⚡

TLDR: make sure that:
* your complications assets have the proper Scale (Individual Scales for Circular and Graphic Extra large, Single Scale for the others)
* all icons are setup to "Single Size", whether it's your main AppIcon or an alternate one

When I first heard about the support for single-size icons at WWDC22, I was thrilled. I had a reasonably good setup to generate the images with Affinity Designer and a Shortcut, but maybe it wasn't worth the trouble.

Starting Size

The first thing I did was to use build an archive and check the size like Antoine van der Lee did, which got me this report (I simplified it). The process is explained by Apple in their Reducing your app’s size guide.

# iPhone 14 Pro
App + On Demand Resources size: 71,8 MB compressed, 82,4 MB uncompressed
App size: 71,8 MB compressed, 82,4 MB uncompressed
On Demand Resources size: Zero KB compressed, Zero KB uncompressed

# Watch Ultra
App + On Demand Resources size: 26,1 MB compressed, 32,2 MB uncompressed
App size: 26,1 MB compressed, 32,2 MB uncompressed

Up until now, I had:

one AppIcon in the Assets.xcassets of the iOS target, with icons for both iOS and watchOS (can't really remember why…)
one AppIcon in the Assets.xcassets of the watchOS target

So the first thing I did was to have:

a single Assets for both iOS and watchOS (add it to the Watch app target): I created a new "Heavy.xcassets" for all assets used for iOS (and soon more…), so that my Assets.xcassets would be limited to assets shared between all apps (I have other catalogs for things shared with apps + extensions). This means I only had the AppIcon in there for now.
delete the AppIcon from the Assets of the Watch target

I build on the Watch to make sure that the change didn't mess anything up.

I created a new archive and went through the process of exporting the archive for development with app thinning again, to see what changed.

# iPhone 14 Pro
App + On Demand Resources size: 71,8 MB compressed, 82,4 MB uncompressed
App size: 71,8 MB compressed, 82,4 MB uncompressed
On Demand Resources size: Zero KB compressed, Zero KB uncompressed

# Watch Ultra
App + On Demand Resources size: 26,1 MB compressed, 32,2 MB uncompressed
App size: 26,1 MB compressed, 32,2 MB uncompressed
On Demand Resources size: Zero KB compressed, Zero KB uncompressed

All good!

Switching to Single Size

I went ahead and proceed to opt for Single Size

I built my app, and it got ugly:

❌

Command CompileAssetCatalog failed with a nonzero exit code

The error message was not rather confusing: why is Core Data involved in this?

AssetCatalogSimulatorAgent *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Object 0x906e9858c73ac8ba <x-coredata://E0662F14-2CE1-4084-BCF3-D261022348C6/SimpleArtworkRenditionSpec/p82> persistent store is not reachable from this NSManagedObjectContext's coordinator'
*** First throw call stack:
(
	0   CoreFoundation                      0x00000001807a3b44 __exceptionPreprocess + 172
	1   libobjc.A.dylib                     0x000000018005c144 objc_exception_throw + 56
	2   CoreData                            0x00000001858739cc _PFRetainedObjectIDCore + 680
	3   CoreData                            0x000000018586a1e4 -[NSManagedObjectContext objectWithID:] + 424
	4   CoreThemeDefinition                 0x000000019d9b5b2c __34-[TDRenditionsDistiller _distill:]_block_invoke + 36
	5   CoreData                            0x0000000185871720 developerSubmittedBlockToNSManagedObjectContextPerform + 156
	6   …
)
2022-11-09 14:00:32.369 AssetCatalogSimulatorAgent[38516:518671] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Object 0x906e9858e65ac95a <x-coredata://E0662F14-2CE1-4084-BCF3-D261022348C6/NamedColorRenditionSpec/p345> persistent store is not reachable from this NSManagedObjectContext's coordinator'
*** First throw call stack:
(…)
2022-11-09 14:00:32.369 AssetCatalogSimulatorAgent[38516:518675] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Object 0x906e9858e5fac8ba <x-coredata://E0662F14-2CE1-4084-BCF3-D261022348C6/SimpleArtworkRenditionSpec/p324> persistent store is not reachable from this NSManagedObjectContext's coordinator'
*** First throw call stack:
(…)
2022-11-09 14:00:32.369 AssetCatalogSimulatorAgent[38516:518672] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Object 0x906e9858c3dac8ba <x-coredata://E0662F14-2CE1-4084-BCF3-D261022348C6/SimpleArtworkRenditionSpec/p117> persistent store is not reachable from this NSManagedObjectContext's coordinator'
*** First throw call stack:
(…)
libc++abi: terminating with uncaught exception of type NSException
/* com.apple.actool.errors */
: error: ERROR: Identical key for two renditions

(E)Key
	element:   	Grouped/Packed Images
	part:      	Artwork Image
	value:     	(null)
	identifier:	0
	dimension1:	0
	state:     	(null)
	scale:     	2
	idiom:   	watch
	subtype:      	0
	gamut:   	sRGB
	target:   	any
	sizeClassHorizontal:   	universal
	sizeClassVertical:   	universal
	graphicsFeatureSetClass:      	GLES2,0
	memoryClass:      	0
	target:    	any
	appearance:      	(null)
	localization:      	(null)
	glyphWeight:	(null)
	tglypSize: 	(null)
	deployment:   	any

I got stuck in that place for about a month:

Did anyone with a Watch app switch to Single Size app icon?

There's something wrong with my Assets and I can't build my app (I get an obscure/misleading Core Data error)https://t.co/KTNNE64yMc
— Arnaud Joubay (@sowenjub) October 11, 2022

I had more urgent matters to tend to, but with the announcement of a new round of "Ask Apple" I decided to give it another shot and so that I could submit a request.

After a little bit more digging, I found the culprit: Complications Assets.

I switched from "Individual Scales" to "Single Scale", updated one of the icons for the Graphic Extra Large, 38mm, but that was it: the build error disappeared.

I thought I was done and tried to archive the app this time, not just build it: it failed.

For the Graphic Extra Large, I changed the Apple Watch Screen Width to be "Any" instead of Individual Widths. This time I received a new error.

❌

CoreData: API Misuse: Attempt to serialize store access on non-owning coordinator (PSC = 0x600003cb9110, store PSC = 0x0)

I opened the Assets for my Alternate App Icons and switched them all from "All Sizes" to "Single Size". It worked!

But I received warnings, because not all formats require the same scale 😤. Here's my setup:

Circular: Individual Scales ⚠️
Extra Large: Single Scale
Graphic Bezel: Single Scale
Graphic Circular: Single Scale
Graphic Corner: Single Scale
Graphic Extra Large: Individual Scales ⚠️
Graphic Large Rectangular: Single Scale
Modular: Single Scale
Utilitarian: Single Scale

The (Initial) Result

# iPhone 14 Pro
App + On Demand Resources size: 74,7 MB compressed, 85,3 MB uncompressed
App size: 74,7 MB compressed, 85,3 MB uncompressed
On Demand Resources size: Zero KB compressed, Zero KB uncompressed

# Watch Ultra
App + On Demand Resources size: 26 MB compressed, 32,2 MB uncompressed
App size: 26 MB compressed, 32,2 MB uncompressed
On Demand Resources size: Zero KB compressed, Zero KB uncompressed

74,7 MB compressed with Single Size, vs 71,8MB compressed with All Sizes.

It is rather disappointing as a reward after so much waiting and debugging! But all is not lost. In the process, I discovered that my app could weigh a lot less! But that's for another post ;)

Anyway, I think I'll keep my current setup for the time being. What would you do? 🙃

I finally managed to use Xcode's new Singe Size app icon option, but my app seems heavier (74,7 MB vs 71,8MB for an iPhone 14 Pro).

What would you do?https://t.co/vgq2PI0whA
— Arnaud Joubay (@sowenjub) November 9, 2022

[Follow-up] The final result!

After this disappointing result, I started to investigate my assets and managed to divide the weight of the app by 3.

And then I discovered something: the reason why the above result was disappointing was that my configuration didn't yet handle all the new icon sizes.

By adding the new sizes, the App size went up

App size: 29,1 MB compressed, 41,2 MB uncompressed

And it was actually with the Single Size that I was able to get back to a reasonable size.

App size: 22,2 MB compressed, 33,4 MB uncompressed

Conclusion

These errors happen especially when you haven't caught up on the latest sizes. I guess Xcode will try to make the bridge between the required sizes and what you offer, but sometimes finds itself in an inextricable situation and spouts these incomprehensible errors.

Anyway, with a little help on my part, Single Size turned out to be a no-brainer:

less work to prepare the files
smaller app size

So, in the end, I'll say: Well done Xcode!