Re: NSSwitch drawRect bug

[H. Nikolaus Schaller]

Hi all,

> Am 14.08.2023 um 08:55 schrieb Fred Kiefer <fredkiefer@gmx.de>:
> 
> 
>> Am 14.08.2023 um 08:36 schrieb H. Nikolaus Schaller <hns@goldelico.com>:
>> 
>>> Am 13.08.2023 um 22:40 schrieb dr_clow@me.com:
>>> 
>>> Nice! Maybe even just drawSwitchInRect:dirtyRect:forState:enabled: Though, 
>>> with your way, you could get more information from the control without 
>>> having to enlarge method arguments every time we wanted to add something 
>>> else. 
>>> 
>>> So maybe -(void) drawSwitch:(NSSwitch*)switch inRect:(CGRect)rect 
>>> dirtyRect:(CGRect)dirtyRect {}
>> 
>> I am not sure if passing the dirtyRect is necessary and useful.
>> The idea is that -drawRect: is usually called after setting a clipping rect 
>> within -display so that drawing with bounds size is correct but will be 
>> clipped away.
> 
> 
> This is not about  correctness, as you wrote the NSView clipping will take 
> care of that, this idea is about performance. When we pass on the dirty 
> rectangle the drawing will be able to decide which parts actually require a 
> redraw. This isn’t important for a small component like an NSSwitch but for 
> something like NSMatrix or NSTableView it allows us to speed up drawing a 
> lot. We only redraw and compute bits that will be visible.
> In general we should always draw a view within the assigned bounds and use 
> the dirty rectangle to speed things up, where this is useful. Somebody should 
> go through the NSView subclasses to see where corrections in the code are 
> required.

Apple has a good description here: 
https://developer.apple.com/documentation/appkit/nsview/1483686-drawrect

The dirty rect is a *hint* for *optimizing* drawing. It is not a bounds 
dimension of a view which means that using it for scaling was also wrong. But 
there may not only be a single dirty rect, there may be dirty regions.

Hence, the best solution would be if the drawing code for a View or Cell knows 
about the view it is drawing into and can use methods like 
-getRectsBeingDrawn:count: or -needsToDrawRect: This can easily be done easily 
within -drawRect: but not within [GSTheme theme].

So passing the control view seems to be the right thing, until we find that 
there is a +focusView in NSView. This should be the view which is currently 
being drawn to.

Hence there is no need to pass down neither the dirtyRect nor a controlView to 
a method like -drawSwitchInRect:forState:enabled: if it wants to optimize 
drawing.

BR, hns