建議在設(shè)置Frame的時候屈梁，建議用一個參照來改變(我這里假導(dǎo)航欄里面的文字用偏移出現(xiàn)了問題嗤练，所以改用假導(dǎo)航欄的透明度)
設(shè)置頭部圖片的Frame要注意，origin值不要改變一旦改變不好調(diào)整在讶。

#pragma mark - UIScrollViewDelegate
- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    //拿到偏移量
    CGFloat offsetY = scrollView.contentOffset.y;
    NSInteger headImvH = kScreenW / 320 * 300 ;
    CGFloat offset = headImvH + offsetY;//計(jì)算偏移量
    
    //設(shè)置導(dǎo)航欄
    self.navView.alpha = (offset / 250);
    if (self.navView.alpha >=1) {
        self.navView.alpha = 1;
    }
    //設(shè)置標(biāo)題文字 設(shè)置距離
    //alpha 0-->1 y 64-->20
    self.titLab.y = 64 - 44 * self.navView.alpha;
    
    //設(shè)置頭部圖片大小
    self.headImv.frame = CGRectMake(0, 0, kScreenW, headImvH-offset);   
}

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