Trying to implement Facebook instant article page but I can't get the image view height to animate

Question

I'm trying to implement the following in my app:

1. A scrollview with an image view and a view as subviews.

2. As I drag down from the top of the scroll view, the image view increases in height, in the aspect fill mode, giving the impression of zooming.

3. When I let go, the image returns to its original height.

So basically, I'm trying to replicate the Facebook app's instant article page.

The problem is that I can't animate the image view returning to its original size. It seems to always jerk back to it. UIView.animateWithDuration() doesn't work here for some reason:/

here is my code: (imageHeight is an IBOutlet of the UIImageView's height constraint)

    func scrollViewDidScroll(scrollView: UIScrollView) {

    let y = self.scrollView.contentOffset.y

    if y < 0 && self.imageHeight.constant < 500
    {
        self.imageHeight.constant += (-1*y)*0.5;
    }

}

func scrollViewDidEndDecelerating(scrollView: UIScrollView) {

    self.stoppedScrolling()

}


func scrollViewDidEndDragging(scrollView: UIScrollView, willDecelerate decelerate: Bool) {


    if !decelerate
    {
        self.stoppedScrolling()
    }

}

func stoppedScrolling()
{

    UIView.animateWithDuration(1) { 

        self.imageHeight.constant = 180

    }

}

Here's a gif of the result Also, it doesn't show in the gif but there's a half-second delay between end touches and resizing

Answer 1

Seems like it cause by UIImageView's height constraint, I try to create the UI by code and use frame property to control it, it works.

Sorry about that I don't know how to write swift code, but I think that I can know what you mean, you must can read my code as well, like this:

#import "MainViewController.h"

@interface MainViewController ()

@property UIScrollView *scrollView;
@property UIImageView *imageView;
@property bool needReceiveDraggingFlag;

@end

@implementation MainViewController

- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view.

_scrollView = [[UIScrollView alloc] initWithFrame:[UIScreen mainScreen].bounds];
_scrollView.backgroundColor = [UIColor grayColor];
_scrollView.delegate = self;
_scrollView.scrollEnabled = YES;
_scrollView.contentSize = CGSizeMake(_scrollView.frame.size.width,_scrollView.frame.size.height*1.2f);
[self.view addSubview:_scrollView];

UILabel *lbInfo = [[UILabel alloc] initWithFrame:CGRectMake(50, 300, 200, 30)];
lbInfo.backgroundColor = [UIColor greenColor];
lbInfo.text = @"Test label";
[_scrollView addSubview:lbInfo];

_imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
_imageView.backgroundColor = [UIColor redColor];
[self.view addSubview:_imageView];

_needReceiveDraggingFlag = YES;
}

- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}

-(void)scrollViewDidScroll:(UIScrollView *)scrollView{
NSLog(@"scrollViewDidScroll");
if(_needReceiveDraggingFlag){
    float y = _scrollView.contentOffset.y;
    if (y<0 && _imageView.frame.size.width<300) {
        float nowImageLength = _imageView.frame.size.width;
        float targetImageLength = nowImageLength + -y*0.5f;
        _imageView.frame = CGRectMake(0, 0, targetImageLength, targetImageLength);
    }
}
}

-(void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate{
NSLog(@"scrollViewDidEndDragging");
[self stoppedScrolling];
}

-(void)stoppedScrolling{
_needReceiveDraggingFlag = NO;
[UIView animateWithDuration:1 animations:^{
    self.imageView.frame = CGRectMake(0, 0, 100, 100);
} completion:^(BOOL isFinished){
    _needReceiveDraggingFlag = YES;
}];
}

@end

Hope it can help you.