UIViews are subviews of UIScrollView. How to trace that a subview is inside defined rectangle?

I add UIViews to UIScrollView one under another. When user scrolls and current UIView passes a rectangle it has to change background color of UIView. How to trace that a subview is inside defined rectangle?

- (v开发者_StackOverflowoid) showViews {

    CGRect scrollRect = CGRectMake(0.0f, 44.0f, 704.0f, 704.0f); 

    UIScrollView *aScroll = [[UIScrollView alloc] initWithFrame:scrollRect];
    float yPos = 20.0f;
    //the rectangle to trace intersection
    CGRect intersectRect = CGRectMake(0.0f, 200.0f, 704.0f, 400.0f);

    for (UIView* view in arrayOfViews) {
        CGRect viewRect = CGRectMake(100.0f, yPos, 320.0f, 480.0f); 
        view.frame = viewRect;
        [aScroll addSubView:view];
        yPos = yPos + 340.0f;
        aScroll.contentSize = CGSizeMake(704.0, yPos);
     }


}

The does CGRectIntersection(); might be what you are looking for.

It returns a CGRect, the intersection, which can be null (check it with CGRectIsNull()). If the rects have same size, and they are one over the second, the result should be a CGRectwith same size.

I've found the solution in delegate method and property contentOffset.

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {

    for (UIView *view in [[scrollView subviews] copy]) {

        if ((view.frame.origin.y - scrollView.contentOffset.y - self.controllStrip.frame.origin.y - self.controllStrip.frame.size.height) < 0  & 

             ( view.frame.origin.y + view.frame.size.height - scrollView.contentOffset.y - self.controllStrip.frame.origin.y) > 0) {

        view.backgroundColor = [UIColor grayColor];

        }

        else 
            view.backgroundColor = [UIColor greenColor];

    }

}