Warning: UICollectionViewFlowLayout has cached frame mismatch for index path 'abc'

Question

This is the code causing the warning:

private override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes? {
    let attributes = super.layoutAttributesForItemAtIndexPath(indexPath)
    let distance = CGRectGetMidX(attributes!.frame) - self.midX;
    var transform = CATransform3DIdentity;
    transform = CATransform3DTranslate(transform, -distance, 0, -self.width);
    attributes!.transform3D = CATransform3DIdentity;
    return attributes
}

The console also prints:

This is likely occurring because the flow layout "xyz" is modifying attributes returned by UICollectionViewFlowLayout without copying them.

How do I fix this warning?

Phelloderm

Answer 1

This is likely occurring because the flow layout "xyz" is modifying attributes returned by UICollectionViewFlowLayout without copying them

And sure enough, that's just what you are doing:

private override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes? {
    let attributes = super.layoutAttributesForItemAtIndexPath(indexPath)
    let distance = CGRectGetMidX(attributes!.frame) - self.midX;
    var transform = CATransform3DIdentity;
    transform = CATransform3DTranslate(transform, -distance, 0, -self.width);
    attributes!.transform3D = CATransform3DIdentity;
    return attributes
}

I expect that if you simply say:

let attributes = 
    super.layoutAttributesForItemAtIndexPath(indexPath).copy() 
    as! UICollectionViewLayoutAttributes

or similar, the problem will go away.

Cymar

In Swift 2, you can't call copy() on super.layoutAttributesForItemAtIndexPath. However you can call it on the individual layout attributes, then put those into an array and return that instead. See https://mcmap.net/q/145845/-warning-uicollectionviewflowlayout-has-cached-frame-mismatch-for-index-path-39-abc-39 – Langsyne

@TrevorAlyn yes you can. Here is an example of me doing it. github.com/mattneub/Programming-iOS-Book-Examples/blob/… and the code I give in my answer is Swift 2 and works fine. – Cymar

@matt: Oops -- I was trying to call copy() on layoutAttributesForElementsInRect, not layoutAttributesForItemAtIndexPath. But Xcode gives me a "has no member copy" error when I try to do this: var attributes = super.layoutAttributesForElementsInRect(rect).copy() – Langsyne

@TrevorAlyn If you don't copy it, is it giving you the same error? – Cymar

Nope. Fortunately I can copy the individual attributes, like so: attributesCopy.append(attributes![0].copy() as! UICollectionViewLayoutAttributes) – Langsyne

@TrevorAlyn Right but if you're not having a problem I don't see why you need to make a copy. – Cymar

@matt: I'm copying them so I can make adjustments to them. – Langsyne

In Swift4.1 I used override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? { var array: [UICollectionViewLayoutAttributes] = [] for object in super.layoutAttributesForElements(in: rect)! { array.append(object.copy() as! UICollectionViewLayoutAttributes) } – Enphytotic

Answer 2

In addition to the great answer above.

I know the example code is written in swift, but I thought that it can be helpful to have the Objective-C version.

For Objective-C, this won't work, because the copy function does only a shallow copy. You will have to do this:

NSArray * original   = [super layoutAttributesForElementsInRect:rect];
NSArray * attributes = [[NSArray alloc] initWithArray:original copyItems:YES];

I have added a temp variable for readability.

Gilda

In addition : my answer is handy when You use an array of attributes, if You want to get only the attributes for a specific indexPath see the answer of Ayman Ibrahim – Gilda

Answer 3

I had this issue when overriding layoutAttributesForElementsInRect. Iterating through each element in the super.layoutAttributesForElementsInRect(rect) array and calling copy wasn't working for me, so I ended up falling back on Foundation classes and using NSArray's copyItems:

override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    // unwrap super's attributes
    guard let superArray = super.layoutAttributesForElementsInRect(rect) else { return nil }

    // copy items
    guard let attributes = NSArray(array: superArray, copyItems: true) as? [UICollectionViewLayoutAttributes] else { return nil }

    // modify attributes

    return attributes
}

Harriettharrietta

Thank you for posting this even though there was another answer. I was iterating through each element in an array and copying as well. Wasn't working for me either. So thanks for posting this solution. – Tarter

@JAL, how to append attributes to it. because its a NSArray – Cooe

@Cooe cast the NSArray as a Swift Array like I do with the attributes variable – Harriettharrietta

@JAL, I solved the copy issue, but I am facing one more issue, I have created a SO question here #38147413 – Cooe

Answer 4

It's not the answer to original question, but can help for layoutAttributesForElements(in rect: CGRect) (Swift 3.0):

let safeAttributes = super.layoutAttributesForElements(in: rect)?.map { $0.copy() as! UICollectionViewLayoutAttributes }
safeAttributes?.forEach { /* do something with attributes*/ }

Gambado

Answer 5

Adding to @Georgi answer

<NSCopying> must be conformed and add copy message call to layoutAttributesForItemAtIndexPath

UICollectionViewLayoutAttributes* attributes = [[super layoutAttributesForItemAtIndexPath:indexPath] copy];

Odericus

Answer 6

Updated response for Swift 3!

for func layoutAttributesForElements

override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

    guard let attributes = super.layoutAttributesForElements(in: rect) else {
        return nil
    }

    guard let attributesToReturn =  attributes.map( { $0.copy() }) as? [UICollectionViewLayoutAttributes] else {
        return nil
    }

    return attributesToReturn

}

for func layoutAttributesForItem

override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {

    guard let currentItemAttributes = super.layoutAttributesForItem(at: indexPath)?.copy() as? UICollectionViewLayoutAttributes else {
        return nil
    }

    return currentItemAttributes
}

If you override both function, you have to call copy on both function!

Good coding!

Flushing

Answer 7

I have subclassed UICollectionViewFlowLayout. Inside layoutAttributesForElementsInRect() I did this change:

change from

guard let attributesForItem: UICollectionViewLayoutAttributes = self.layoutAttributesForItemAtIndexPath(indexPath) else {
    return
}

change to

guard let attributesForItem = self.layoutAttributesForItemAtIndexPath(indexPath)?.copy() as? UICollectionViewLayoutAttributes else {
    return
}

Russellrusset