> On Mar 23, 2020, at 8:01 AM, Gabriel Zachmann via Cocoa-dev
> <cocoa-dev@lists.apple.com> wrote:
>
>>>> I would not do this. It is widely documented that AppKit API that produce
>>>> UI elements, like NSOpenPanel, are _not_ thread safe.
>>>
>>> Right, and that is why I have to execute any UI code in the main thread,
>>> or so I thought.
>>> As far as I understand, the dispatch_get_main_queue is executed by the main
>>> thread, isn't it?
>> Yes
>
> So, why is
> dispatch_sync( dispatch_get_main_queue(), block opening a NSOpenPanel )
> a bad idea?
It’s not a bad idea. I misread the code. I didn’t see
dispatch_get_main_queue. Sorry about that. This will invoke the open panel on
the main thread and not return until the panel is dismissed.
>>> When the user selects a directory, I collect all files in that sub-tree
>>> (aka "scan").
>>> That might involve aliases. which means I need to ask the user to open the
>>> directory that alias points to.
>>> Because that scan can take several seconds (10-20),
>>> I wanted to do that scan in a secondary thread so that it can update
>>> progress info in the UI.
>>
>> This is all reasonable… I guess there may be two options, ask the user on
>> the main thread, before you are on your background thread,
>
> Would be doable, but would complicate the code, because I would first need to
> scan
> the whole directory tree for aliases, get user permission , then pass the
> array of directories to a secondary thread.
>
> Also, I would like to give the user progress info while the secondary thread
> is collecting.
>
>> and only start the background thread after the user answers, or dispatch the
>> UI work to the main thread from your background thread as your code sample
>> showed.
>
> So my solution is viable ?
Yes.
—Rob
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