Is it possible to add fixed content to a UIScrollView?

Question

I want to create a subclass of UITableView or UIScrollView that will have some shading at the top when the content offset is > 0 to indicate that the content is scrollable. (See image attached)

The way I'm implementing it right now is using the UIViewController that is the delegate of the tableView. I simply have a GradientView on top of the tableView, and I intercept scrollViewDidScroll: to animate the visibility of that top gradient.

My problem with this implementation is that it's not "clean". I want my UIViewControllers to take care of logic, and not to deal with applying gradients and stuff. I wish I could just drop a subclass of UITableView that will do that for me.

The challenge for me is that I can't figure out how the tableView could add to itself a fixed content on top of the scrollable content.

Another question is what method/s of UIScrollView should I override to intercept the scrolling event. Obviously I don't want the tableView to be the delegate of itself...

Any ideas?

Thanks!

Answer 1

I've managed to figure out a much simpler way of doing this then what Avraham did.

I use the fact that the UIScrollView calls scrollViewDidScroll: ever pixel the scrolling changes to set the object at the location of the offset. Below is my full code to keep a gray bar at the top of the scrollview as you move around:

- (void)viewDidLoad {
    UIScrollView* scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(5.0, 50.0, self.bounds.size.width - 15.0, self.bounds.size.height - 60.0)];
    [scrollView setBackgroundColor:[UIColor colorWithRed:251.0/255.0 green:251.0/255.0 blue:251.0/255.0 alpha:1.0]];
    [scrollView setContentSize:CGSizeMake(scrollView.frame.size.width + 500, 1000.0)];
    [scrollView setDelegate:self];
    [self addSubview:scrollView];

    UIView* header = [[UIView alloc] initWithFrame:CGRectMake(0.0, 0.0, scrollView.contentSize.width, 40.0)];
    [header setTag:100];
    [header setBackgroundColor:[UIColor darkGrayColor]];
    [scrollView addSubview:header];
}

-(void)scrollViewDidScroll:(UIScrollView *)scrollView {
    UIView* header = [self viewWithTag:100];
    [header setFrame:CGRectMake(0.0, scrollView.contentOffset.y, header.bounds.size.width, header.bounds.size.height)];
}

Answer 2

Ok, so I found the solution on Apple's WWDC 2011 Session 104 video - Advanced Scroll View Techniques.

There is a whole section in this video about "Stationary Views" inside a scroll view. According to Apple, the way to go here is to override layoutSubviews and put there all the code to position whatever you want - wherever you want.

I tried it and it's actually pretty easy and it's working as expected.

So for example if I would like a shadowed header on top of the table when the content is being scrolled, this is the code I should write:

-(void) layoutSubviews
{
    [super layoutSubviews];
    [self positionTopShadow];
}

-(void) positionTopShadow
{
    CGFloat yOffset = self.contentOffset.y;
    // I'm doing some limiting so that the maximum height of the shadow view will be 40 pixels
    yOffset = MIN(yOffset, 40);
    yOffset = MAX(0, yOffset);

    CGRect frame = self.topShadowView.frame;
    // The origin should be exactly like the content offset so it would look like
    // the shadow is at the top of the table (when it's actually just part of the content) 
    frame.origin = CGPointMake(0, self.contentOffset.y);
    frame.size.height = yOffset;
    frame.size.width = self.frame.size.width;
    self.topShadowView.frame = frame;

    if (self.topShadowView.superview == nil)
    {
        [self addSubview:self.topShadowView];
    }
    [self bringSubviewToFront:self.topShadowView];
}