[Drawkit] Keeping objects sorted

[James Maxwell]

Hi Graham,

Thanks for the tips. Thinking about it more, I figured it probably  
won't be a huge deal to simply offer the sorted versions to the  
methods that need them, when they need them. I was a bit worried about  
it taking extra time to do this, but I don't imagine it will even be  
noticeable, really... I've been poking around with the new fast  
enumeration, which should help!

I'm really loving the DrawKit, btw, now that I'm getting comfortable  
with using it!
Great stuff!!

cheers,

J.



On 3-Jul-08, at 6:22 PM, Graham Cox wrote:

> The guide layer doesn't require sorted guides for what it was  
> designed to do, so you need to subclass it to keep the guides  
> sorted. You can't sort an NSArray because it's immutable, but if  
> you're subclassing, you'll notice that in fact the guide arrays are  
> NSMutableArrays internally, so they can indeed be sorted "in place".  
> You just need to use -sortUsingFunction:context: and write a short  
> comparison function, e.g:
>
> static int cmpGuidePositions (DKGuide* lh, DKGuide* rh, void *context)
> {
> 	#pragma unused(context)
> 	
> 	float lp = [lh position];
> 	float rp = [rh position];
> 	
> 	if (lp < rp)
> 		return NSOrderedAscending;
> 	else if (lp > rp)
> 		return NSOrderedDescending;
> 	else
> 		return NSOrderedSame;
> }
>
>
> You can also sort drawables by position in a similar manner, either  
> by subclassing the layer on which they live, or just implementing an  
> external method in a suitable place. Bear in mind that the ordering  
> of objects in a layer is actually their z-position or stacking  
> order, so you would be forcing the x and z positions to be the same.  
> You'd need to grab the objects into a mutable copy, sort them, then  
> put them back (-setObjects:)
>
> A much better alternative approach would be just to keep another  
> list of objects sorted by x position, and leave the normal stuff "as  
> is". Whenever you add an object to the layer, you'd also add it to  
> your sorted list, and vice versa on removal. Additional lists of  
> objects ordered in a different way is an efficient way to do things  
> because a) the objects themselves are not copied, just retained, and  
> the sorting of the additional list doesn't require any redrawing or  
> other potentially time-consuming side effects to be triggered - it's  
> your private list.
>
> The drawback is making sure the lists are synchronised with respect  
> to the objects they contain. Since there are a number of ways that  
> objects can get added or removed from layers, a possible approach  
> would be just to make a new copy of the list of objects every time  
> that happened. The new notifications suggested by Linan Wang will  
> ease this operation in the next version, but it's still doable in  
> beta3 by various means. On balance I'd say subclassing  
> DKObjectDrawingLayer is probably the best option, so you can simply  
> override the various add/remove methods to manage your sorted array,  
> then call super for the normal stuff. The subclass would also own  
> the sorted list so you can declare methods to simply get the sorted  
> objects or even -objectNearestToTime: type stuff.
>
> hth,
>
> cheers, Graham
>
> On 4 Jul 2008, at 4:06 am, James Maxwell wrote:
>
>> Hi All,
>>
>> In my particular app, it would be really helpful if I could keep  
>> drawn objects sorted by x position.
>> Does anyone know a simple and straightforward way of doing that?  
>> For example, I want my vertical guides to always be sorted by - 
>> position:, so I can efficiently get the closest guide "before" or  
>> "after" a particular object (this is a music app, so time runs left  
>> to right). I also want to do this with my main drawing layer's  
>> objects - keeping them ordered by x position (i.e., time). Is there  
>> no simple way to sort an array, like m_vGuides, for example, "in- 
>> place", without making a new sorted array and assigning it?
>
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