Calculations of the visible rect can be a heavy operation as it depends of the superviews
visible rect. They have to be transformed and intersected together to create the result.
This calculation is a recursive operation that travels up the view hierarchy all the way
to the top.
Updating the tracking areas is also a recursive operation. But it travels down the view hierarchy
instead. It will calculate the new tracking area using the visible rect for the current view. For
each view it updates it will get the visible rect that has to travel all the way up the view
hierarchy to calculate it by transform and intersect them together. This results in that the visible
rect will be calculated for each view multible times. This makes it a very slow operation, specially
for deeper view hierarchies.
This fix will send along the visible rect for the superview when traveling down the
view hierarchy. This means that the visible rect does not need to travel up the view hierarchy
to be calculated. It will just need transform and intersect the provided rect for its superview
with the one from itself.
For some (yet unknown) reason, when declaring a view with tracking area
in a CIB, there’s some weird things happening that try to use the
_trackingAreas array when it’s not yet initialized… So, replacing
_trackingAreas.length by [_trackingAreas count] resolves the problem.
Martin CarlbergandGitHub
Didier Korthoudt