Intro

This an example of UICollectionView with custom layout, using UICollectionViewInvalidationContext to work with self-sizing cells.

TextCell

Simple subclass of UICollectionViewCell which display cell with UILabel inside.

DataSource

This object is a collection view data source that will return n sections with one cell inside (colored, with random text inside).

MyLayoutAttributes

In this project I'm using custom subclass of UICollectionViewLayoutAttributes. This sublass has one property called preferredWidth which is width of collection view delegated form UICollectionViewLayout. TextCell class use this value in self.preferredLayoutAttributesFitting(_:) to activate additional width constraint. As a result of this operation each cell width is equal to collection view width.

MyLayout

It's a subclass of UICollectionViewLayout. I've tried to comment each line in this file to explain everything I know. Method self.layoutAttributesForItem(at:) returns newly created subclass of MyLayoutAttributes with already calculated frame. In method self.layoutAttributesForElements(in:) I'm iterating over available indexPaths, finding index paths of layout attributes that frames are intersecting with current bounds.

I'm keeping cached cell size under array called cachedSizes where index is cell's section index. If key is out of bounds, i'm using estimatedHeight property as default cell height.

Now the fun part, context invalidation.

1. First i'm checking if cached size of cell is different from preferredAttributes.size in self.shouldInvalidateLayout(forPreferredLayoutAttributes:withOriginalAttributes:). If true, that means I need to invalidate this cell...

2. In self.invalidationContext(forPreferredLayoutAttributes:withOriginalAttributes:) I'm caching new value of calculated cell size and invalidating item at given this index path and all index paths below (since at least frame origin of next cells will change).

3. This process will repeat until all cells have proper size.

My Problems:

1. When you run project and scroll slowly everything looks great, self-sizing cells works! (https://cl.ly/1U2L1h3y2u43) But when you scroll more dynamically strage things are happening, like some cell size is wrong, or strange spacing between cells occures (https://cl.ly/0T023b1T0u3l)

2. I'm randomizing cell's label text to be 1-3 line text, so cell height is between 50pt - 77pt, that's why layout's estimatedHeight property is equal to 50pt which is resonable in this case. But when we increase this value to much bigger like 150 starange things are happening (https://cl.ly/2M452a2s0G0N). In production evniroment cell have more that three lines... What's the best approach here?

3. preferredLayoutAttributesFitting(_:) in TextCell is calling several times, in more complicated layouts this makes some performance issues / frames droppings. How can I improve/avoid this?

4. How does adding self-sizing cell work with custom layouts, since initial frame is estimated? From my reaserch, after insert cell is first layouted/displayed and then CollectionViewLayout try to invalidate cell to set proper frame. How can I get poper frame size a priori?