Deprecate `.observe()`

[sindresorhus]

.updates() is better in every way. Having two ways to do the same is also confusing. We cannot remove .observe() for a long time, but maybe we could add availability annotations so that apps that target macOS 10.15+ (and equivalent iOS/tvOS/watchOS) gets a deprecation message.

[Maschina]

Since I see the point to get rid of dead wood and streamline the API, I see a strong benefit of using .observe() and that is the KeyChange object, which comprises KeyChange.oldValue. AsyncStream would loose this functionality.

[Maschina]

Would it make sense to extend the AsyncStream implementation with a KeyChange next to Value?

[sindresorhus]

I wanted the main method to be simple, because from extensive use, I almost never needed the old value. However, I do agree we should support providing the old value somehow. Maybe a updatesWithPrevious() method or something. It would return a tuple of old/new values though. I considered overloading .updates(), but it may create some ambiguities. I will need to think about it.

---

For now, you could use these utilities:

extension AsyncStream {
	func withPrevious() -> AsyncStream<(previous: Element?, current: Element)> {
		.init { continuation in
			Task {
				var previous: Element?
				for await element in self {
					continuation.yield((previous, element))
					previous = element
				}

				continuation.finish()
			}
		}
	}

	func withPrevious(initialPreviousValue: Element) -> AsyncStream<(previous: Element, current: Element)> {
		.init { continuation in
			Task {
				var previous = initialPreviousValue
				for await element in self {
					continuation.yield((previous, element))
					previous = element
				}

				continuation.finish()
			}
		}
	}
}

The obvious benefit of having support built-in is that the previous value would be non-optional.