Ranjit Ranjit - 5 months ago 472
iOS Question

UICollectionView custom flow layout crashes on scrolling

I am creating a custom flowLayout with section headers.

here is my flowLayout.swift

import UIKit

class FlowLayout: UICollectionViewFlowLayout {

override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
// let attributesForElementsInRect = super.
var newAttributesForElementsInRect = [UICollectionViewLayoutAttributes]()

// unwrap super's attributes
guard let attributesForElementsInRect = super.layoutAttributesForElementsInRect(rect) else { return nil }

// modify attributes

var leftMargin: CGFloat = 0.0;



for attributes in attributesForElementsInRect {

let itemAttributesCopy = attributes.copy() as! UICollectionViewLayoutAttributes

if( itemAttributesCopy.frame.size.width + leftMargin > self.collectionView?.frame.size.width)
{
leftMargin = 8.0
}
if (itemAttributesCopy.frame.origin.x == self.sectionInset.left) {
leftMargin = self.sectionInset.left
} else {
var newLeftAlignedFrame = itemAttributesCopy.frame
newLeftAlignedFrame.origin.x = leftMargin
itemAttributesCopy.frame = newLeftAlignedFrame
}
leftMargin += itemAttributesCopy.frame.size.width + 8
print(itemAttributesCopy.frame)

newAttributesForElementsInRect.append(itemAttributesCopy)
print(itemAttributesCopy)
}

return newAttributesForElementsInRect
}

}


here is my ViewController with delegate function for section header

func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {


if(sectionsArray.count == 0)
{
return 1
}
else
{
return sectionsArray.count
}
}


func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath) -> UICollectionReusableView {

let headerView: TagHeader = collectionView.dequeueReusableSupplementaryViewOfKind(kind, withReuseIdentifier: "TagHeader", forIndexPath: indexPath) as! TagHeader

headerView.sectionLabel.text = sectionsArray[indexPath.section].header
return headerView
}


So when I run the app, it shows me the section headers and everything works fine.

But when I scroll, app crashes with following error from debugger

2016-07-01 18:52:12.051 TagFlowLayout[15025:4704387] *** Assertion failure in -[UICollectionViewData validateLayoutInRect:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKit/UIKit-3512.60.7/UICollectionViewData.m:408
2016-07-01 18:52:12.054 TagFlowLayout[15025:4704387] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'layout attributes for supplementary item at index path (<NSIndexPath: 0x17e96f70> {length = 2, path = 0 - 0}) changed from <UICollectionViewLayoutAttributes: 0x17e7ab00> index path: (<NSIndexPath: 0x17d8fb30> {length = 2, path = 0 - 0}); element kind: (UICollectionElementKindSectionHeader); frame = (8 0; 314 50); zIndex = 10; to <UICollectionViewLayoutAttributes: 0x17e9d770> index path: (<NSIndexPath: 0x17e96f70> {length = 2, path = 0 - 0}); element kind: (UICollectionElementKindSectionHeader); frame = (0 0; 314 50); zIndex = 10; without invalidating the layout'

Answer

You need to invalidate the existing layout before updating, see the end of the error message:

without invalidating the layout

You should override the method at the subclass of UICollectionViewFlowLayout:

override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
        return true
    }

For more reference see Apple Documentation for UICollectionViewLayout