The HTML importer should not be called from a background thread
(that is, the options dictionary
includesNSDocumentTypeDocumentAttribute with
a value of NSHTMLTextDocumentType).
It will try to synchronize with the main thread, fail, and time out. Calling it
from the main thread works (but can still time out if the HTML contains
references to external resources, which should be avoided at all costs). The
HTML import mechanism is meant for implementing something like markdown (that
is, text styles, colors, and so on), not for general HTML
import.
Multicore considerations:
Since OS X
v10.4, NSAttributedString has used WebKit
for all import (but not for export) of HTML documents. Because WebKit document
loading is not thread safe, this has not been safe to use on background threads.
For applications linked on OS X v10.5 and later,
if NSAttributedString imports HTML documents
on any but the main thread, the use of WebKit is transferred to the main thread
via performSelectorOnMainThread:withObject:waitUntilDone:.
This makes the operation thread safe, but it requires that the main thread be
executing the run loop in one of the common modes. This behavior can be
overridden by setting the value of the standard user
default NSRunWebKitOnAppKitThread to
either YES(to obtain the new behavior regardless
of linkage) or NO (to obtain the old
behavior regardless of linkage).
Why NSAttributedString import html must be on main thread?
原文：http://www.cnblogs.com/ioriwellings/p/3556990.html