Experimental `onChange(of:initial:perform:)`

Observation.swift

import Foundation
import Observation

func onChange<Value>(
    of value: @autoclosure () -> Value,
    initial: Bool = false,
    perform: (_ oldValue: Value, _ newValue: Value) -> Void,
    _: isolated Actor = #isolation
) async throws {
    let (stream, continuation) = AsyncThrowingStream.makeStream(of: Void.self)

    var iterator = stream.makeAsyncIterator()
    var oldValue: Value?
    repeat {
        let newValue = withObservationTracking {
            value()
        } onChange: {
            continuation.yield()
        }

        if let _oldValue = oldValue {
            perform(_oldValue, newValue)
        } else {
            oldValue = newValue
            if initial {
                // NOTE: This behavior same as `onChange(of:initial:perform:)` on SwiftUI `View`.
                perform(newValue, newValue)
            }
        }
        oldValue = newValue

        if Task.isCancelled {
            // This is only finishing point, by cancelling Task.
            continuation.finish(throwing: CancellationError())
        }
    } while try await iterator.next() != nil

    // Should not reach here because previous `try` throws with `CancellationError`
    // is the only finishing point.
    fatalError("Should not reach here.")
}