A publisher that contains the previous value

[vladyslavsosiuk]

Hello,

I recently came across a use case where I couldn't use the onChange reducer operator with a shared value and had to explicitly observe changes via the publisher property of the Shared type. Unlike the onChange modifier, the publisher property only provides the most recent value. Would there be interest in having a publisher property that emits both the previous and new values? I'd be happy to open a PR for it.
The interface will look like this:

extension Shared {
    public var publisherWithOldValue: some Publisher<(old: Value, new: Value), Never> { ... }
}

[stephencelis]

@vladyslavsosiuk Thanks for the discussion. Because the publisher emits during willSet, you should be able to get the previous value by accessing it directly on the shared:

$shared.publisher.map { (shared, $0) } // (old, new)

Does this help satisfy your needs?