PoC for an accessory view that tracks scrolling a la Path

ScrollieScrollView.m

//  ScrollieScrollView
//  PoC for an accessory view that tracks scrolling
//  Created by Matt Drance (@drance) on 12/12/11.

@interface ScrollieScrollView : UIScrollView ()
@property (strong, nonatomic) UIView *thumb;
@end

@implementation ScrollieScrollView

@synthesize thumb = _thumb;

- (void)layoutSubviews {
    [super layoutSubviews];
    
    if (self.thumb == nil) {
        self.thumb = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 80.0, 20.0)];
        self.thumb.backgroundColor = [UIColor greenColor];
        [self addSubview:self.thumb];
    }
    
    CGRect thumbFrame = self.thumb.frame;
    CGFloat scrollDistance = self.contentSize.height - self.frame.size.height;
    CGFloat percentScrolled = self.contentOffset.y / scrollDistance;
    
    CGFloat projectedThumbY = floor(self.contentOffset.y + (self.frame.size.height * percentScrolled));
    CGFloat maxThumbY = (self.contentOffset.y + self.frame.size.height) - thumbFrame.size.height;
    CGFloat minThumbY = 0;
    
    CGFloat thumbY = MAX(projectedThumbY, minThumbY);
    thumbY = MIN(thumbY, maxThumbY);
    
    if (self.contentOffset.y < 0) {
        thumbY = self.contentOffset.y;
    } else {
        CGFloat overshoot = self.contentOffset.y - (self.contentSize.height - self.frame.size.height);
        if (overshoot > 0) {
            thumbY = (self.contentSize.height - thumbFrame.size.height) + overshoot;
        }
    }
    
    thumbFrame.origin.y = thumbY;
    self.thumb.frame = thumbFrame;
}

@end