AutoLayout滚动到缩放动画

Question

我有一个内部有UIImageView的UIScrollview。当用户拖动UIScrollView时，UIImageView会动画如下：

这是我正在使用的代码，很简单：

- (void)viewDidLoad {
    [super viewDidLoad];
    self.scrollView = [[UIScrollView alloc] initWithFrame:CGRectZero];
    self.scrollView.delegate = self;
    [self.view addSubview:self.scrollView];

    self.zoomView = [[UIImageView alloc] initWithFrame:CGRectZero];
    [self.scrollView addSubview:self.zoomView];
}

- (void)viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];

    self.scrollView.frame = self.view.bounds;
    self.scrollView.backgroundColor = [UIColor purpleColor];
    self.view.backgroundColor = [UIColor yellowColor];
    self.scrollView.alwaysBounceVertical = YES;
    self.zoomView.frame = CGRectMake(0, 0, self.view.frame.size.width, 100);
    self.zoomView.backgroundColor = [UIColor redColor];
    self.zoomView.image = [UIImage imageNamed:@"Spawn.jpg"];
    self.zoomView.contentMode = UIViewContentModeScaleAspectFill;
    self.cachedImageViewSize = self.zoomView.frame;
}


#pragma mark - UIScrollViewDelegate Methods

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    CGFloat y = -scrollView.contentOffset.y;
    if (y < 0) {
        return;
    }

    self.zoomView.frame = CGRectMake(0, scrollView.contentOffset.y, self.cachedImageViewSize.size.width + y, self.cachedImageViewSize.size.height + y);
    self.zoomView.center = CGPointMake(self.view.center.x, self.zoomView.center.y);
}

我的问题是，如何使用autoLayout执行完全相同的效果？ 我通过代码使用autolayout，就像这样，但我不知道在UIScrollView委托上添加什么：

 self.zoomView.translatesAutoresizingMaskIntoConstraints = NO;
    self.scrollView.translatesAutoresizingMaskIntoConstraints = NO;

    NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings(_zoomView, _scrollView);
    NSMutableArray *constraint = [[NSMutableArray alloc] init];
    [constraint addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[_zoomView]|" options:0 metrics:nil views:viewsDictionary]];
    [constraint addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[_scrollView]|" options:0 metrics:nil views:viewsDictionary]];
    [constraint addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[_scrollView]|" options:0 metrics:nil views:viewsDictionary]];
    [constraint addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[_zoomView(==30)]" options:0 metrics:nil views:viewsDictionary]];
    [self.view addConstraints:constraint];

似乎Autolayout对这种动画并不擅长。