Returns the indices of the specified container objects.
- macOS 10.0+
An object specifier for the container objects for which to obtain the indices.
A zero-based array of
NSNumber objects that identify the zero-based indices of the container objects that match
nil if no matching objects were found.
Containers that want to evaluate some specifiers on their own should implement this method. If this method returns
nil, the object specifier will go on to do its own evaluation, so you should only return
nil if that's the behavior you want, or if an error occurs. If this method returns an array, the object specifier will use the
NSNumber objects in it as the indices. So, if you evaluate the specifier and there are no objects that match, you should return an empty array, not
nil. If you find only one object, you should still return its index in an array. Returning an array with a single index where the index is –1 is interpreted to mean all the objects.