UICollectionViewLayout (Converting from Swift to Objective-C) - ios

Question

I'm currently following this tutorial (https://www.raywenderlich.com/99087/swift-expanding-cells-ios-collection-views) which creates a custom UICollectionViewLayout, the problem is that the tutorial is written in Swift but I'm trying to convert it to Objective-C for my project.
The first part of reproducing the tutorial into Objective-C was fine and i've gotten to this stage.
Though, in the 2nd part when we're suppose to create a custom UICollectionViewLayout, and upon changing in the storyboard the Layout of CollectionView to Custom and setting the custom class, a blank screen appears.
Below are the codes that I'd reproduced from the tutorial from Swift to Objective-C:
#implementation TimetableCustomLayout{
NSMutableArray *cache;
}
-(NSInteger)featuredItemIndex{
CGFloat dragOffset = 180;
return MAX(0, self.collectionView.contentOffset.y - dragOffset);
}
-(CGFloat)nextItemPercentageOffset{
CGFloat dragOffset = 180;
return (self.collectionView.contentOffset.y / dragOffset) - [self featuredItemIndex];
}
-(CGFloat)width{
return CGRectGetWidth(self.collectionView.bounds);
}
-(CGFloat)height{
return CGRectGetHeight(self.collectionView.bounds);
}
-(NSInteger)numberOfItems{
//Will be replaced with dynamic value later
return 5;
}
-(CGSize)collectionViewContentSize{
CGFloat dragOffset = 180;
return CGSizeMake([self height], [self width]);
}
-(void)prepareLayout{
[super prepareLayout];
cache = [[NSMutableArray alloc]initWithObjects:[UICollectionViewLayoutAttributes class], nil];
self.standardHeight = 100;
self.featuredHeight = 280;
CGRect frame = CGRectZero;
CGFloat y = 0;
for(int item = 0; item < 5; item ++){
NSIndexPath *indexPath = [NSIndexPath indexPathForItem:item inSection:0];
UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
attributes.zIndex = item;
CGFloat height = self.standardHeight;
if(indexPath.item == [self featuredItemIndex]){
CGFloat yOffset = self.standardHeight * [self nextItemPercentageOffset];
y = self.collectionView.contentOffset.y - yOffset;
height = self.featuredHeight;
}else if(indexPath.item == ([self featuredItemIndex] + 1) && indexPath.item != [self numberOfItems]){
CGFloat maxY = y + self.standardHeight;
height = self.standardHeight + MAX((self.featuredHeight - self.standardHeight) * [self nextItemPercentageOffset], 0);
y = maxY - height;
}
frame = CGRectMake(0, y, [self width], [self height]);
attributes.frame = frame;
[cache addObject:attributes];
y = CGRectGetMaxY(frame);
}
}
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
[super layoutAttributesForElementsInRect:rect];
NSMutableArray *layoutAttributes = [[NSMutableArray alloc]initWithObjects:[UICollectionViewLayoutAttributes class], nil];
NSLog(#"%#", cache);
for(UICollectionViewLayoutAttributes *attributes in cache){
if(CGRectIntersectsRect(attributes.frame, rect)){
[layoutAttributes addObject:attributes];
}
}
return layoutAttributes;
}
-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds{
return true;
}
#end
I'm also getting error, Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[UICollectionViewLayoutAttributes frame]: unrecognized selector.
I believe that the error is possibly due to incorrect translation from Swift to Objective-C, particularly this line,
Swift:
var cache = [UICollectionViewLayoutAttributes]()
Objective-C:
NSMutableArray *layoutAttributes = [[NSMutableArray alloc]initWithObjects:[UICollectionViewLayoutAttributes class], nil];
This is my first question on StackOverflow, any feedback and help would be greatly appreciated, thanks in advance.

Answer 1

You are adding a reference to the UICollectionViewLayoutAttributes class to the cache and layoutAttributes array on initialization. And then, calling frame property to the class reference (notice the + on the error message that denotes a class method, where instance methods use -).
Replace this line:
NSMutableArray *layoutAttributes = [[NSMutableArray alloc]initWithObjects:[UICollectionViewLayoutAttributes class], nil];
With this:
NSMutableArray *layoutAttributes = [[NSMutableArray alloc] init];
The same applies to the cache variable initialization.
You can also use type-safe arrays in Objective-C using generics:
NSMutableArray<UICollectionViewLayoutAttributes*> *layoutAttributes = [[NSMutableArray alloc] init];
More information here.