Warning: UICollectionViewFlowLayout has cached fra

问题:

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?

回答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.

回答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.

回答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
}

回答4:

Adding to @Georgi answer

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

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

回答5:

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*/ }

回答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!

回答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
}