[ListMonitor] Fix performance of subscript(safeSectionIndex:safeItemIndex:).

[ruslanskorb]

There is a performance problem in Swift when getting an object from the Objective-C array with a large number of objects using subscript. It requires casting the array between Objective-C and Swift, that is pretty slow.

[JohnEstropia]

Do you have benchmarks for this? I am under the impression that ObjC arrays are toll-free bridged, especially since we are just accessing the indexer which should be calling NSArray's objectAtIndexedSubscript: method

[ruslanskorb]

I guess this is because of the type of the elements of the array. This is what I have.

With the current implementation:

With the proposed implementation:

[JohnEstropia]

I see, that's interesting. Can you try if casting it to NSArray first would work?

        return ObjectType.cs_fromRaw(
            object: (section.objects! as NSArray).object(at: itemIndex) as! NSManagedObject
        )

If this works then I'm pretty sure it would be faster than using IndexPath as that would search the internal sections again.

[ruslanskorb]

This is faster than the current implementation, but still quite slow.

[JohnEstropia]

Wow, that's actually counter-intuitive. Thanks for the tests!