FObjectHandles can optionally support tracking. Because of the low level nature of object handles, anything that registers itself for these callbacks should ensure that it is: 1) error free (ie: should not cause exceptions even in unusual circumstances) 2) fault tolerant (ie: could be called at a time when an exception has happened) 3) thread-safe (ie: could be called from any thread) 4) high performance (ie: will be called many times) Callback notifying when an object value is read from a handle. Fired regardless of whether the handle was resolved as part of the read operation or not and whether the object being read is null or not.