I'm trying to get self sizing UICollectionViewCells
working with Auto Layout, but I can't seem to get the cells to size themselves to the content. I'm having trouble understanding how the cell's size is updated from the contents of what's inside the cell's contentView.
Here's the setup I've tried:
UICollectionViewCell
with a UITextView
in its contentView.UITextView
is disabled.UITextView
is pinned to the left of the cell with an explicit width of 320.UITextView
pinned to the top of the cell.UITextView
has a height constraint set to a constant which the UITextView
reports will fit the text.Here's what it looks like with the cell background set to red, and the UITextView
background set to Blue:
I put the project that I've been playing with on GitHub here.
preferredLayoutAttributesFittingAttributes
renamed to preferredLayoutAttributesFitting
and use auto sizing
systemLayoutSizeFittingSize
renamed to systemLayoutSizeFitting
After seeing my GitHub solution break under iOS 9 I finally got the time to investigate the issue fully. I have now updated the repo to include several examples of different configurations for self sizing cells. My conclusion is that self sizing cells are great in theory but messy in practice. A word of caution when proceeding with self sizing cells.
TL;DR
Check out my GitHub project
Self sizing cells are only supported with flow layout so make sure thats what you are using.
There are two things you need to setup for self sizing cells to work.
estimatedItemSize
on UICollectionViewFlowLayout
Flow layout will become dynamic in nature once you set the estimatedItemSize
property.
self.flowLayout.estimatedItemSize = UICollectionViewFlowLayout.automaticSize
This comes in 2 flavours; Auto-Layout or custom override of preferredLayoutAttributesFittingAttributes
.
I won't go to in to detail about this as there's a brilliant SO post about configuring constraints for a cell. Just be wary that Xcode 6 broke a bunch of stuff with iOS 7 so, if you support iOS 7, you will need to do stuff like ensure the autoresizingMask is set on the cell's contentView and that the contentView's bounds is set as the cell's bounds when the cell is loaded (i.e. awakeFromNib
).
Things you do need to be aware of is that your cell needs to be more seriously constrained than a Table View Cell. For instance, if you want your width to be dynamic then your cell needs a height constraint. Likewise, if you want the height to be dynamic then you will need a width constraint to your cell.
preferredLayoutAttributesFittingAttributes
in your custom cellWhen this function is called your view has already been configured with content (i.e. cellForItem
has been called). Assuming your constraints have been appropriately set you could have an implementation like this:
//forces the system to do one layout pass
var isHeightCalculated: Bool = false
override func preferredLayoutAttributesFitting(_ layoutAttributes: UICollectionViewLayoutAttributes) -> UICollectionViewLayoutAttributes {
//Exhibit A - We need to cache our calculation to prevent a crash.
if !isHeightCalculated {
setNeedsLayout()
layoutIfNeeded()
let size = contentView.systemLayoutSizeFitting(layoutAttributes.size)
var newFrame = layoutAttributes.frame
newFrame.size.width = CGFloat(ceilf(Float(size.width)))
layoutAttributes.frame = newFrame
isHeightCalculated = true
}
return layoutAttributes
}
NOTE On iOS 9 the behaviour changed a bit that could cause crashes on your implementation if you are not careful (See more here). When you implement preferredLayoutAttributesFittingAttributes
you need to ensure that you only change the frame of your layout attributes once. If you don't do this the layout will call your implementation indefinitely and eventually crash. One solution is to cache the calculated size in your cell and invalidate this anytime you reuse the cell or change its content as I have done with the isHeightCalculated
property.
At this point you should have 'functioning' dynamic cells in your collectionView. I haven't yet found the out-of-the box solution sufficient during my tests so feel free to comment if you have. It still feels like UITableView
wins the battle for dynamic sizing IMHO.
Be very mindful that if you are using prototype cells to calculate the estimatedItemSize - this will break if your XIB uses size classes. The reason for this is that when you load your cell from a XIB its size class will be configured with Undefined
. This will only be broken on iOS 8 and up since on iOS 7 the size class will be loaded based on the device (iPad = Regular-Any, iPhone = Compact-Any). You can either set the estimatedItemSize without loading the XIB, or you can load the cell from the XIB, add it to the collectionView (this will set the traitCollection), perform the layout, and then remove it from the superview. Alternatively you could also make your cell override the traitCollection
getter and return the appropriate traits. It's up to you.
Let me know if I missed anything, hope I helped and good luck coding