I have filed a bug report for this (FB17734946), but I'm posting it here verbatim in case others have the same issue and in hopes of getting attention from an Apple engineer sooner.
When calling setNeedsDisplayInRect on a CATiledLayer - or a UIView whose backing layer is CATiledLayer - one would expect to re-draw only a region identified by the rect passed to the method. This is even written in the documentation for the class:
"Regions of the layer may be invalidated using the setNeedsDisplayInRect: method however the update will be asynchronous. While the next display update will most likely not contain the updated content, a future update will."
However, upon calling this method, CATiledLayer redraws whole contents instead of just the tile at the specified rect, and it flashes when doing so. It behaves exactly the same as if one had called setNeedsDisplay without passing any rect; all contents are cleared and re-drawn again. I'm 100% sure I've passed in the correct rect of the exact tile that I need to redraw. I have even tried passing much smaller rects, but still the same. (And yes, the rect I've passed accounts for the current level of detail.)
I have found this GitHub repo https://github.com/frankus/NetPhotoScroller, which based on discussion from here https://forums.macrumors.com/threads/catiledlayer-blanks-out-tiles-when-redrawing.1333948/ aims at solving these issues by using two private methods on CATiledLayer class:
(void)setNeedsDisplayInRect:(CGRect)r levelOfDetail:(int)level;
(BOOL)canDrawRect:(CGRect)rect levelOfDetail:(int)level;
I have explored the repo in detail, however I wasn't able to test exactly this code from the GitHub repo. I have tried using those two private methods myself (through an Objective-C class that defines the methods in the header file and then a swift class which inherits it), but I couldn't solve the issue; the flashing and the full re-draw is still there.
After doing a lot of research, the conclusion seems to be that one cannot use CATiledLayer with contents that are downloaded remotely, on demand, as tiles are being requested.
I have, however, found one interesting thing which seems to work so far: before calling setNeedsDisplayInRect (or just setNeedsDisplay, as they behave the same for CATiledLayer in my testing), cache the current layer's contents, and after calling setNeedsDisplay (or setNeedsDisplayInRect), restore the contents back to the layer. This prevents flashing and preserves any tiles that were drawn at the time of the re-draw.
let c = tiledLayer.contents
tiledLayer.setNeedsDisplay(tileRect)
tiledLayer.contents = c
However! Docs clearly state the warning:
Do not attempt to directly modify the contents property of a CATiledLayer object. Doing so disables the ability of a tiled layer to asynchronously provide tiled content, effectively turning the layer into a regular CALayer object.
I believe this message implies modifying the contents property with some raw content, like image data, and that it may be safe to re-apply the existing contents (which are in my testing of type CAImageProvider) -- but I can't rely on an implementation detail in my production app.
I have tested this and confirmed that the bug appears on:
iPhone 14 Pro, iOS 18.5
iPhone 13 Pro, iOS 17.5.1
iPhone 5s, iOS 15.8.3
iPad Pro 1st gen, iPadOS 18.4.1
a couple simulator versions
I can also confirm that the fix (to re-apply contents property) is also working properly on all these versions.
Is this expected behavior, that tiled layer redraws itself entirely instead of redrawing specific tiles?
Is it safe to modify contents of a CATiledLayer by re-applying the existing contents?
If not, is there an alternative to avoid flashing?