NSDiffableDataSourceSnapshot doesn't call Hashable methods if ItemIdentifierType is class

Question

Created Aug ’20

Replies 9

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I have a collection view with UICollectionViewDiffableDataSource, which has class PHAssetVisualAssetsPickerItem as it’s ItemIdentifierType. When I create instance of NSDiffableDataSourceSnapshot with the same items that dataSource already has, and then apply it, I see that collection view blinks even tho nothing changed. It actually blinks no matter what new snapshot is.

My investigation led to a fact that methods == (Equatable) and hasher(into:) (Hashable) are not called on items if items are classes. If I change them to structs, methods are called, dataSource understands what to do, nothing blinks.

https://www.icloud.com/iclouddrive/0vnB3auwp0ShEvPQmpVOAy6fg#ApplySnapshotIndexPaths

Here is the code where you can simply change HashableType to either class or struct and see that collectionView update looks different when you tap button.

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Answer 1

Claude31 OP

Aug ’20

Could you show the complete definition of the class ?

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Answer 2

Nezhyborets OP

Aug ’20

Sure! Here's the class that is used as ItemIdentifierType for UICollectionViewDiffableDataSource:

Code Block swiftclass HashableType: Hashable {
    let string: String
    init(string: String) {
        self.string = string
    }
    func hash(into hasher: inout Hasher) {
        hasher.combine(string)
    }
    static func == (lhs: HashableType, rhs: HashableType) -> Bool {
        return lhs.string == rhs.string
    }
}

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Answer 3

Claude31 OP

Aug ’20

I do not see a good reason for it not to work.

How did you define the dataSource ?
May look at this tutorial if needed:
https :// www.raywenderlich. com/8241072-ios-tutorial-collection-view-and-diffable-data-source

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Answer 4

Nezhyborets OP

Aug ’20

Seems the same. Overall the implementation linked in my post is pretty simple and default, I haven't found where it could go wrong.

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Answer 5

blazej.biesiada-toptal OP

Dec ’20

Same here! 🙋‍♂️

However, the collection view doesn't actually blink when updated in my case. Not sure why. Anyway, I only discovered it when trying to figure out why does a collection view creates way more cells then needed and makes most of them invisible. Turns out that each collection view reload creates a new set of cells because it thinks that data actually changed.

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Answer 6

PatrickDotStar OP

Jun ’21

Same for me. Both methods are never called when the model is a class but it's mandatory to implement them.

1

Answer 7

logancautrell OP

Mar ’22

There is a work around, in iOS 15 at least. You can wrap your class in a struct like this ->


struct ItemWrapper: Hashable {
    let item: Item

    func hash(into hasher: inout Hasher) {
        hasher.combine(item)
    }

    static func == (lhs: ItemWrapper, rhs: ItemWrapper) -> Bool {
        lhs.item == rhs.item
    }
}

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Answer 8

DaleOne OP

Jul ’23

I just ran into this issue. If I try to use a class instead of a struct as a diffable data source object, the hashable methods are not called causing issues. I haven't found anything documented about this limitation. It would be nice to get an official feedback about this. As a temporary fix, logancautrell solution works perfectly, thank you for sharing that.

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Answer 9

kevraypt OP

Oct ’24

Here's the stack overflow explaining the problem and the answer: https://stackoverflow.com/questions/67324651/why-do-diffable-datasources-treat-class-and-struct-types-differently

Need to inherit from NSObject and override isEqual(_ object: Any?) -> Bool and var hash: Int

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