How do I tell if a UIView is visible and on screen?

Question

If I have a UIView (or UIView subclass) that is visible, how can I tell if it's currently being shown on the screen (as opposed to, for example, being in a section of a scroll view that is currently off-screen)?

To maybe give you a better idea of what I mean, UITableView has a couple of methods for determining the set of currently visible cells. I'm looking for some code that can make a similar determination for any given UIView.

Answer 1

Not tried any of this yet. But CGRectIntersectsRect(), -[UIView convertRect:to(from)View] and -[UIScrollView contentOffset] seem to be your basic building blocks here.

Answer 2

Here's what I used to check which UIViews were visible in a UIScrollView:

for(UIView* view in scrollView.subviews) {
    if([view isKindOfClass:[SomeView class]]) {

        // the parent of view of scrollView (which basically matches the application frame)
        CGRect f = self.view.frame; 
        // adjust our frame to match the scroll view's content offset
        f.origin.y = _scrollView.contentOffset.y;

        CGRect r = [self.view convertRect:view.frame toView:self.view];

        if(CGRectIntersectsRect(f, r)) {
            // view is visible
        }
    }
}

Answer 3

I recently had to check whether my view was onscreen. This worked for me:

CGRect viewFrame = self.view.frame;
CGRect appFrame = [[UIScreen mainScreen] applicationFrame];

// We may have received messages while this tableview is offscreen
if (CGRectIntersectsRect(viewFrame, appFrame)) {
    // Do work here
}

Answer 4

if you are primarily worried about releasing an object that is not in the view hierarchy, you could test to see if it has a superview, as in:

if (myView.superview){
 //do something with myView because you can assume it is on the screen
}
else {
 //myView is not in the view hierarchy
}