Add update support for diff algorithm

[Tayphoon]

DifferenceKit use ContentEquatable protocol to produce items update. DataSource wrap items into struct which implement Differentiable protocol. In fact diff algorithm never produce update events, there is only one way - call reload method in DataSource. We would like to implement ContentEquatable for DataSource items and get ChangeSet with updates.

[Ryo Aoyama]

This is the same specification as Apple's official DiffableDataSource, because it uses "Hashable" instead of "Identifiable".

[Olli Tapaninen]

Just run into this issue. Could you consider introducing a wrapper struct to help out?

/// A struct whose hash value is based on
/// the differenceIdentifier of the content.
public struct Diffable<Item: Differentiable & Equatable>: Hashable {
    public var content: Item

    public init(_ content: Item) {
        self.content = content
    }
    public func hash(into hasher: inout Hasher) {
        hasher.combine(content.differenceIdentifier)
    }
}

[Winston Du]

@ollitapa , I have a PR over from my fork of this library that solves this issue. You can feel free to pull from my fork until it gets merged in.

#18

The larger issue, I think, is Apple's engineers clearly wrote their library wrong (they don't have any updates). So the question is whether we should have this library conform to Apple's mistakes, or we do it right.

[Olli Tapaninen]

@winstondu Seems good! I agree that the Apple provided API is a bit confusing, so I opted to use the Diffable wrapper I showed above. I thought it worked fine, but now that you mention the issue, with the hashValue handling, I can see that those updates are also needed.

[Olli Tapaninen]

Btw, it's curious that SwiftUI List seems to use Identifiable protocol for "differenceIdentifier" and view graph equality for the "isContentEqual"

[Winston Du]

@ollitapa , agreed. I don't understand why Apple's official DiffableDataSources uses the Hashable protocol when they have the Identifiable Protocol