On Jul 20, 2008, at 4:01 PM, Torsten Curdt wrote:
AFAIU accessing UI elements from within an NSThread is a big no-no.
There are exceptions, but that's a good rule of thumb.
Here are Apple's specific guidelines: http://developer.apple.com/documentation/Cocoa/Conceptual/Multithreading/ThreadSafetySummary/chapter_950_section_2.html#/
/apple_ref/doc/uid/10000057i-CH12-123351-BBCFIIEB
So in the following naive code I would have to either wrap the calls
to the progressIndicator object through a
performSelectorOnMainThread or could send NSNotifications to update
it on the main thread.
NSNotifications are delivered on the same thread from which they are
sent/posted. So, they don't help -- in and of themselves -- for
moving work from a secondary thread to the main thread.
I'm pretty sure that, at some point, you have to use
performSelectorOnMainThread:... or the equivalent to pass the
information about whatever progress has been made from the secondary
thread to the main thread. Since some of the methods you invoke on
progressIndicator take scalar values, and
performSelectorOnMainThread:... requires a selector which takes an
object pointer as its argument, you need to create wrapper methods in
your own class to invoke on the main thread. These wrappers take
their object parameter (likely an NSNumber) and "unpack" a value from
it to pass along to methods of progressIndicator.
I would recommend that you do the -setMaxValue: and -startAnimation:
calls from the main thread before detaching the background thread.
Likewise, you should have a final method on your class which the
background thread invokes using performSelectorOnMainThread:... just
before it exits. That method signals that the background thread has
completed its work. Among other things, that's where you would invoke
-stopAnimation:.
Taken all together, you end up defining a small ad-hoc protocol
between your background thread and the main thread. The background
thread never actually touches the progressIndicator. Instead, it just
messages self via performSelectorOnMainThread:... and the methods
invoked in that way are responsible for updating progressIndicator.
I am also wondering whether using NSRunCriticalAlertPanel() in the
thread is OK or not.
Good question. I don't find a specific note about that. However, the
above-linked guide says this:
"You can create a modal window on a secondary thread. The Application
Kit blocks the calling secondary thread while the main thread runs the
modal loop."
I think that means that NSRunCriticalAlertPanel is safe to use from a
secondary thread.
Cheers,
Ken
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