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

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];
}