Offscreen UITableViewCells (for size calculations) not respecting size class?

Update on Dec 2015:

Apple now discourages overriding -traitCollection. Please consider using other workarounds. From the doc:

IMPORTANT

Use the traitCollection property directly. Do not override it. Do not provide a custom implementation.

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Original Answer:

The existing answer is great. It explained that the problem is that:

• -dequeueReusableCellWithIdentifier: returns a cell without a valid cell.traitCollection, and
• cell.traitCollection is readonly.

The proposed workaround is to temporarily add the cell to the table view. However, this does NOT work if we are in, say, -viewDidLoad, in which the traitCollection of the table view, or the view of the view controller, or even the view controller itself, is not valid yet.

Here, I propose another workaround, which is to override traitCollection of the cell. To do so:

1. Create a custom subclass of UITableViewCell for the cell (which you probably did already).

2. In the custom subclass, add a - (UITraitCollection *)traitCollection method, which overrides the getter of the traitCollection property. Now, you can return any valid UITraitCollection you like. Here’s a sample implementation:

   // Override getter of traitCollection property
   // https://stackoverflow.com/a/28514006/1402846
   - (UITraitCollection *)traitCollection
   {
       // Return original value if valid.
       UITraitCollection* originalTraitCollection = [super traitCollection];
       if(originalTraitCollection && originalTraitCollection.userInterfaceIdiom != UIUserInterfaceIdiomUnspecified)
       {
           return originalTraitCollection;
       }
   
       // Return trait collection from UIScreen.
       return [UIScreen mainScreen].traitCollection;
   }
   

   Alternatively, you can return a suitable UITraitCollection created using any one of its create methods, e.g.:

   + (UITraitCollection *)traitCollectionWithDisplayScale:(CGFloat)scale
   + (UITraitCollection *)traitCollectionWithTraitsFromCollections:(NSArray *)traitCollections
   + (UITraitCollection *)traitCollectionWithUserInterfaceIdiom:(UIUserInterfaceIdiom)idiom
   + (UITraitCollection *)traitCollectionWithHorizontalSizeClass:(UIUserInterfaceSizeClass)horizontalSizeClass
   + (UITraitCollection *)traitCollectionWithVerticalSizeClass:(UIUserInterfaceSizeClass)verticalSizeClass
   

   Or, you can even make it more flexible by doing this:

   // Override getter of traitCollection property
   // https://stackoverflow.com/a/28514006/1402846
   - (UITraitCollection *)traitCollection
   {
       // Return overridingTraitCollection if not nil,
       // or [super traitCollection] otherwise.
       // overridingTraitCollection is a writable property
       return self.overridingTraitCollection ?: [super traitCollection];
   }
   

This workaround is compatible with iOS 7 because the traitCollection property is defined in iOS 8+, and so, in iOS 7, no one will call its getter, and thus our overriding method.