Re: clearing (not erasing) when drawing from a thread

FROM : Murat Konar
DATE : Mon Nov 26 20:24:01 2007

Oops. That's what I get for reading messages in the wrong order. Yes, 
I had already come to the conclusions you outlined.

_murat

On Nov 26, 2007, at 2:32 AM, Alastair Houghton wrote:

> On 25 Nov 2007, at 23:57, m wrote:
>

>> Apologies in advance if this has already been answered. While 
>> questions similar to mine have been posted, I wasn't able to 
>> locate actual  working answers.
>>
>> On Tiger, when repeatedly drawing to a view from a thread, how do 
>> I "clear out" what I drew before? NSEraseRect() won't work as it 
>> erases to white. NSRectFillUsingOperation() with [NSColor 
>> clearColor] results in a black rect. Tricks like erasing to the 
>> background color won't work for me as there is no "the" background 
>> color (my drawing will be placed over an arbitrary and possibly 
>> changing background).

>
> AFAIK you'd have to force the underlying views to refresh that part 
> of the display.  That would be bad, because it would result in 
> their drawing code running from a thread, and not all views will be 
> thread-safe.
>
> Layer-backed views would offer one way out if you were on Leopard, 
> I think, because they're composited together rather than simply 
> drawn in order into the same buffer.  If you're on Tiger, I think 
> you're either going to have to use a child window, or manually save 
> the background, and draw that as the first step in your code.  But 
> the latter won't work if the background is animating, so if that's 
> a possibility then you probably want to use a child window.
>