iOS convertPoint:toView: return inconsistent values - ios

Question

I'm trying to get the offset of my viewControllers' views relative to the top of the screen.
I thought I could convert the origin of the view to the window's coordinates so I tried something like this in my viewControllers:
CGPoint basePoint = [self.view convertPoint:self.view.frame.origin toView:nil];
CGFloat offset = basePoint.y;
It works as expected in some cases, however in other cases it returns different values even when the parameters are the same.
Any ideas on what's going on behind the scenes of this convertPoint:toView: that might be resulting in different return values?
Or if you have any other suggestions to get the offset of my viewControllers' views relative to the top of the screen it would be very much appreciated!
Thanks

Answer 1

You are converting your point to a point in a nil view.
The initial window, has a rootViewController which has a view and so you can access to that view:
UIView *firstView = [[[(YourClassAppDelegate *)[UIApplication sharedApplication] window] rootViewController] view];
CGPoint basePoint = [self.view convertPoint:self.view.frame.origin toView:firstView];
CGFloat offset = basePoint.y;
remember to import in the implementation, your class of the app delegate.

Answer 2

I end up crawling up the chain of superviews and adding up all the offsets.y from each view. Not sure if it's the best approach, especially performance wise, but for now it works.
UIView *view = self.view;
CGFloat offset = view.frame.origin.y;
while (view.superview != nil) {
offset += view.superview.frame.origin.y;
view = view.superview;
}

Answer 3

try change this
CGPoint basePoint = [self.view convertPoint:self.view.frame.origin toView:nil];
to
CGPoint basePoint = [self.view.superview convertPoint:self.view.frame.origin toView:nil];

Answer 4

Your two views must have common superview (for example UIWindow), otherwise you'll get weird results.
This might happen when you try to use UIViewController's view in -viewDidLoad with convertPoint, because view is not yet added to the view hierarchy.
- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(#"%#", self.view.superview); // nil, not added to view hierachy
// If you try to convert point using self.view (or any self.view subview),
// and any other view, which is not self.view subview (for example, UIWindow),
// it will fail, because self.view is not yet added to the view hierachy.
}